Allosteric regulation

In allosteric regulation, a regulatory molecule, called an effector or modulator, binds to an enzyme at an allosteric site, a pocket separate from the active site where the substrate binds. Binding shifts the shape of the enzyme, and the change is carried through the protein to the active site. An allosteric activator helps the active site bind its substrate or work faster; an allosteric inhibitor does the opposite.

Many allosteric enzymes are built from several subunits, each with its own active site, and the whole enzyme can switch between a less active tense (T) form and a more active relaxed (R) form. A shape change in one subunit spreads to the others, so these enzymes often show cooperativity: their reaction rate climbs along an S-shaped curve as substrate rises, instead of the simple curve described by Michaelis–Menten kinetics. This lets activity switch sharply over a narrow range of conditions.

Allosteric control is a main way cells tune metabolic pathways from moment to moment. In feedback inhibition, a pathway's end product binds an enzyme near the start and slows its own production, as isoleucine does to the first enzyme of its own synthesis. ATP inhibits, and ADP and AMP activate, the glycolytic enzyme phosphofructokinase. Hemoglobin's cooperative oxygen binding is the classic non-enzyme example.