Factors Affecting Enzyme Activity

The rate of an enzyme-catalysed reaction is governed by several factors:

1. Concentration of substrate: As substrate concentration is increased (with enzyme held constant), the velocity of the reaction rises — but only up to a point. Plotting velocity (V) against substrate concentration (S) gives a rectangular-hyperbola curve with three recognisable phases: at low substrate concentration the velocity rises almost linearly (the enzyme's active sites are mostly free); at intermediate concentration the rise slows; and at high substrate concentration the velocity levels off at a maximum value, Vmax, because essentially all of the enzyme's active sites are now occupied ('saturated') by substrate.

The substrate concentration at which the reaction proceeds at exactly half of Vmax is called the Michaelis–Menten constant, Km. Km is a constant, characteristic value for a given enzyme–substrate pair, and serves as a measure of how tightly the enzyme binds its substrate: a low Km means the enzyme has a strong (high) affinity for its substrate (only a little substrate is needed to reach half-maximal velocity), while a high Km means a weak affinity. For most enzymes, Km falls somewhere between 10⁻⁵ and 10⁻² moles/litre.

2. Concentration of enzyme: With substrate present in excess, the rate of reaction is directly proportional to the concentration of substrate and rises with — and is also related to the square root of — the concentration of enzyme present: more enzyme molecules means more active sites available to work simultaneously, so the reaction proceeds faster; reducing enzyme concentration correspondingly slows the reaction.

3. Temperature: Enzyme-catalysed reactions proceed fastest at or around 37°C — the normal body temperature of homeotherms (warm-blooded animals) — the optimum temperature. Like any chemical reaction, the rate increases as temperature rises, but only over a limited range: above about 40°C enzymes rapidly denature and lose activity, while activity is also reduced at temperatures below the optimum. …