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What does it mean when all of an enzyme’s active sites are saturated?

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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.

When an enzyme’s active sites are saturated, it means that all available active sites on the enzyme molecules are occupied by substrate molecules.123

At this point, the enzymatic reaction has reached its maximum velocity ( $V_{M}$ ). Because every active site is engaged, the reaction rate becomes independent of further increases in substrate concentration; the speed of the reaction is then limited solely by how quickly the active sites can convert the bound substrate into product. This relationship is a fundamental component of Michaelis-Menten kinetics, which describes the velocity of enzyme-catalyzed reactions.31245