Structural Proteins

Structural Proteins Definition

Organic compounds are essentially chemical compounds that contain carbon bonds. Carbon is essential for life, as it quickly forms bonds with other molecules and components, allowing life to occur readily.

Proteins consist of building blocks, or monomers, called amino acids. The amino acids bind together like beads on a pearl necklace to form proteins, as shown in Figure 1. They consist of an alpha (\(\alpha\)) carbon bonded to an amino group (\(NH_2\)), a carboxyl group (\(COOH\)), hydrogen (\(H\)), and a variable side chain named (\(R\)) which gives it different chemical properties.

Figure 1: Amino acid structure. Daniela Lin, Study Smarter Originals.

Structural Proteins Function

Proteins come in different sizes and shapes. The shape of proteins determines the protein's function, making it essential.

There are generally two shapes of proteins: globular and fibrous.

• Globular proteins are spherical, usually act as Enzymes or transport materials, are generally soluble in water, have an irregular amino acid sequence, and are usually more sensitive to heat and pH changes than fibrous ones. A globular protein is hemoglobin, as shown in Figure 2.

• Fibrous proteins are narrower and more prolonged, usually are structural in function, are generally not soluble in water, have a regular amino acid sequence, and are usually less sensitive to heat and pH changes than globular ones. An example of a fibrous protein is keratin, as shown in Figure 2. Fibrous proteins can also be referred to as scleroproteins.

Figure 2: Examples of different protein shapes. Daniela Lin, Study Smarter Originals.

When a few amino acid chains bind together, they create peptide bonds. In contrast, when longer chains of amino acids bind together, they synthesize polypeptide bonds.

Since structural proteins are a type of protein, they all have primary, secondary, and tertiary structures. Some of them also have quaternary structures (Figure 3), such as collagen.

• Primary structure: A protein's primary structure is its amino acid sequences linked into a polypeptide chain. This sequence determines a protein's shape. This is very important as a protein's shape determines its function.

• Secondary structure: The secondary structure is caused by folding amino acids from the primary structure. The most common structures proteins fold into in the secondary level are alpha (\(\alpha\)) helices and beta (\(\beta\)) pleated sheets, which are held together by hydrogen bonds.

• Tertiary structure: The tertiary structure is a protein's three-dimensional structure. This three-dimensional structure is formed by the interactions between the variable R groups.

• Quaternary structure: Not all proteins have a quaternary structure. But some proteins can form quaternary structures that consist of multiple polypeptide chains. These polypeptide chains can be referred to as subunits.

_{Figure 3: Protein structure (primary, secondary, tertiary, and quaternary). Daniela Lin, Study Smarter Originals.}

Collagen proteins are naturally fibrous. This sheet-like elongated shape helps collagen serve its structural and protective role in the cell. This is because collagen's rigidity and ability to resist being pulled or stretched make it the perfect support for our bodies

In the next section, we'll go over some of the most common types of structural proteins in more detail.

Types of structural proteins

Some common examples of proteins are enzymes and defense proteins. Enzymes speed up reactions while defense proteins protect your body by eliminating threats.

Collagen

Within nature, structural proteins are the most common types of proteins. Collagen is the most common structural protein found in mammals, making up around 30% of the total proteins present in the body.

Collagen is located in the extracellular matrix and our bodies' connective tissues.

Collagen is a fibrous protein that supports Cells and their tissues and provides Cells with their shape and structure. Specifically, it's an elongated fibrous protein made of amino acids that bind together to form triple helix-shaped long rod structures that are usually referred to as fibrils.

Collagen can be found all over the body, including in ligaments, bones, tendons, and epithelial tissue in general. Collagen can be rigid to less rigid depending on which parts they are in. Bone collagen, for example, is very rigid when compared to tendons.

We use collagen industrially in supplements and gelatin, which can be found in desserts such as gummies and Jell-O.

There are around five common types of collagen, but type I comprises 96% of the body. Type I refers to skin, bones, tendons, and organs. Collagen Type I is shown in a thin section of mammalian lung tissue in Figure 5.

Keratin

Keratin is a structural fibrous protein found in vertebrates. It's the primary component that makes up nails, hair, skin, and feathers.

Keratin is insoluble in water, and its monomers form rigid filaments that comprise the lining of organs and other body parts. Higher keratin levels can correlate with certain cancers, such as breast and lung cancer.

Alpha (\(\alpha\)) keratin is the type of keratin found in vertebrates, and it’s usually softer compared to Beta (\(\beta\)) keratin. In general, keratin can be compared to chitin, a complex carbohydrate in arthropods and fungi.

• There are two alpha keratins: Type I is acidic, while Type II is basic. There are 54 keratin Genes in humans, 28 of which belong to type I and 26 to type II.

Beta keratin is found in birds and reptiles and consists of beta sheets compared to alpha keratin, which consists of alpha helices. Silk that spiders and insects make is usually classified as keratin and is made of beta-pleated sheets (\(\beta\)).

Fibrinogen

Fibrinogen is a structural fibrous protein made in the liver that circulates the blood of vertebrates. When injuries occur, enzymes convert fibrinogen into fibrin to help blood clotting.

Actin and Myosin

Actin and Myosin are proteins that play a vital role in muscle contraction illustrated in Figure 4. They can be both globular or fibrous.

• Myosin converts chemical energy or ATP into mechanical energy that generates work and movement.
• Actin performs many critical cellular functions. Still, in muscle contraction, actin associates with myosin, allowing myosin to slide along and causing muscle fibers to contract.

Figure 4: Human muscle anatomy showing myosin and actin. Image by brgfx on Freepik.

Structural Proteins Examples

Within this section, we will focus on the structural proteins located in viruses.

Most biologists think that viruses aren't alive. This is because viruses aren't made up of cells. Instead, viruses consist of Genes bundled into the capsid.

Viruses also cannot copy their own genes, as they don't have the structures to do so. This means viruses must take over the host's cells to make copies of themselves!

Viruses, like humans, have proteins. For viruses, their structural proteins make up the capsid and the envelope of the virus. This is because structural proteins are types of proteins that protect and maintain the viruses' shape.

The capsid is vital to the virus as it stores the genetic material of the virus, protecting it from being broken down by the host. Capsids are also the way viruses attach to their host.

• Many oligomers, or polymers with a few repeating units, together form a capsomere. Capsomeres are subunits that come together to form the capsid of a virus. Capsomeres usually assemble into many different shapes, including helical and icosahedral.

Envelopes are present in some viruses and surround the capsid. Usually, envelopes from proteins come from the host's cell membrane, which they acquire when they bud off of it. The envelopes are made from proteins that bind to the membranes of the host's cells. These proteins located on the envelopes are glycoproteins, proteins attached to carbohydrates.

Examples of some common virus structures are shown in Figure 6.

Figure 6: Types of virus structures illustrated. Image by brgfx on Freepik.

Figure 7: Illustration of how the COVID-19 looks like. Image by starline on Freepik.

Structural Proteins in the Body

Structural proteins are proteins that are naturally found in the body, and this is because they have functions that are integral to all living organisms. Structural proteins maintain cell shape and form and comprise bones and even tissues! We can essentially compare structural proteins to the skeletons of our cells.

We've already gone over some of the body's most essential and abundant structural proteins, such as collagen, keratin, actin, and myosin. Thus, this section will cover a few more examples of structural proteins found in human bodies.

• Tubulin is a globular protein that combines or polymerizes into chains that form microtubules. Microtubules are fibers utilized for cell transport and cell division or mitosis. Tubulin comes in an (\(\alpha\)) and (\(\beta\)) form. Another function of microtubules is to serve as a "skeleton" for our cells.

• Elastin is also part of the extracellular matrix and works with other structural proteins, such as collagen, in connective tissues. In arteries, elastin helps blood flow. The degeneration of elastin in our tissues can lead to many side effects, including premature aging, as excessive sun exposure breaks down collagen and elastin in connective tissue.

• Titin is the largest protein consisting of around 27,000 amino acids. After actin and myosin, titin is the most common protein in muscles. Titin plays a vital role in the function of striated muscles as it provides shape and flexibility. Striated muscles are heart or cardiac and skeletal muscles, as shown in Figure 8. Unlike smooth muscles, striated muscles have sarcomeres or repeating units that help with muscle contraction. Titin interacts with actin and myosin to stabilize sarcomeres as you move or your body functions causing the muscles to contract and relax.

Figure 8: Types of muscle cells illustrated. Image by brgfx on Freepik