Enzymes speed up reactions by lowering the activation energy of the reactants. The speed at which the enzyme turns reactants into products is called the rate of reaction. Several factors impact the rate of reaction, including the number of available reactants. Enzyme kinetics is the study of how an enzyme changes the rate of a reaction.
Scientists typically study enzyme kinetics with a fixed amount of enzyme in the controlled environment of a test tube. When more reactant, or substrate, is added to a fixed amount of enzyme, the rate of the reaction increases as the enzyme can make more product. As a result, when the reaction rate is graphed against substrate concentration, increasing the substrate increases the reaction rate. However, once all the active sites of the enzyme are occupied, the reaction rate plateaus. The concentration of substrate at which the maximum rate of reaction is reached is called Vmax. The number of present enzyme molecules limits Vmax. If the amount of enzyme is increased, Vmax increases, but adding more substrate has no effect.
The graph of rate of reaction versus substrate concentration can reveal other important characteristics of an enzyme’s kinetics. The substrate concentration at which the reaction rate is halfway to Vmax (i.e., ½ Vmax) is called the Michaelis constant (Km). Km is a representation of the affinity between an enzyme and a substrate. Enzymes with a lower Km require less substrate to reach Vmax and therefore have a higher affinity for their substrate. Interestingly, for many enzymes, the value of Km is very close to the cellular concentration of the substrate. Near Km, slight changes in substrate concentration can significantly impact the reaction rate, so small changes in cellular substrate availability can impact the function of an entire biological pathway.
Not all enzymes produce the hyperbolic-shaped substrate-rate graph known as Michaelis Menten kinetics. Michaelis Menten kinetics assumes that the enzyme catalyzes a single substrate. Enzymes that are regulated allosterically have multiple active sites and tend to produce a sigmoid-shaped graph when the reaction rate is plotted versus the substrate concentration.
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