Lesson Explainer: Structure of Proteins Biology • Third Year of Secondary School

In this explainer, we will learn how to describe the basic structure of a protein and identify structural and regulatory proteins.

What texture is your hair? Is it straight or curly? You might be surprised to hear that the texture of your hair is influenced by sulfur. More exactly, it is influenced by the amino acid cysteine, which contains a sulfur atom in its side chain. Cysteines can form bonds with other cysteines in your hair to make it curly. This is all a property of the structural protein keratin that is found in hair.

We will come back to this and explain all the details in a moment, but first, let’s recall some of the key elements of a protein’s structure.

Proteins are polymers of amino acids that have been linked together. You will recall that the basic structure of an amino acid has a central carbon atom (the alpha carbon) that can form four bonds: one with an amino group (), one with a carboxyl group (), one with a hydrogen, and one with a side chain as indicated in Figure 1 below. The side chain is sometimes called an “R group” and differentiates one amino acid from another.

Example 1: Recalling the Structure of an Amino Acid

Part of a basic outline of an amino acid is provided. What functional group is missing?

Answer

Proteins are important biological molecules that are assembled from amino acids.

Amino acids have the same basic structure: a central carbon attached to the amino group (), a carboxyl group (), a hydrogen atom, and a side chain. Each amino acid can have a different side chain to give that amino acid its specific properties.

The carboxyl and amino groups are considered functional groups because they also have specific functions for the chemistry of the amino acid. Amino acids can be joined between amino and carboxyl groups to form polymers that ultimately fold into proteins.

This basic structure of an amino acid is shown below.

Therefore, the missing group in the amino acid as shown above is the amino group ().

There are twenty amino acids in the human body that are all identical with the exception of their side chains. Side chains can range in complexity. Glycine is the simplest amino acid with its side chain being a hydrogen atom, while lysine has a longer chain with a charged amino group. These complexities exist in order to build proteins with very specific shapes. You can see a few examples of amino acids and their chemical properties below in Figure 2.

Each amino acid has unique chemical properties because of the R group. For example, the amino acid serine has a hydroxyl group () that makes its side chain polar. You will recall that polarity in chemistry refers to a molecule that has experienced a separation of electric charge, where one side has a slightly negative charge and the other side has a slightly positive charge. Polarity is important for forming hydrogen bonds between side chains. Some amino acids, like aspartate, have a fully negative-charged side group. This has to do with the specific chemical properties of the side chain and the pH environment of the body. Each protein has a unique shape that dictates its function, and this shape depends on the sequence of amino acids, and their chemical properties, that fold to form the protein.

So, how are these amino acids linked together?

Each amino acid can be joined to another amino acid by forming a peptide bond. The carboxyl group of one amino acid can react with the hydroxyl group of another amino acid to form what is called a peptide bond. This chemical reaction is called a condensation, or dehydration, reaction because a molecule of water is formed in the process. You can see this in Figure 3.

Peptide bonds are what link amino acids together. A long chain of amino acids linked in this way forms a polypeptide.

The polypeptide can then fold into a specific 3D shape to form the protein. This folding is based on the chemical interactions of the amino acids. For example, a common structure in proteins is the alpha-helix. Here, a hydrogen bond is formed between a hydrogen of the amino group and an oxygen of the carboxyl group that is about 4 amino acids away. This makes the polypeptide assume a helical shape that you can see in Figure 4.

Another example of how amino acids contribute to the folding of a protein is with the amino acid cysteine. Cysteine has a sulfhydryl group () that is a special functional group that can join together with another cysteine to form a disulfide group (). These are strong bonds that play important roles in protein folding and maintaining the shape’s stability. You can see this in Figure 5 below.

Example 2: Recalling How Amino Acids Form a Protein

Proteins are biological polymers. What monomers will join together to form a protein?

Answer

Proteins serve critical functions in nearly all biological processes. Each protein has a unique shape that dictates its function. This shape depends on the sequence of amino acids that fold to form the protein.

Amino acids all have the same basic structure. Each amino acid has an amino group, a carboxyl group, and a side chain as shown below.

These amino acids can be joined together by the chemical reaction of the amino group and the carboxyl group to form a peptide bond. You can see this below. Additional amino acids can be added by peptide bonds to form a polypeptide that can fold into the protein.

Therefore, monomers of amino acids can be joined together to form a protein.

The way the polypeptide folds and the resulting 3D shape of the protein that is formed determines the function of the protein. One way to characterize the different types of proteins is based on their function, as either a structural protein or a regulatory protein.

Structural proteins are the building materials of the body. As we will see, the 3D shape of these proteins makes them ideal for forming fibers that can give cells mechanical stability.

Actin and myosin are two structural proteins that are important components of muscle cells. Collagen is the most abundant protein in mammals and provides structure in connective tissues like ligaments, cartilage, and skin. Keratin is another example of a type of structural protein that is found predominantly in hair, skin, and nails.

Let’s look at keratin in more detail as an example of a structural protein.

Keratin is actually a group of proteins that are critical components of the cell’s cytoskeleton. The cytoskeleton is a complex network of interlinking protein filaments that gives the cell its shape and structure. You can see the cytoskeleton stained red, while the nucleus stained purple, in the picture below.

Due to the amino acid sequence in a keratin polypeptide, a keratin polypeptide can fold to form a helix structure as we described earlier. These individual keratin helices can then associate with each other to form dimers as shown in the image below. A dimer is the complex formed by the association of two proteins. Higher level structures can form through additional interactions to form filaments.

The amino acid cysteine plays an important role in holding keratin helices together. In polypeptides, the sulfur group in cysteines are able to react with one another to form disulfide bonds. These are strong interactions and can help stabilize the keratin filament structure.

Keratin is a major component of hair, and the texture of hair can be influenced by disulfide bonds. These bonds can be temporarily broken by the application of heat, which is how a hair straightener can straighten curly hair! You can see this in Figure 8. Sulfur is also the reason why burnt hair has such a strong odor—because it is liberated from the cysteine amino acids in keratin.

Example 3: Recalling Some Examples of Structural Proteins

Which of the following is an example of a structural protein?

1. Keratin
2. Cytokines
3. Adrenaline
4. Amylase
5. Testosterone

Answer

Proteins play a critical role in all of life’s biological processes. Proteins can be broadly categorized as structural or regulatory.

Structural proteins are the building materials of the body. Actin and myosin are important components of muscle tissue. Collagen gives structure to connective tissues like ligaments or skin. Keratin is an example of a structural protein that is found in hair and nails. All of these proteins provide structure to the cells of the tissue.

You can see how the shape of keratin forms a helical structure as shown below.

These helical structures can be further combined to form higher level structures such as filaments. These filaments are the basis of the cytoskeleton in the cell that gives it its mechanical stability. You can see the cytoskeleton stained red below.

Besides structural proteins, another group of proteins are the regulatory proteins. Regulatory proteins are proteins that regulate different processes and activities within the organism. These can include enzymes that can speed up chemical reactions (like amylase), proteins involved in the immune system (like cytokines), or hormones that can act as chemical messengers to influence target cells (like adrenaline, insulin, or glucagon).

Therefore, an example of a structural protein is keratin.

Structural proteins often have repeating units and shapes that give them their mechanical strength. In contrast, regulatory proteins have specific 3D shapes that often need to be compatible with other molecules or proteins to perform their functions and may not have repeating structures like structural proteins.

Regulatory proteins are proteins that regulate different processes and activities within the organism. There are several categories of regulatory proteins. Enzymes, like peptidase or amylase, can help speed up chemical reactions. Antibodies or cytokines play an important role in the immune system. Hormones are chemical messengers that can be delivered to target cells to affect gene expression to influence the cell’s function. Some examples of hormones include insulin, glucagon, adrenaline, and testosterone. Let’s look at the hormone insulin in more detail.

Insulin is a major hormone involved in regulating blood sugar levels. When blood sugar levels are high, beta cells in the pancreas secrete insulin that then circulates in the blood. Insulin can then act on its target cells to instruct them to absorb glucose to be used for energy in cellular respiration.

Insulin targets cells of the liver, fat, and muscle. These cells have the insulin receptor on their cell surface. Once insulin binds to the receptors, this causes the receptor to change its shape as you can see below. In this image, the receptor is embedded in the cell’s membrane and is blue, while insulin is orange. In the bottom portion of the image, notice how the shape of the receptor has changed after binding to insulin.

This change in the shape of the insulin receptor after binding to insulin acts as a signal to other proteins inside the cell to ultimately lead to the export of GLUT4 to the cell’s surface. GLUT4 is a glucose transport protein that embeds itself into the cell membrane to act as a channel to let glucose into the cell. So, by binding to insulin, the cell is able to take up glucose.

Mutations can occur in the insulin gene that can result in a change in the protein’s amino acid sequence and structure. This can have a serious impact on the 3D shape of insulin and may prevent it from binding to its receptor. Without insulin binding to its receptor, the cell does not take up glucose, and blood glucose levels rise, causing diabetes. This can be treated by injections with normal insulin.

Example 4: Recalling Some Examples of Regulatory Proteins

Which of the following is an example of a regulatory protein?

1. Collagen
2. Adrenaline
3. Keratin
4. Myosin
5. Actin

Answer

Proteins play a critical role in all of life’s biological processes. Proteins can be broadly categorized as structural or regulatory.

Structural proteins are the building materials of the body. Actin and myosin are important components of muscle tissue. Collagen gives structure to connective tissues like ligaments or skin. Keratin is an example of a structural protein that is found in hair and nails. All of these proteins provide structure to the cells of the tissue.

Regulatory proteins are proteins that regulate different processes and activities within the organism. The 3D shape of regulatory proteins often needs to be compatible with other molecules or proteins.

Insulin is a protein that is involved in regulating blood glucose levels. It tells cells to take up glucose, so the cell can use it for energy. Insulin is sensed by the cell with the presence of insulin receptors. The 3D shape of insulin is compatible with the receptor, so when insulin (orange) binds to the receptor (blue), this causes a change in the shape of the receptor as shown below.

This change in shape can be relayed to the cell and interpreted as a signal to take up glucose.

Besides insulin, other examples of regulatory proteins include enzymes that can speed up chemical reactions (like peptidase or amylase) or hormones that can act as chemical messengers to influence target cells (like adrenaline or glucagon).

Therefore, an example of a regulatory protein is adrenaline.

Let’s recap some of the key points we have covered in this explainer.