3.5 Lipid Structure and Function
Melissa Hardy and Christelle Sabatier
Learning Objectives
By the end of this section, you will be able to do the following:
- Describe the four major types of lipids
- Explain the role of fats in storing energy
- Differentiate between saturated and unsaturated fatty acids
- Define the basic structure of a steroid and some steroid functions
Lipids include a diverse group of compounds that are largely nonpolar in nature. This is because they are primarily composed of hydrocarbons that include mostly nonpolar carbon–carbon or carbon–hydrogen bonds. Non-polar molecules are hydrophobic (“water fearing”). Lipids perform many different functions in a cell. Cells store energy for long-term use in the form of fats. Lipids also provide insulation from the environment for plants and animals. For example, they help keep aquatic birds and mammals dry when forming a protective layer over fur or feathers because of their water-repellant hydrophobic nature. Lipids are also the building blocks of many hormones and are an important constituent of all cellular membranes. Lipids include fats, oils, waxes, phospholipids, and steroids.
Fats and Oils
A fat molecule consists of two main components—glycerol and fatty acids. Glycerol is an organic compound (alcohol) with three carbons, five hydrogens, and three hydroxyl (OH) groups. Fatty acids have a long chain of hydrocarbons to which a carboxyl group is attached, hence the name “fatty acid.” The number of carbons in the fatty acid may range from 4 to 36. The most common are those containing 12–18 carbons.
In a fat molecule, the fatty acids attach to each of the glycerol molecule’s three carbons via dehydration synthesis.
During this ester bond formation, three water molecules are released. The three fatty acids in the triacylglycerol may be similar or dissimilar. We also call fats triacylglycerols or triglycerides because of their chemical structure. Some fatty acids have common names that specify their origin. For example, palmitic acid, a saturated fatty acid, is derived from the palm tree. Arachidic acid is derived from Arachis hypogea, the scientific name for groundnuts or peanuts.
Fatty acids may be saturated or unsaturated. In a fatty acid chain, if there are only single bonds between neighboring carbons in the hydrocarbon chain, the fatty acid is saturated. Saturated fatty acids are saturated with hydrogen. In other words, the number of hydrogen atoms attached to the carbon skeleton is maximized. When a fatty acid has no double bonds, it is a saturated fatty acid because it is not possible to add more hydrogen to the chain’s carbon atoms.
A fat may contain similar or different fatty acids attached to glycerol. Long straight fatty acids with single bonds generally pack tightly and are solid at room temperature. Animal fats with stearic acid and palmitic acid (common in meat) and the fat with butyric acid (common in butter) are examples of saturated fats. Mammals store fats in specialized cells, or adipocytes, where fat globules occupy most of the cell’s volume.
When the hydrocarbon chain contains a double bond, the fatty acid is unsaturated. Oleic acid is an example of an unsaturated fatty acid.
Most unsaturated fats are liquid at room temperature. We call these oils. If there is one double bond in the molecule, then it is a monounsaturated fat (e.g., olive oil), and if there is more than one double bond, then it is a polyunsaturated fat (e.g., canola oil).
Plants commonly store fat or oil in many seeds and use them as a source of energy during seedling development. Unsaturated fats or oils are usually of plant origin and contain cis unsaturated fatty acids. Cis and trans indicate the configuration of the molecule around the double bond. If hydrogens are present in the same plane, it is a cis fat. If the hydrogen atoms are on two different planes, it is a trans fat. The cis double bond causes a bend or a “kink” that prevents the fatty acids from packing tightly, and forming as many van der Waals interactions between them, keeping them liquid at room temperature. Olive oil, corn oil, canola oil, and cod liver oil are examples of unsaturated fats.
Trans Fats
The food industry artificially hydrogenates oils to make them semi-solid and of a consistency desirable for many processed food products. Simply speaking, hydrogen gas is bubbled through oils to solidify them. During this hydrogenation process, double bonds of the cis– conformation in the hydrocarbon chain may convert to double bonds in the trans– conformation.
Margarine, some types of peanut butter, and shortening are examples of artificially hydrogenated trans fats. Recent studies have shown that an increase in trans fats in the human diet may lead to higher levels of low-density lipoproteins (LDL), or “bad” cholesterol, which in turn may lead to plaque deposition in the arteries, resulting in heart disease. Many fast food restaurants have recently banned using trans fats, and food labels are required to display the trans fat content.
Waxes
Wax covers some aquatic birds’ feathers and some plants’ leaf surfaces. Because of waxes’ hydrophobic nature, they prevent water from sticking on the surface.
Phospholipids
Phospholipids are the major components of the plasma membrane. Like fats, most phospholipids are comprised of fatty acid chains attached to a glycerol backbone. However, instead of three fatty acids attached as in triglycerides, there are two fatty acids, and a modified phosphate group is attached to the glycerol’s third carbon.
A phospholipid is an amphipathic molecule, meaning it has a hydrophobic and a hydrophilic part. The fatty acid chains are hydrophobic and cannot interact with water; whereas, the phosphate-containing group is hydrophilic and interacts with water. You will have a chance to explore phospholipids in more details in Section 4.1 Plasma Membrane Structure and Components
Steroids
Unlike phospholipids and fats, steroids have a fused ring structure. Although they do not resemble the other lipids, scientists group them with them because they are also hydrophobic and insoluble in water. All steroids have four linked carbon rings and several of them, like cholesterol, have a short tail. Many steroids also have a hydroxyl group, which makes them alcohols (sterols).
Cholesterol is the most common steroid. The liver synthesizes cholesterol. It is the precursor to many steroid hormones such as testosterone and estradiol, as well as Vitamin D, and bile salts. Cholesterol is absolutely necessary for the body’s proper functioning. Sterols (cholesterol in animal cells, phytosterol in plants) are components of the plasma membrane of cells and are found within the phospholipid bilayer.
Media Attributions
- Great Crested Grebe © Dr. Georg Wiestschorke is licensed under a CC0 (Creative Commons Zero) license
- fat structure is licensed under a Public Domain license
- Stearic acid © OpenStax is licensed under a CC BY (Attribution) license
- oleic acid © OpenStax is licensed under a CC BY (Attribution) license
- Trans fat © OpenStax is licensed under a CC BY (Attribution) license
- Raindrops on a leaf © Denis Doukhan is licensed under a CC0 (Creative Commons Zero) license
- steroids © OpenStax is licensed under a CC BY (Attribution) license
Macromolecules that are predominantly nonpolar and hydrophobic. Made up of fatty acids.
Phospholipids and steroids are also in this category and have different properties.
Learning Objectives
By the end of this chapter, you will be able to do the following:
- Predict the functional effects of mutations in β-galactosidase
Proteins are one of the most abundant biological macromolecules in living systems and have the most diverse range of functions of all macromolecules. Proteins may be structural, regulatory, contractile, or protective. They may serve in transport, storage, or membranes; or they may be toxins or enzymes. Each cell in a living system may contain thousands of proteins, each with a unique function. Their structures, like their functions, vary greatly, and by investigating their structures, we can make predictions about their functions.
1. Protein structure
A protein's shape is critical to its function. For example, an enzyme can bind to a specific substrate at an active site. If this active site is altered because of local changes or changes in overall protein structure, the enzyme may be unable to bind to the substrate. To understand how the protein gets its final shape or conformation, we need to understand the four levels of protein structure: primary, secondary, tertiary, and quaternary.
Primary Structure
The amino acid sequence in a polypeptide chain is its primary structure. For example, the primary sequence of hemoglobin may be found on Uniprot, entry P69905. The N-terminal amino acid is methionine (Met, M), and the C-terminal amino acid is arginine (Arg, R) (Figure 1). The amino acid sequence of hemoglobin is the same every time it is expressed, and hemoglobin is the only protein that has exactly this sequence of amino acids.
MVLSPADKTNVKAAWGKVGAHAGEYGAEALERMFLSFPTTKTYFPHFDLSHGSAQVKGHGKKVADALTNAVAHVDDMPNALSALSDLHAHKLRVDPVNFKLLSHCLLVTLAAHLPAEFTPAVHASLDKFLASVSTVLTSKYR
Figure 1: Primary structure of human hemoglobin α chain. The α chain of human hemoglobin has 142 amino acids, all linked together in sequence with peptide bonds.
The gene encoding the protein ultimately determines the unique sequence of amino acids for every protein. A change in nucleotide sequence of the gene’s coding region may lead to adding a different amino acid to the growing polypeptide chain, causing a change in protein structure and sometimes, therefore function. In sickle cell anemia, the hemoglobin β chain (a small portion of which is shown in Figure 2) has a single amino acid substitution, causing a change in the protein's structure and function. Specifically, valine in the β chain is substituted with the amino acid, glutamate. What is most remarkable to consider is that a hemoglobin molecule is comprised of two alpha and two beta chains that each consist of about 150 amino acids. The molecule, therefore, has about 600 amino acids. The structural difference between a normal hemoglobin molecule and a sickle cell molecule – which dramatically decreases life expectancy – is two amino acids of the 600.
Figure 2: Structure and function of hemoglobin. Because of one change in the primary, amino acid sequence of the beta chain of hemoglobin, hemoglobin molecules form long fibers that distort the biconcave, or disc-shaped, red blood cells and causes them to assume a crescent or “sickle” shape, which clogs blood vessels. In normal hemoglobin, the amino acid at position six is glutamate, but in sickle cell hemoglobin, it is valine. (Credit: Rao, A., Tag, A. Ryan, K. and Fletcher, S. Department of Biology, Texas A&M University) [Image Description]
Secondary Structure
The local folding of the polypeptide in some regions gives rise to the secondary structure of the protein. The most common are the α-helix and β-pleated sheet structures (Figure 3). Both structures are held in shape by backbone hydrogen bonds. Hydrogen bonds form between the oxygen atom in the carbonyl group in one amino acid and hydrogen and nitrogen atoms in the amide group of another amino acid that is four amino acids away in sequence.
Figure 3: The α-helix and β-pleated sheet are secondary structures formed in proteins. These structures occur when hydrogen bonds form between the carbonyl oxygen and the amino hydrogen and nitrogen in the peptide backbone of two amino acids in a protein. Black = carbon, White = hydrogen, Blue = nitrogen, and Red = oxygen. Credit: Rao, A., Ryan, K. Fletcher, S. and Tag, A. Department of Biology, Texas A&M University. [Image Description]
Tertiary Structure
The polypeptide's unique three-dimensional structure is its tertiary structure (Figure 4). This structure is primarily due to chemical interactions between the side chains of amino acids in the polypeptide chain. The chemical nature of the side chain in the amino acids involved determine which amino acids are energetically favorable to be next to other amino acids. For example, side chains with like charges repel each other and those with unlike charges are attracted to each other (ionic bonds). The sulfur atoms in cysteine side chains can form disulfide linkages in the presence of oxygen, the only covalent bond that forms during protein folding. When protein folding takes place, the nonpolar amino acids' hydrophobic side chains repel water in the protein's environment and pack into the protein's interior; whereas, the hydrophilic side chains tend position on the surface of the protein, interacting with water. In general, whenever a protein is translated, it always folds into the same tertiary structure, as determined by the primary structure of its amino acids.
Figure 4: A variety of chemical interactions determine the proteins' 3D, tertiary structure. These include hydrophobic interactions, ionic bonding, hydrogen bonding, and disulfide linkages. [Image Description]
Quaternary Structure
In nature, some proteins form from several polypeptides, or subunits, and the interaction of these subunits forms the quaternary structure of the protein. Weak interactions between the subunits help to stabilize the overall structure. For example, the α and β chains of human hemoglobin, a globular protein, fold into a their tertiary structures, and then two copies of the α chain come to interact with two copies of the β chain to form a tetramer of four chains (Figure 5). Silk, a fibrous protein, however, has a β-pleated sheet structure that is the result of hydrogen bonding between many different chains.
Figure 5: Primary, secondary, tertiary, and quaternary structure of hemoglobin. The primary structure of a hemoglobin is its amino acid sequence. It secondary structure is entirely α helices. Its tertiary structure is globular. Four protein chains come together to form the quaternary structure that is the functional hemoglobin protein. Credit: Rao, A. Ryan, K. and Tag, A. Department of Biology, Texas A&M University. [Image Description]
2. Amino acids
Amino acids are the monomers that comprise the polymeric molecules, proteins. Each amino acid has the same fundamental structure, which consists of a central carbon atom, or the alpha carbon (Cα), bonded to an amino group (NH2), a carboxyl group (COOH), and a hydrogen atom. These atoms are considered the backbone of the amino acid. Every amino acid also has another atom or group of atoms bonded to the central Cα atom known as the R group or side chain (Figure 6).
Figure 6: Structure of an amino acid. Amino acids have a central asymmetric carbon (Cα) to which an amino group, a carboxyl group, a hydrogen atom, and a side chain (R group) are covalently bonded. [Image Description]
Scientists use the name "amino acid" because these acids contain both an amino group and a carboxyl-acid-group in their basic structure. As we mentioned, there are 20 common amino acids present in proteins. For each amino acid, the side chain (or R group) is different (Figure 7). The chemical nature of the side chain determines the amino acid's nature (that is, whether it is acidic, basic, polar, or nonpolar). Each amino acid has both a single-letter and a three-letter abbreviation. For example, valine is abbreviated with the letter V or the three-letter symbol, Val.
Figure 7: The 20 common amino acids. The chemical structure for each amino acid is given, grouped by chemical property. The single- and three-letter abbreviations are also provided. Adapted from "Molecular structures of the 21 proteinogenic amino acids.svg" by Dan Cojocari licensed under CC-BY-SA. [Image Description]
The sequence and the number of amino acids ultimately determine the protein's shape, size, and function. A covalent bond, or peptide bond, attaches to each amino acid, which a dehydration reaction forms. One amino acid's carboxyl group and the incoming amino acid's amino group combine, releasing a water molecule. The resulting bond is the peptide bond (Figure 8).
Figure 8: Peptide bond formation. The carboxyl group of one amino acid is linked to the incoming amino acid's amino group. In the process, it releases a water molecule. [Image Description]
The products that such linkages form are peptides. As more amino acids join to this growing chain, the resulting chain is a polypeptide. Each polypeptide has a free amino group at one end. This end is called the N terminus, or the amino terminus, and the other end has a free carboxyl group, also called the C or carboxyl terminus. When a polypeptide is built by the ribosome, amino acids are added from the N terminus to the C terminus. When polypeptide sequences are written out, they are written from N to C terminus. While the terms polypeptide and protein are sometimes used interchangeably, a polypeptide is technically a polymer of amino acids, whereas the term protein is used for a polypeptide that is folded into its functional form.
Each of the 20 most common amino acids has specific chemical characteristics and a unique role in protein structure and function. Based on the propensity of the side chains to be in contact with water (polar environment), amino acids can be classified into three groups:
- Those with polar side chains.
- Those with hydrophobic side chains.
- Those with charged side chains.
Below we look at each of these classes and briefly discuss their role in protein structure and function.
Polar amino acids
When considering polarity, some amino acids are straightforward to define as polar, while in other cases, we may encounter disagreements. For example, serine (Ser, S), threonine (Thr, T), and tyrosine (Tyr, Y) are polar since they carry a hydroxylic (-OH) group (Figure 9). Furthermore, this group can form a hydrogen bond with another polar group by donating or accepting a proton (a table showing donors and acceptors in polar and charged amino acid side chains can be found at the FoldIt site. Tyrosine is also involved in metal binding in many enzymatic sites. Asparagine (Asn, N) and glutamine (Gln, Q) also belong to this group and may donate or accept a hydrogen bond.
Histidine (His, H), on the other hand, depending on the environment and pH, can be polar or carry a charge. It has two –NH groups with a pKa value of around 6. At pHs below 6, when both groups are protonated, the side chain has a charge of +1. Within protein molecules, the pKa may be modulated by the environment so that the side chain may give away a proton and become neutral or accept a proton, becoming charged. This ability makes histidine useful in enzyme active sites when the chemical reaction requires proton extraction.
Figure 9: The polar amino acids. Adapted from "Molecular structures of the 21 proteinogenic amino acids.svg" by Dan Cojocari licensed under CC-BY-SA. [Image Description]
Hydrophobic amino acids
The hydrophobic amino acids include alanine (Ala, A), valine (Val, V), leucine (Leu, L), isoleucine (Ile, I), proline (Pro, P), phenylalanine (Phe, F) and cysteine (Cys, C) (Figure 10). These residues typically form the hydrophobic core of proteins, which is isolated from the polar solvent. The side chains within the core are tightly packed and participate in van der Waals interactions, which are essential for stabilizing the structure. In addition, Cys residues are involved in three-dimensional structure stabilization through the formation of disulfide (S-S) bridges, which sometimes connect different secondary structure elements or different subunits in a complex. Another essential function of Cys is metal binding, sometimes in enzyme active sites and sometimes in structure-stabilizing metal centers.
The aromatic amino acids tryptophan (Trp, W) and Tyr and the non-aromatic methionine (Met, M) are sometimes called amphipathic due to their ability to have both polar and nonpolar character. In protein molecules, these residues are often found close to the interface between a protein and solvent. We should also note here that the side chains of histidine and tyrosine, together with the hydrophobic phenylalanine and tryptophan, can also form weak hydrogen bonds of the types OH−π and CH−O, using electron clouds within their ring structures. A characteristic feature of aromatic residues is that they are often found within the core of a protein structure, with their side chains packed against each other. They are also highly conserved within protein families, with Trp having the highest conservation rate.
Figure 10: The hydrophobic amino acids. Adapted from "Molecular structures of the 21 proteinogenic amino acids.svg" by Dan Cojocari licensed under CC-BY-SA. [Image Description]
Charged amino acids
The charged amino acids at neutral pH (around 7.4) carry a single charge in the side chain. There are four of them; the two basic ones include lysine (Lys, K) and arginine (Arg, R), with a positive charge at neutral pH. The two acidic residues include aspartate (Asp, D) and glutamate (Glu, E), which carry a negative charge at neutral pH (Figure 11). A so-called salt bridge is often formed by the interaction of closely located positively and negatively charged side chains. Such bridges are often involved in stabilizing three-dimensional protein structure, especially in proteins from thermophilic organisms, organisms that live at elevated temperatures, up to 80-90 C, or even higher. The binding of positively charged metal ions is another function of the negatively charged carboxylic groups of Asp and Glu. Metalloproteins and the role of metal centers in protein function is a fascinating field of structural biology research.
Figure 11: The charged amino acids. Adapted from "Molecular structures of the 21 proteinogenic amino acids.svg" by Dan Cojocari licensed under CC-BY-SA. [Image Description]
Glycine & proline
Glycine (Gly), one of the common amino acids, does not have a side chain – its R group is just a hydrogen atom – and is often found at the surface of proteins within loop or coil regions (regions without defined secondary structure), providing high flexibility to the polypeptide chain. This flexibility is required in sharp polypeptide turns in loop structures. Proline (Pro), although considered hydrophobic, is also found at the surface, presumably due to its presence in turn and loop regions. In contrast to Gly, which provides the polypeptide chain high flexibility, Pro provides rigidity by imposing certain torsion angles on the segment of the structure. The reason for this is that its side chain makes a covalent bond with the main chain, which constrains the backbone shape of the polypeptide in this location. Sometimes Pro is called a helix breaker since it is often found at the end of α-helices.
Figure 12: The special amino acids. Adapted from "Molecular structures of the 21 proteinogenic amino acids.svg" by Dan Cojocari licensed under CC-BY-SA. [Image Description]
Figure Descriptions
Figure 2: The image is a comparative illustration of the structural and functional differences between normal hemoglobin and sickle-cell hemoglobin across various levels of protein structure. The layout is divided into two vertical sections labeled "Normal" and "Sickle-Cell," each with subsections depicting the primary, secondary, tertiary, quaternary structures, and function.
- Primary Structure:
- Normal: Seven circular molecules labeled sequentially from 1 to 7 with the respective amino acids: Val, His, Leu, Thr, Pro, Glu, Glu.
- Sickle-Cell: Same seven circular molecules labeled sequentially with the amino acids: Val, His, Leu, Thr, Pro, Val, Glu. The sixth molecule, Glu, is replaced with Val, highlighted in red.
- Secondary and Tertiary Structures:
- Normal: A blue 3D ellipsoid shape representing the normal β subunit.
- Sickle-Cell: A reddish-brown 3D ellipsoid shape representing the sickle-cell β subunit.
- Quaternary Structure:
- Normal: Combination of blue and purple ellipsoid shapes to form normal hemoglobin.
- Sickle-Cell: Combination of reddish-brown and purple ellipsoid shapes to form sickle-cell hemoglobin.
- Function:
- Normal: Depicts individual globular hemoglobin molecules scattered and unassociated, each capable of carrying oxygen.
- Sickle-Cell: Illustrates abnormal aggregation of hemoglobin molecules into fibers, impairing oxygen-carrying capacity.
Figure 3: The image illustrates two types of secondary protein structures against a light blue background: an alpha-helix and a beta-pleated sheet. The illustration is divided horizontally into two sections.
- Top Section: Alpha Helix
- A right-handed helical structure is shown in orange, twisting in a clockwise direction.
- The helix is depicted with a string of colored spheres (atoms) connected by lines (chemical bonds) representing the molecular structure.
- Hydrogen bonds are represented by dashed lines connecting parts of the helix.
- The labels include "α Helix" and "Hydrogen Bond".
- Bottom Section: Beta Pleated Sheet
- Several strands are aligned next to each other, forming a pleated sheet structure in orange.
- Similar to the helix, the strands are composed of colored spheres (atoms) connected by lines (chemical bonds).
- Hydrogen bonds are depicted as dashed lines running perpendicular to the strands, connecting adjacent strands.
- The labels include "β Pleated Sheet," "β Strand," and "Hydrogen Bond".
Figure 4: The image depicts a simplified diagram of a polypeptide backbone, illustrating various interactions and bonds that occur within a protein structure. The backbone is represented by a red, ribbon-like structure that loops and twists, showing the complex folding of the protein.
- Polypeptide Backbone: The main red ribbon represents the polypeptide backbone which loops around the image.
- Ionic Bond: There is a highlighted section showing a segment with a labeled "Ionic Bond," featuring an NH₃⁺ group connected to an O⁻ group.
- Hydrogen Bond: A light blue segment indicates a "Hydrogen bond" between O-H groups.
- Disulfide Linkage: An adjacent part shows a connection labeled "Disulfide linkage" marked by two sulfur atoms connected by a line (represented by "S-S").
- Hydrophobic Interactions: Another section indicates "Hydrophobic interactions," involving CH₃ groups interacting with one another.
Figure 6: The image is a diagram depicting the structure of an amino acid. The diagram is divided into three sections vertically, from left to right, labeled "Amino group," "Side chain," and "Carboxyl group." The amino group section contains a nitrogen atom (N) colored blue at the center, bonded to two hydrogen atoms (H) represented in white and labeled. Moving rightwards, the central section contains a carbon atom (C) depicted in black, bonded to one hydrogen atom (H) in white and to an "R" group representing the side chain. The carbon is also bonded to another carbon atom (C), also in black, positioned to the right in the carboxyl group section. This carbon is double-bonded to an oxygen atom (O) colored in red, and single-bonded to another oxygen (O) with a single hydrogen (H) attached. An arrow points to the central carbon labeled "α carbon." [Return to Figure]
Figure 7: The image is an educational chart titled "20 Common Amino Acids." It is divided into four main sections by backgrounds of different colors: Polar Uncharged (light blue), Hydrophobic (light green), Charged (light pink), and Special Cases (light yellow).
- Polar Uncharged (light blue background):
- Contains six amino acids: Serine (S), Threonine (T), Histidine (H), Asparagine (N), Glutamine (Q), and Tyrosine (Y).
- Each amino acid is depicted with its chemical structure and a red circle indicating its one-letter code inside the circle.
- Hydrophobic (light green background):
- Contains nine amino acids: Alanine (A), Cysteine (C), Valine (V), Isoleucine (I), Leucine (L), Methionine (M), Phenylalanine (F), and Tryptophan (W).
- Each amino acid is depicted with its chemical structure and a red circle indicating its one-letter code inside the circle.
- Charged (light pink background):
- Divided into Positive and Negative sections.
- The Positive section includes Arginine (R) and Lysine (K).
- The Negative section includes Aspartic Acid (D) and Glutamic Acid (E).
- Each amino acid is depicted with its chemical structure and a red circle indicating its one-letter code inside the circle.
- Special Cases (light yellow background):
- Contains two amino acids: Glycine (G) and Proline (P).
- Each amino acid is depicted with its chemical structure and a red circle indicating its one-letter code inside the circle.
- The top left structure represents an amino acid, featuring an amino group (H2N), a central carbon (C) bonded to a hydrogen atom (H), a variable side chain (R), and a carboxyl group (COOH). The hydroxyl group (OH) in the carboxyl group is highlighted in red.
- The top right structure represents another amino acid with a similar structure but differing variable side chains (R).
- The two structures at the top are separated by a space and linked by an arrow pointing to a single structure at the bottom.
- The bottom structure represents the resulting dipeptide with a peptide bond formed. The peptide bond is highlighted within a blue rectangle, showing the linkage between the carbon (C) of one amino acid and the nitrogen (N) of the other amino acid.
- The term "Peptide Bond" is written below the blue rectangle.
Figure 9: The image categorizes polar uncharged amino acids and visually represents their structures. It displays six amino acids: Serine, Threonine, Histidine, Asparagine, Glutamine, and Tyrosine. Each amino acid shows its backbone and distinct side chain. The background is light blue, with the structures depicted in black. Each amino acid name is followed by its three-letter and one-letter code, represented within a red circle. [Return to Figure]
Figure 10: The image is a diagram depicting the molecular structures of eight hydrophobic amino acids. The background is light green, and each amino acid is illustrated with its chemical structure, the three-letter abbreviation, and the single-letter code. The amino acids are aligned horizontally. From left to right, the amino acids are Alanine (Ala, A), Cysteine (Cys, C), Valine (Val, V), Isoleucine (Ile, I), Leucine (Leu, L), Methionine (Met, M), Phenylalanine (Phe, F), and Tryptophan (Trp, W). Each single-letter code is presented in a red circle. [Return to Figure]
Figure 11: The image is a diagram that categorizes amino acids based on their charge properties and atomic structure. The background is a light pink color, and there is a shaded rectangular area in the center where the chemical structures are displayed. The diagram is divided into two main groups labeled “Positive” and “Negative”. Under the “Positive” group, two amino acids are listed: Arginine (Arg) and Lysine (Lys), each represented with their respective chemical structures and a red circle with the letters "R" and "K". Under the “Negative” group, two amino acids are listed: Aspartic Acid (Asp) and Glutamic Acid (Glu), each represented with their respective chemical structures and a red circle with the letters "D" and "E". [Return to Figure]
The image has a yellow background and is titled "Special Cases" at the top in black font. Below the title, there are two sections dedicated to the amino acids Glycine (Gly) and Proline (Pro).
To the left, under the heading "Glycine (Gly)" in black text, there is a red circle with a white uppercase letter "G" inside. Below this, a structural formula of Glycine is depicted within a beige rectangle. The formula shows a carbon atom bonded to an amine group (NH₂), a carboxyl group (COOH), and two hydrogen atoms.
To the right, under the heading "Proline (Pro)" in black text, there is a red circle with a white uppercase letter "P" inside. Below this, a structural formula of Proline is also shown within the same beige rectangle. The Proline structure shows a carbon atom bonded to a carboxyl group (COOH), an amine group in a five-membered ring structure, and single hydrogen atoms.
Licenses and Attributions
"Protein Structure & Function" by Michelle McCully is adapted from "3.4 Proteins" by Mary Ann Clark, Matthew Douglas, Jung Choi for OpenStax Biology 2e under CC-BY 4.0 and "The 20 Amino Acids and Their Role in Protein Structures" by Salam Al-Karadaghi under CC-BY-SA 4.0. "Protein Structure & Function" is licensed under ???.
Learning Objectives
By the end of this chapter, you will be able to do the following:
- Predict the functional effects of mutations in β-galactosidase
Proteins are one of the most abundant biological macromolecules in living systems and have the most diverse range of functions of all macromolecules. Proteins may be structural, regulatory, contractile, or protective. They may serve in transport, storage, or membranes; or they may be toxins or enzymes. Each cell in a living system may contain thousands of proteins, each with a unique function. Their structures, like their functions, vary greatly, and by interrogating their structures, we can make predictions about their functions.
1. Protein structure
A protein's shape is critical to its function. For example, an enzyme can bind to a specific substrate at an active site. If this active site is altered because of local changes or changes in overall protein structure, the enzyme may be unable to bind to the substrate. To understand a protein's shape or conformation, we need to understand the four levels of protein structure: primary, secondary, tertiary, and quaternary.
Primary Structure
The amino acid sequence in a polypeptide chain is its primary structure. For example, the primary sequence of the β chain of human hemoglobin may be found on Uniprot, entry P68871. The N-terminal amino acid is valine (Val, V), and the C-terminal amino acid is histidine (His, H) (Figure 1). The amino acid sequence of hemoglobin is the same every time it is expressed, and hemoglobin is the only protein that has exactly this sequence of amino acids.
VHLTPEEKSAVTALWGKVNVDEVGGEALGRLLVVYPWTQRFFESFGDLSTPDAVMGNPKVKAHGKKVLGAFSDGLAHLDNLKGTFATLSELHCDKLHVDPENFRLLGNVLVCVLAHHFGKEFTPPVQAAYQKVVAGVANALAHKYH
Figure 1: Primary structure of human hemoglobin β chain. The β chain of human hemoglobin has 146 amino acids, all linked together in sequence with peptide bonds.
The gene encoding the protein ultimately determines the unique sequence of amino acids for every protein. A change in nucleotide sequence in the gene’s coding region may lead to change in the amino acid sequence, causing a change in the protein's structure and sometimes, therefore its function. In people who have sickle cell anemia, the hemoglobin β chain (a small portion of which is shown in Figure 2) has a single amino acid substitution, causing a change in the protein's structure and function. Specifically, at the sixth position in the primary sequence of the β chain, the wild type amino acid, glutamate (Glu, E) is substituted by valine (Val, V). What is most remarkable to consider is that a hemoglobin molecule is comprised of two α and two β chains that each consist of about 150 amino acids. The full hemoglobin protein, therefore, has about 600 amino acids. The structural difference between a normal hemoglobin molecule and a sickle cell molecule – which dramatically decreases life expectancy – is two amino acids of the ~600.
Figure 2: Structure and function of hemoglobin. Because of one change in the primary, amino acid sequence of the β chain of hemoglobin, hemoglobin proteins form long fibers that distort normally disc-shaped, red blood cells and causes them to assume a crescent or “sickle” shape, which clogs blood vessels. In wild type hemoglobin, the amino acid at position six is glutamate, but in sickle cell hemoglobin, it is valine. (Credit: Rao, A., Tag, A. Ryan, K. and Fletcher, S. Department of Biology, Texas A&M University) [Image Description]
Secondary Structure
The local folding of the polypeptide in some regions gives rise to the secondary structure of the protein. The most common are the α-helix and β-pleated sheet structures (Figure 3). Both structures are held in shape by backbone hydrogen bonds. In α-helices, for example, hydrogen bonds form between the oxygen atom in the carbonyl group in one amino acid and hydrogen and nitrogen atoms in the amide group of another amino acid that is four amino acids away in the primary sequence.
Figure 3: The α-helix and β-pleated sheet are secondary structures formed in proteins. These structures occur when hydrogen bonds form between the carbonyl oxygen and the amino hydrogen and nitrogen in the peptide backbone of two amino acids in a protein. Black = carbon, White = hydrogen, Blue = nitrogen, and Red = oxygen. Credit: Rao, A., Ryan, K. Fletcher, S. and Tag, A. Department of Biology, Texas A&M University. [Image Description]
Tertiary Structure
The polypeptide's unique three-dimensional structure is its tertiary structure (Figure 4). This structure forms primarily due to chemical interactions between the side chains of amino acids in the polypeptide chain. The chemical nature of the side chain in the amino acids involved determines which amino acids are energetically favorable to be near other amino acids. For example, side chains with like charges repel each other and those with opposite charges are attracted to each other (ionic bonds). The sulfur atoms in cysteine side chains can form disulfide linkages in the presence of oxygen, the only covalent bond that forms during protein folding. When protein folding takes place, the nonpolar amino acids' hydrophobic side chains repel water from the protein's environment and pack into the protein's interior; whereas, the hydrophilic side chains tend position on the surface of the protein as the protein folds, interacting with water. In general, whenever a protein is translated, it always folds into the same tertiary structure, as determined by the primary structure of its amino acids.
Figure 4: A variety of chemical interactions determine the proteins' 3D, tertiary structure. These include hydrophobic interactions, ionic bonding, hydrogen bonding, and disulfide linkages. [Image Description]
Quaternary Structure
In nature, some – but not all – proteins form from several polypeptides, or subunits, and the interaction of these subunits forms the quaternary structure of the protein. Weak interactions between the subunits help to stabilize the overall structure. For example, the α and β chains of human hemoglobin, a globular protein, fold into a their tertiary structures, and then two copies of the α chain come into interact with two copies of the β chain to form a tetramer of four chains (Figure 5). Silk, a fibrous protein, however, has a β-pleated sheet structure that is the result of hydrogen bonding between many different chains.
Figure 5: Primary, secondary, tertiary, and quaternary structure of hemoglobin. The primary structure of a hemoglobin is its amino acid sequence. It secondary structure is entirely α helices. Its tertiary structure is globular. Four protein chains come together to form the quaternary structure that is the functional hemoglobin protein. Credit: Rao, A. Ryan, K. and Tag, A. Department of Biology, Texas A&M University. [Image Description]
Practice Questions
2. Amino acids
Amino acids are the monomers that comprise the polymeric molecules, proteins. Each amino acid has the same fundamental structure, which consists of a central carbon atom, or the alpha carbon (Cα), bonded to an amino group (NH2), a carboxyl group (COOH), and a hydrogen atom. These atoms are considered the backbone of the amino acid. Every amino acid also has another atom or group of atoms bonded to the central Cα atom known as the R group or side chain (Figure 6).
Figure 6: Structure of an amino acid. Amino acids have a central asymmetric carbon (Cα) to which an amino group, a carboxyl group, a hydrogen atom, and a side chain (R group) are covalently bonded. [Image Description]
Practice Question
Scientists use the name "amino acid" because these acids contain both an amino group and a carboxyl-acid-group in their basic structure. As we mentioned, there are 20 common amino acids present in proteins. For each amino acid, the side chain (or R group) is different (Figure 7). The chemical nature of the side chain determines the amino acid's nature (that is, whether it is acidic, basic, polar, or nonpolar). Each amino acid has both a single-letter and a three-letter abbreviation. For example, valine is abbreviated with the letter V or the three-letter symbol, Val.
Figure 7: The 20 common amino acids. The chemical structure for each amino acid is given, grouped by chemical property. The single- and three-letter abbreviations are also provided. Adapted from "Molecular structures of the 21 proteinogenic amino acids.svg" by Dan Cojocari licensed under CC-BY-SA. [Image Description]
The sequence and the number of amino acids ultimately determine the protein's shape, size, and function. A covalent bond, or peptide bond, attaches to each amino acid, which a dehydration reaction forms. One amino acid's carboxyl group and the incoming amino acid's amino group combine, releasing a water molecule. The resulting bond is the peptide bond (Figure 8).
Figure 8: Peptide bond formation. The carboxyl group of one amino acid is linked to the incoming amino acid's amino group. In the process, it releases a water molecule. [Image Description]
The products that such linkages form are peptides. As more amino acids join to this growing chain, the resulting chain is a polypeptide. Each polypeptide has a free amino group at one end. This end is called the N terminus, or the amino terminus, and the other end has a free carboxyl group, also called the C or carboxyl terminus. When a polypeptide is built by the ribosome, amino acids are added from the N terminus to the C terminus. When polypeptide sequences are written out, they are written from N to C terminus. While the terms polypeptide and protein are sometimes used interchangeably, a polypeptide is technically a polymer of amino acids, whereas the term protein is used for a polypeptide that is folded into its functional form.
Each of the 20 most common amino acids has specific chemical characteristics and a unique role in protein structure and function. Based on the propensity of the side chains to be in contact with water (polar environment), amino acids can be classified into three groups: 1) those with polar side chains, 2) those with hydrophobic side chains, and 3) those with charged side chains. Below we look at each of these classes and briefly discuss their role in protein structure and function.
Polar amino acids
When considering polarity, some amino acids are straightforward to define as polar, while in other cases, we may encounter disagreements. For example, serine (Ser, S), threonine (Thr, T), and tyrosine (Tyr, Y) are polar since they carry a hydroxylic (-OH) group (Figure 9). Furthermore, this group can form a hydrogen bond with another polar group by donating or accepting a proton (a table showing donors and acceptors in polar and charged amino acid side chains can be found at the FoldIt site. Tyrosine is also involved in metal binding in many enzymatic sites. Asparagine (Asn, N) and glutamine (Gln, Q) also belong to this group and may donate or accept a hydrogen bond.
Histidine (His, H), on the other hand, depending on the environment and pH, can be polar or carry a charge. It has two –NH groups with a pKa value of around 6. At pHs below 6, when both groups are protonated, the side chain has a charge of +1. Within protein molecules, the pKa may be modulated by the environment so that the side chain may give away a proton and become neutral or accept a proton, becoming charged. This ability makes histidine useful in enzyme active sites when the chemical reaction requires proton extraction.
Figure 9: The polar amino acids. Adapted from "Molecular structures of the 21 proteinogenic amino acids.svg" by Dan Cojocari licensed under CC-BY-SA. [Image Description]
Hydrophobic amino acids
The hydrophobic amino acids include alanine (Ala, A), valine (Val, V), leucine (Leu, L), isoleucine (Ile, I), proline (Pro, P), phenylalanine (Phe, F) and cysteine (Cys, C) (Figure 10). These residues typically form the hydrophobic core of proteins, which is isolated from the polar solvent. The side chains within the core are tightly packed and participate in van der Waals interactions, which are essential for stabilizing the structure. In addition, Cys residues are involved in three-dimensional structure stabilization through the formation of disulfide (S-S) bridges, which sometimes connect different secondary structure elements or different subunits in a complex. Another essential function of Cys is metal binding, sometimes in enzyme active sites and sometimes in structure-stabilizing metal centers.
The aromatic amino acids tryptophan (Trp, W) and Tyr and the non-aromatic methionine (Met, M) are sometimes called amphipathic due to their ability to have both polar and nonpolar character. In protein molecules, these residues are often found close to the interface between a protein and solvent. We should also note here that the side chains of histidine and tyrosine, together with the hydrophobic phenylalanine and tryptophan, can also form weak hydrogen bonds of the types OH−π and CH−O, using electron clouds within their ring structures. A characteristic feature of aromatic residues is that they are often found within the core of a protein structure, with their side chains packed against each other. They are also highly conserved within protein families, with Trp having the highest conservation rate.
Figure 10: The hydrophobic amino acids. Adapted from "Molecular structures of the 21 proteinogenic amino acids.svg" by Dan Cojocari licensed under CC-BY-SA. [Image Description]
Charged amino acids
The charged amino acids at neutral pH (around 7.4) carry a single charge in the side chain. There are four of them; the two basic ones include lysine (Lys, K) and arginine (Arg, R), with a positive charge at neutral pH. The two acidic residues include aspartate (Asp, D) and glutamate (Glu, E), which carry a negative charge at neutral pH (Figure 11). A so-called salt bridge is often formed by the interaction of closely located positively and negatively charged side chains. Such bridges are often involved in stabilizing three-dimensional protein structure, especially in proteins from thermophilic organisms, organisms that live at elevated temperatures, up to 80-90 C, or even higher. The binding of positively charged metal ions is another function of the negatively charged carboxylic groups of Asp and Glu. Metalloproteins and the role of metal centers in protein function is a fascinating field of structural biology research.
Figure 11: The charged amino acids. Adapted from "Molecular structures of the 21 proteinogenic amino acids.svg" by Dan Cojocari licensed under CC-BY-SA. [Image Description]
Glycine & proline
Glycine (Gly), one of the common amino acids, does not have a side chain – its R group is just a hydrogen atom – and is often found at the surface of proteins within loop or coil regions (regions without defined secondary structure), providing high flexibility to the polypeptide chain. This flexibility is required in sharp polypeptide turns in loop structures. Proline (Pro), although considered hydrophobic, is also found at the surface, presumably due to its presence in turn and loop regions. In contrast to Gly, which provides the polypeptide chain high flexibility, Pro provides rigidity by imposing certain torsion angles on the segment of the structure. The reason for this is that its side chain makes a covalent bond with the main chain, which constrains the backbone shape of the polypeptide in this location. Sometimes Pro is called a helix breaker since it is often found at the end of α-helices. (Figure 12)
Figure 12: The special amino acids. Adapted from "Molecular structures of the 21 proteinogenic amino acids.svg" by Dan Cojocari licensed under CC-BY-SA. [Image Description]
Practice Questions
Figure Descriptions
Figure 2: The image is a comparative illustration of the structural and functional differences between normal hemoglobin and sickle-cell hemoglobin across various levels of protein structure. The layout is divided into two vertical sections labeled "Normal" and "Sickle-Cell," each with subsections depicting the primary, secondary, tertiary, quaternary structures, and function.
- Primary Structure:
- Normal: Seven circular molecules labeled sequentially from 1 to 7 with the respective amino acids: Val, His, Leu, Thr, Pro, Glu, Glu.
- Sickle-Cell: Same seven circular molecules labeled sequentially with the amino acids: Val, His, Leu, Thr, Pro, Val, Glu. The sixth molecule, Glu, is replaced with Val, highlighted in red.
- Secondary and Tertiary Structures:
- Normal: A blue 3D ellipsoid shape representing the normal β subunit.
- Sickle-Cell: A reddish-brown 3D ellipsoid shape representing the sickle-cell β subunit.
- Quaternary Structure:
- Normal: Combination of blue and purple ellipsoid shapes to form normal hemoglobin.
- Sickle-Cell: Combination of reddish-brown and purple ellipsoid shapes to form sickle-cell hemoglobin.
- Function:
- Normal: Depicts individual globular hemoglobin molecules scattered and unassociated, each capable of carrying oxygen.
- Sickle-Cell: Illustrates abnormal aggregation of hemoglobin molecules into fibers, impairing oxygen-carrying capacity.
Figure 3: The image illustrates two types of secondary protein structures against a light blue background: an alpha-helix and a beta-pleated sheet. The illustration is divided horizontally into two sections.
- Top Section: Alpha Helix
- A right-handed helical structure is shown in orange, twisting in a clockwise direction.
- The helix is depicted with a string of colored spheres (atoms) connected by lines (chemical bonds) representing the molecular structure.
- Hydrogen bonds are represented by dashed lines connecting parts of the helix.
- The labels include "α Helix" and "Hydrogen Bond".
- Bottom Section: Beta Pleated Sheet
- Several strands are aligned next to each other, forming a pleated sheet structure in orange.
- Similar to the helix, the strands are composed of colored spheres (atoms) connected by lines (chemical bonds).
- Hydrogen bonds are depicted as dashed lines running perpendicular to the strands, connecting adjacent strands.
- The labels include "β Pleated Sheet," "β Strand," and "Hydrogen Bond".
Figure 4: The image depicts a simplified diagram of a polypeptide backbone, illustrating various interactions and bonds that occur within a protein structure. The backbone is represented by a red, ribbon-like structure that loops and twists, showing the complex folding of the protein.
- Polypeptide Backbone: The main red ribbon represents the polypeptide backbone which loops around the image.
- Ionic Bond: There is a highlighted section showing a segment with a labeled "Ionic Bond," featuring an NH₃⁺ group connected to an O⁻ group.
- Hydrogen Bond: A light blue segment indicates a "Hydrogen bond" between O-H groups.
- Disulfide Linkage: An adjacent part shows a connection labeled "Disulfide linkage" marked by two sulfur atoms connected by a line (represented by "S-S").
- Hydrophobic Interactions: Another section indicates "Hydrophobic interactions," involving CH₃ groups interacting with one another.
Figure 5: The image illustrates the hierarchical structure of proteins from the primary structure to the quaternary structure, using hemoglobin as an example. The background is a gradient blue, transitioning from a darker blue at the top to a lighter blue at the bottom.
From left to right:
- Primary Structure: Depicts a sequence of amino acids connected via peptide bonds. Four amino acids are shown (labeled 1, 2, 3, and 4). Each amino acid consists of an amino group (NH2), carboxyl group (COOH), hydrogen atom (H), and side chain (R1, R2, R3, R4).
- Secondary Structure (α Helix): Shows the formation of an alpha helix from the amino acid chain. The helix is represented by an orange spiraling ribbon with dotted lines indicating hydrogen bonds stabilizing the structure.
- Tertiary Structure: Illustrates a β-globin polypeptide chain folded into a specific three-dimensional shape. It appears as a purple, looped, and twisted structure.
- Quaternary Structure: Demonstrates the assembly of multiple polypeptide chains. The β-globin (purple) and α-globin (yellow, green, and blue) polypeptides combine to form a hemoglobin molecule.
Figure 6: The image is a diagram depicting the structure of an amino acid. The diagram is divided into three sections vertically, from left to right, labeled "Amino group," "Side chain," and "Carboxyl group." The amino group section contains a nitrogen atom (N) colored blue at the center, bonded to two hydrogen atoms (H) represented in white and labeled. Moving rightwards, the central section contains a carbon atom (C) depicted in black, bonded to one hydrogen atom (H) in white and to an "R" group representing the side chain. The carbon is also bonded to another carbon atom (C), also in black, positioned to the right in the carboxyl group section. This carbon is double-bonded to an oxygen atom (O) colored in red, and single-bonded to another oxygen (O) with a single hydrogen (H) attached. An arrow points to the central carbon labeled "α carbon." [Return to Figure]
Figure 7: The image is an educational chart titled "20 Common Amino Acids." It is divided into four main sections by backgrounds of different colors: Polar Uncharged (light blue), Hydrophobic (light green), Charged (light pink), and Special Cases (light yellow).
- Polar Uncharged (light blue background):
- Contains six amino acids: Serine (S), Threonine (T), Histidine (H), Asparagine (N), Glutamine (Q), and Tyrosine (Y).
- Each amino acid is depicted with its chemical structure and a red circle indicating its one-letter code inside the circle.
- Hydrophobic (light green background):
- Contains nine amino acids: Alanine (A), Cysteine (C), Valine (V), Isoleucine (I), Leucine (L), Methionine (M), Phenylalanine (F), and Tryptophan (W).
- Each amino acid is depicted with its chemical structure and a red circle indicating its one-letter code inside the circle.
- Charged (light pink background):
- Divided into Positive and Negative sections.
- The Positive section includes Arginine (R) and Lysine (K).
- The Negative section includes Aspartic Acid (D) and Glutamic Acid (E).
- Each amino acid is depicted with its chemical structure and a red circle indicating its one-letter code inside the circle.
- Special Cases (light yellow background):
- Contains two amino acids: Glycine (G) and Proline (P).
- Each amino acid is depicted with its chemical structure and a red circle indicating its one-letter code inside the circle.
- The top left structure represents an amino acid, featuring an amino group (H2N), a central carbon (C) bonded to a hydrogen atom (H), a variable side chain (R), and a carboxyl group (COOH). The hydroxyl group (OH) in the carboxyl group is highlighted in red.
- The top right structure represents another amino acid with a similar structure but differing variable side chains (R).
- The two structures at the top are separated by a space and linked by an arrow pointing to a single structure at the bottom.
- The bottom structure represents the resulting dipeptide with a peptide bond formed. The peptide bond is highlighted within a blue rectangle, showing the linkage between the carbon (C) of one amino acid and the nitrogen (N) of the other amino acid.
- The term "Peptide Bond" is written below the blue rectangle.
Figure 9: The image categorizes polar uncharged amino acids and visually represents their structures. It displays six amino acids: Serine, Threonine, Histidine, Asparagine, Glutamine, and Tyrosine. Each amino acid shows its backbone and distinct side chain. The background is light blue, with the structures depicted in black. Each amino acid name is followed by its three-letter and one-letter code, represented within a red circle. [Return to Figure]
Figure 10: The image is a diagram depicting the molecular structures of eight hydrophobic amino acids. The background is light green, and each amino acid is illustrated with its chemical structure, the three-letter abbreviation, and the single-letter code. The amino acids are aligned horizontally. From left to right, the amino acids are Alanine (Ala, A), Cysteine (Cys, C), Valine (Val, V), Isoleucine (Ile, I), Leucine (Leu, L), Methionine (Met, M), Phenylalanine (Phe, F), and Tryptophan (Trp, W). Each single-letter code is presented in a red circle. [Return to Figure]
Figure 11: The image is a diagram that categorizes amino acids based on their charge properties and atomic structure. The background is a light pink color, and there is a shaded rectangular area in the center where the chemical structures are displayed. The diagram is divided into two main groups labeled “Positive” and “Negative”. Under the “Positive” group, two amino acids are listed: Arginine (Arg) and Lysine (Lys), each represented with their respective chemical structures and a red circle with the letters "R" and "K". Under the “Negative” group, two amino acids are listed: Aspartic Acid (Asp) and Glutamic Acid (Glu), each represented with their respective chemical structures and a red circle with the letters "D" and "E". [Return to Figure]
The image has a yellow background and is titled "Special Cases" at the top in black font. Below the title, there are two sections dedicated to the amino acids Glycine (Gly) and Proline (Pro).
To the left, under the heading "Glycine (Gly)" in black text, there is a red circle with a white uppercase letter "G" inside. Below this, a structural formula of Glycine is depicted within a beige rectangle. The formula shows a carbon atom bonded to an amine group (NH₂), a carboxyl group (COOH), and two hydrogen atoms.
To the right, under the heading "Proline (Pro)" in black text, there is a red circle with a white uppercase letter "P" inside. Below this, a structural formula of Proline is also shown within the same beige rectangle. The Proline structure shows a carbon atom bonded to a carboxyl group (COOH), an amine group in a five-membered ring structure, and single hydrogen atoms.
Licenses and Attributions
"Protein Structure & Function" by Michelle McCully is adapted from "3.4 Proteins" by Mary Ann Clark, Matthew Douglas, Jung Choi for OpenStax Biology 2e under CC-BY 4.0 and "The 20 Amino Acids and Their Role in Protein Structures" by Salam Al-Karadaghi under CC-BY-SA 4.0. "Protein Structure & Function" is licensed under ???.
