structural proteins

Meaning: Structural proteins give cells and tissues shape, support and strength. Collagen is a triple helix of three chains with glycine at every third position, and it resists stretching in skin, tendon and cartilage. Elastin is cross-linked so tissue can stretch and recoil. Keratin forms intermediate filaments in epithelial cells, hair and nails, and disulfide bonds between cysteines add strength. Inside cells, actin (microfilaments) and tubulin (microtubules) polymerize into the cytoskeleton.

On the MCAT: Expect questions that match a protein to its tissue or role, or that ask you to predict what happens when it is disrupted. A glycine substitution in collagen disturbs the tight triple helix, and breaking keratin's disulfide bonds makes hair easier to reshape. Passages may also ask which cytoskeletal filament a mutation or treatment affects.

Common confusion: Structural does not mean fibrous or static. Collagen and keratin are fibrous, but actin and tubulin start as globular subunits and assemble and disassemble dynamically. Motor proteins such as myosin, kinesin and dynein are usually listed as a separate class: they use ATP to move along filaments rather than to build them.