Britannica Sources used in this answer

How do enzyme reaction rates change when substrate concentration is low?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

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.

According to Michaelis-Menten kinetics, the rate of an enzyme-catalyzed reaction is directly influenced by the concentration of the substrate.

When substrate concentration is low, the reaction rate is proportional to the substrate concentration. Mathematically, this relationship is expressed as:

$Rate \approx \frac{V [S]}{K}$

In this equation, $V$ and $K$ (the Michaelis constant) are constants specific to the enzymatic process, and $[S]$ represents the substrate concentration. Because the rate is proportional to $[S]$ at these low levels, any increase in the amount of substrate available will result in a corresponding increase in the velocity of the reaction. This contrasts with high substrate concentrations, where the enzyme's active sites become saturated, causing the reaction rate to become essentially independent of further increases in substrate concentration.1