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Why does an enzyme reaction reach a maximum rate at high substrate concentrations?
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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 enzyme reaction reaches a maximum rate at high substrate concentrations because the enzyme becomes saturated.1
According to the Michaelis-Menten hypothesis, enzymes catalyze reactions by forming an intermediate complex with the substrate at a specific region called the active site. As the concentration of the substrate increases, the reaction velocity rises because more active sites are occupied.123
The reaction reaches its maximum velocity () when all available active sites on the enzyme molecules are engaged with substrate. At this point, the rate of the reaction is no longer limited by the availability of the substrate, but rather by the speed at which the active sites can convert the bound substrate into product.312