The catalytic action of an enzyme on a given substrate can be described by two parameters: Vmax, the maximal velocity of the reaction at saturating substrate concentrations, and Km (the Michaelis constant), a measure of the affinity of an enzyme for its substrate (Figure 3-19). The Km is defined as the substrate concentration that yields a half-maximal reaction rate (i.e., ^ Vmax). The smaller the value of Km, the more avidly an enzyme can bind substrate from a dilute solution and the smaller the substrate concentration needed to reach half-maximal velocity.
The concentrations of the various small molecules in a cell vary widely, as do the Km values for the different enzymes that act on them. Generally, the intracellular concentration of a substrate is approximately the same as or greater than the Km value of the enzyme to which it binds.
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