Lesson Video: Structure of Proteins Biology

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

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Video Transcript

In this video, we will learn how to describe the basic structure of a protein as a polymer of amino acid subunits. We will further learn the difference between structural and regulatory proteins and discuss an example of each in more detail.

What texture is your hair? Is it straight or is it curly? You might be surprised to hear that the texture of your hair is influenced by sulfur. More specifically, it’s influenced by the amino acid cysteine. Cysteine 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 the property of the structural protein keratin that’s 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 structure.

Proteins are polymers of amino acids that have been bonded together. You might recall the basic structure of an amino acid. The central carbon atom, also called the alpha carbon, can form four different bonds. One is with the amino group, one is with the carboxyl group, one is with the hydrogen, and one is with a side chain. The side chain is sometimes called an R group and differentiates one amino acid from another. There are 20 amino acids that are all identical with the exception of their side chain, which can range in complexity.

Let’s look at a few different examples of amino acids. Glycine is the simplest amino acid with its side chain being a hydrogen atom. Lysine is more complex and has a longer side chain with a charged amino group at the end. Here’re some other amino acids with different side chains. These complexities in their side chain exist in order to build proteins with very specific shapes. The different shapes arise because the side chains give the amino acids different chemical properties. Aspartic acid normally has a hydrogen atom but is acidic, and it can lose this hydrogen atom because of the pH environment of the body. This ionic and negatively charged form is called aspartate.

The chemical property of lysine, on the other hand, is basic as its side chain is positively charged. We can also distinguish between polar and nonpolar amino acids. 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.

An example of a polar amino acid is serine. Serine contains a hydroxyl group in its side chain that has a slight negative charge, making this amino acid polar, whereas alanine contains a methyl group that makes this side chain nonpolar. Polarity is important for forming hydrogen bonds between side chains of different amino acids in order to build proteins.

Now, let’s see how these amino acids can be joined together to form proteins. Although this might look like a bunch of scribbles, let’s pretend that this is a protein. Each protein has a unique shape that dictates its function. Let’s zoom in on this section of the protein to see what it’s made up of. As we can see, this protein is made up of different amino acids that are linked together. The protein shape depends on the sequence of amino acids and their chemical properties. These amino acids can interact with each other in different ways to fold and form this protein. These amino acids are linked together by peptide bonds.

Let’s see how these are formed. We can see the basic structure of an amino acid here, but let’s redraw this to expand the carboxyl and amino groups so we can see how a peptide bond is formed. Here’s two amino acids that are colored differently. You could see the amino group here, the R group here, and the carboxyl group here.

A peptide bond is created when the carboxyl group of one amino acid reacts with the amino group of another amino acid. This will form a water molecule in the process as well as a peptide bond, so let’s see what this looks like. So here we can see the two amino acids joined together by a peptide bond. We can also see the molecule of water that’s formed. This chemical reaction can be called a condensation or dehydration reaction because this molecule of water is formed in the process. By linking together amino acids with peptide bonds, a long chain of amino acids can form called the polypeptide.

In our protein example here, if we were to unfold it, we would have a long linear chain of amino acids. Because this polypeptide is made up of different amino acids with different side chains that have different chemical properties, it will not stay as a long linear molecule. Some amino acids might be attracted to each other, while others might repel each other. And this will cause the polypeptide structure to change. So suppose these amino acids, represented here as these blue circles, are attracted to each other. This might cause the polypeptide to fold in a certain way, which now produces this unique shape. This shape is what gives the protein its function.

There are certain structures that are found repeatedly in proteins, for instance, this helix shape structure. If we look a bit closer, we can see it here. This is actually made up of amino acids that are joined by peptide bonds. If we uncover a bit of this, we can see a part of the carboxyl group here and a part of an amino group here. This is from two separate amino acids that are about four amino acids apart. And because of their respective polarities, they’re able to hydrogen bond to each other. This same type of hydrogen bonding occurs throughout the helix and is the reason that it takes on this shape. This alpha helix is a good example of how a protein can fold from its primary structure, where it was just a linear chain of amino acids, to the secondary structure, where it takes on the shape of this helix.

Let’s look at another example. In this part of the protein, you can see a sharp bend. Let’s take a closer look. So let’s say we have a polypeptide, and these green amino acids represent cysteine. Cysteine is interesting because its side chain contains a sulfhydryl group. We can also see this indicated down here with these two cysteine amino acids. These two sulfhydryl groups can come together to form a covalent bond known as a disulfide bond, sometimes called a disulfide bridge. These can form bends in the protein and can even help to form the protein’s 3D or tertiary structure.

To summarize, thanks to the chemical properties of the amino acids and their order in the polypeptide, a polypeptide can fold into a protein. How it folds and the resulting 3D shape of the protein is important because it determines the function of the protein.

Now let’s talk more about the functions of proteins. One way to characterize different types of proteins is based on their function as either a structural protein or a regulatory protein. We’ll first talk about structural proteins. Structural proteins are the building materials of the body. They often have repeating units and shapes that give them their mechanical strength to provide structure to cells. As we’ll see, the 3D shape of these proteins makes them ideal for forming fibers that can give cells stability.

Examples of structural proteins include actin and myosin, which are two structural proteins that are important components in muscle cells. 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 actually describes a group of proteins that are critical components of the cells’ cytoskeleton. If we look at cells under the microscope and color the cytoskeleton and the nucleus, we can actually see them nicely. It’s a complex network of interlinking protein filaments that gives the cell its shape and structure.

Let’s take a closer look at what one of these filaments might look like. Here you can see two helices that are formed from two separate keratin proteins. Due to the amino acid sequence in a keratin polypeptide, it can fold to form a helix structure as we described earlier. These individual keratin helices can then associate with each other to form dimers, which are complexes formed by the association of two proteins. These dimers can then associate with other keratin dimers to form filaments.

So how are these helices and dimers held together? It turns out that keratin contains a lot of the amino acid cysteine. And the sulfur group in cysteine can form disulfide bonds, like the ones we just talked about, between the helices and between the dimers in the filaments. These disulfide bonds are strong interactions and can help stabilize these structures.

Remember at the beginning of the video where we explained that the appearance of hair strongly depends on the presence of sulfur. Because keratin is a major component of hair, the texture of hair can be influenced by disulfide bonds. Curly hair has many disulfide bonds that can change the shape of the keratin filaments to make them more curly. These bonds can be temporarily broken by the application of heat, which is how a hair straightener can straighten curly hair. Sulfur is also the reason why burnt hair has such a strong odor because it is liberated from the cysteine amino acids in keratin.

Now let’s talk about regulatory proteins. Regulatory proteins are proteins that regulate different processes and activities within the organism. They 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.

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 and influence the cells’ 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 or glucose 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. Glucose can then be used by the cell for energy in cellular respiration. Insulin targets cells of the liver, fat, and muscle. These cells have the insulin receptor on their cell surface.

The insulin receptor extends into the inside of the cell, so it has an extracellular component and an intracellular component. When insulin binds to the receptor, this causes the intracellular component to change shape. This change in shape acts as a signal to a series of other proteins inside the cell to ultimately lead to the export of a protein called GLUT4. GLUT4 is a glucose transport protein that embeds itself in the cell membrane. This acts as a channel to let glucose inside the cell. So when the insulin receptor binds to insulin, the cell is ultimately able to take up glucose, and this process is mediated by the change in shape of proteins.

Now that we’ve discussed how proteins are built up and learned about the differences between structural and regulatory proteins, let’s test our knowledge with a practice question.

There are 20 different amino acids in humans that share a similar basic structure. Which group differs between all amino acids?

This question is asking us about amino acids in humans, so let’s discuss the basic structure and function of amino acids to answer this question. Humans have 20 different amino acids that are the building blocks to make up all the proteins in the body.

Each of these amino acids contains the same basic structure, so let’s look at the individual parts. In the center is the central carbon, which bonds to four different chemical structures. The first is a carboxyl group, then a hydrogen, an amino group, and an R group, sometimes called the side chain. The R group is the component of an amino acid that differs between all amino acids. Some R groups are relatively simple like glycine, which contains a hydrogen for its R group, while some are more complex like lysine, which has a hydrocarbon chain that terminates with a charged amino group.

The difference in R groups is what contributes to the complex interactions among amino acids, which causes specific folding of the polypeptide. Now that we’ve reviewed the structure and function of amino acids, we are able to answer our question correctly. The group that differs between all amino acids is the R group.

Now, let’s review some of the key points that we covered in this video. Proteins are made of polymers of amino acid subunits. Amino acids are joined by peptide bonds to form a polypeptide. The R group or side chain is what’s different between amino acids. The chemical properties of the R group can influence how the polypeptide folds to form a protein. Structural proteins provide structure to cells, while regulatory proteins regulate different processes in activities within the organism.

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