Nonenzymatic protein function

Enzymes get most of the attention, but a large share of the proteome does its work by binding and holding shape rather than by speeding reactions. Structural proteins are the clearest example: collagen provides tensile strength in connective tissue, elastin allows tissues to recoil, keratin forms hair and nails, and actin and tubulin polymerize into the microfilaments and microtubules of the cytoskeleton.

Transport and motor proteins convert binding into movement. Hemoglobin carries oxygen in blood with cooperative, sigmoidal binding kinetics, while myoglobin stores it in muscle with hyperbolic kinetics and higher affinity. Albumin transports fatty acids, drugs, and hormones and maintains plasma oncotic pressure. Membrane channels and pumps move ions and solutes across bilayers, and the motor proteins myosin, kinesin, and dynein use ATP hydrolysis to walk along cytoskeletal tracks — driving muscle contraction and intracellular cargo transport.

Other nonenzymatic roles are recognition-based. Cell adhesion molecules — cadherins, integrins, and selectins — hold tissues together and mediate leukocyte rolling. Immunoglobulins bind antigens with extraordinary specificity through their variable regions. Receptors and their peptide ligands carry signals across membranes, and biosignaling cascades depend on G proteins and second messengers. In every case, function follows from tertiary and quaternary structure, so denaturation abolishes the activity just as it does for an enzyme.

The MCAT chemical and physical foundations section groups nonenzymatic protein function with protein structure and lipids, and the same content resurfaces in the biological and biochemical foundations section. High-yield comparisons include hemoglobin versus myoglobin binding curves, the distinction between motor and structural proteins, and how antibody structure produces specificity.