Michaelis-Menten Constant (Km) and Its Significance

When the initial rate (velocity) of an enzyme reaction is measured across a range of substrate concentrations, using a fixed amount of enzyme, and the results are plotted on a graph, a characteristic curve emerges. At low substrate concentrations, the velocity increases rapidly as substrate concentration rises, because there are plenty of free active sites available. As substrate concentration continues to rise, however, the rate of increase progressively slows, and above a certain substrate concentration the curve flattens out almost completely, with little further increase in rate - this shows that the enzyme is now working at its maximum velocity, Vmax, with essentially all of its active sites continuously occupied. The Michaelis-Menten constant, Km, is defined as the substrate concentration at which the reaction proceeds at exactly half of Vmax; it is a measure of how readily the enzyme binds its substrate - a LOW Km indicates the enzyme reaches half-maximal speed at a low substrate concentration (i.e., it has a high affinity for its substrate), while a HIGH Km indicates the enzyme n …