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How does competitive inhibition prevent an enzyme from binding its normal substrate?

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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.

Competitive inhibition prevents an enzyme from binding its normal substrate by utilizing a molecule that is structurally very similar to the substrate. This inhibitor molecule "competes" directly with the substrate for access to the enzyme's active site.123

Because the inhibitor occupies the active site, it physically blocks the actual substrate from binding. Once the inhibitor is bound to the active site, it typically cannot undergo the reaction, effectively preventing the enzyme from performing its catalytic function.341

This process is reversible; because the inhibitor and substrate are competing for the same location, the inhibitory effect can be overcome by increasing the concentration of the substrate, which increases the likelihood that substrate molecules will encounter and bind to the active site instead of the inhibitor.2