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How can an activator molecule increase enzyme activity through an allosteric site?

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A multi-panel diagram illustrates how a substrate binds to an enzyme's active site through induced fit, leading to product formation, and shows how non-substrates and allosteric regulators can alter enzyme activity by binding to different sites on the enzyme. — Enzyme In the induced-fit theory of enzyme-substrate binding, a substrate approaches the surface of an enzyme (step 1 in box A, B, C) and causes a change in the enzyme shape that results in the correct alignment of the catalytic groups (triangles A and B; circles C and D represent substrate-binding groups on the enzyme that are essential for catalytic activity). The catalytic groups react with the substrate to form products (step 2). The products then separate from the enzyme, freeing it to repeat the sequence (step 3). Boxes D and E represent examples of molecules that are too large or too small for proper catalytic alignment. Boxes F and G demonstrate binding of an inhibitor molecule (I and I′) to an allosteric site, thereby preventing interaction of the enzyme with the substrate. Box H illustrates binding of an allosteric activator (X), a nonsubstrate molecule capable of reacting with the enzyme.

An activator molecule increases enzyme activity through an allosteric site by inducing a conformational change in the enzyme's structure.12

According to the induced-fit theory, the binding of a substrate to an enzyme typically causes a shape change that aligns the enzyme's catalytic groups to facilitate a reaction. However, some substrates are unable to induce this necessary alignment on their own. When an activator molecule binds to an allosteric site—a location distinct from the active site—it alters the enzyme's shape to properly align the active site for the substrate.32

This process allows the enzyme to effectively bind and catalyze reactions with substrates that would otherwise be unable to trigger the required structural shift. By modulating the enzyme's shape, these activators—which can include hormones or metabolic products—enable the cell to rapidly regulate its metabolism and ensure that essential substances are produced only when needed.321