Substrate concentration

The rate of an enzyme-catalyzed reaction increases to a maximum rate, Vmax, with increasing substrate concentration. Vmax represents saturation of the enzyme with substrate. Km, the Michaelis constant, represents substrate concentration which produces half of Vmax. It is a measure of affinity of the enzyme for its substrate: the smaller Km, the greater is the affinity.

Practically, this means:

• high concentrations of substrate should be used when testing in vitro activity of an enzyme
• Vmax and Km values can be determined in vitro and in vivo to compare the contribution of a given enzyme to a specified metabolic reaction
• a plot of the reciprocal of substrate concentration (x axis) against reciprocal of rate of reaction (y axis) can give Vmax - the reciprocal of the y-axis intercept - and Km - the negative reciprocal of the x-axis intercept. This is termed a Lineweaver-Burk plot.