Structural Proteins

5 min read Updated Apr 18, 2026

Structural proteins are the beams, cables, and sheets of the body. They are designed to resist force. Most are long, repetitive, and fibrous rather than globular. The MCAT focuses on three: collagen, keratin, and elastin, plus a passing nod to the cytoskeletal proteins (actin, microtubules, intermediate filaments).

Collagen

Collagen is the most abundant protein in the body - about 25-30 percent of total protein. It is the main component of bone, tendons, cartilage, skin, blood vessels, and basement membranes. Its job is to resist stretching.

Collagen has a distinctive triple helix structure. Three polypeptide chains (called alpha chains) wrap around each other in a right-handed superhelix. Each chain has the repeating sequence Gly-X-Y, where X is often proline and Y is often hydroxyproline. Glycine’s tiny R group (just -H) is essential because only glycine is small enough to fit in the tight interior of the triple helix. Every third position MUST be glycine.

Three polypeptide chains wrapping around each other in a right-handed triple helix with glycine residues tucked into the interior, demonstrating the collagen structure — Collagen triple helix. Three alpha chains, each with a Gly-X-Y repeat, wrap around one another. Every third residue is glycine. *Credit: Wikimedia Commons, CC BY-SA*

Why Collagen Needs Vitamin C

The hydroxyprolines and hydroxylysines are not encoded in DNA. They are made after translation by enzymes called prolyl hydroxylase and lysyl hydroxylase, which need vitamin C (ascorbate) as a cofactor. Vitamin C keeps the iron in these enzymes in the Fe2+ state, which is required for hydroxylation.

Without hydroxylation, collagen fibers cannot form the hydrogen bonds that stabilize the triple helix. Weak collagen means weak blood vessels, gums, and connective tissue. That is scurvy: bleeding gums, loose teeth, poor wound healing, fragile skin.

Keratin

Keratin is the protein in hair, nails, horns, feathers, and the outer layer of skin. Two types:

Alpha-keratin is rich in alpha helices coiled around each other in coiled coils. Found in hair, wool, nails, and epidermis.
Beta-keratin is rich in beta sheets. Found in feathers, scales, and beaks.

Keratins also contain many cysteines, which form cross-linking disulfide bonds between chains. The more disulfide bonds, the tougher the keratin (nails and claws are harder than hair).

Elastin

Elastin is what allows stretchy tissues (skin, lungs, large arteries) to snap back to their original shape after being deformed. It is a rubber-like protein. Elastin forms a loose, cross-linked network rather than a rigid fiber. When the tissue stretches, the chains pull apart; when force is released, they spring back - entropy drives the recoil.

Elastin is cross-linked by an unusual amino acid called desmosine (formed from 4 lysines). The AAMC outline does not require you to memorize desmosine, but you should know that elastin is extensively cross-linked.

Cell Adhesion Molecules

Proteins at the cell surface decide what sticks to what. The MCAT tests three families - cadherins, integrins, and selectins - and their specific roles in tissue architecture, wound healing, and leukocyte migration.

CAM family	What it binds	Calcium-dependent?	Signature role
Cadherins	Identical cadherins on a neighboring cell (homophilic, cell-cell)	Yes (Ca²⁺ required)	Hold epithelial tissues together at adherens junctions
Integrins	Extracellular matrix proteins (fibronectin, laminin, collagen) via RGD motifs	Yes (Ca²⁺/Mg²⁺)	Anchor cells to the ECM; trigger inside-out and outside-in signaling
Selectins	Carbohydrate ligands on other cells (e.g., sialyl-Lewis X)	Yes	Initial tethering and rolling of leukocytes on endothelium

Cytoskeletal Proteins

Inside cells, three main polymer systems form the cytoskeleton:

Polymer	Monomer	Diameter	Jobs
Actin / microfilaments	G-actin	~7 nm	Cell shape, muscle contraction, cytokinesis
Microtubules	Tubulin dimers (alpha + beta)	~25 nm	Tracks for motor proteins, mitotic spindle
Intermediate filaments	Varies (keratin, lamin, vimentin, neurofilament)	~10 nm	Mechanical strength, nuclear envelope support

Actin and microtubules are dynamic (assembly and disassembly are regulated). Intermediate filaments are mostly stable and provide tensile strength.

Why must every third amino acid in collagen's polypeptide chains be glycine?

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Glycine's R group is just a hydrogen. In the tightly wound triple helix, the third position points into the interior where no larger side chain could fit. Any mutation that replaces a glycine disrupts the helix and causes connective tissue disease (e.g., osteogenesis imperfecta).

How does vitamin C deficiency cause scurvy?

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Vitamin C is required by prolyl and lysyl hydroxylases that hydroxylate proline and lysine residues in collagen. Without hydroxylation, the collagen triple helix cannot form stable hydrogen bonds, so new collagen is weak. Connective tissues fail - bleeding gums, loose teeth, poor wound healing.

What structural feature of keratin makes it so mechanically tough?

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Disulfide cross-links between cysteine residues. Alpha-keratin is based on coiled coils of alpha helices, and beta-keratin on beta sheets, but both rely on abundant cysteine to form covalent S-S bridges that lock the structure. The more cysteines, the harder the keratin (compare soft hair vs. hard nails).