How fast enzymes work

Temperature and pH each give a bell-shaped curve: activity rises to a peak, the optimum, then falls. Human enzymes work best near 37°C. Taq DNA polymerase, from the hot-spring bacterium Thermus aquaticus, survives very high heat. Most enzymes prefer pH 6-8, but pepsin, in the acid of the stomach, works at pH 1-2, and alkaline phosphatase at pH 10-11.

Adding more substrate speeds the reaction, but only up to a point: once every enzyme molecule is busy (saturated), the rate stays at its maximum.

Enzyme activity is measured in units. One enzyme unit converts 1 micromole of substrate per minute; one katal converts 1 mole per second. Specific activity (units per mg of protein) shows how pure an enzyme preparation is: the higher it is, the purer.

How do enzymes speed reactions? Every reaction must climb an energy hill to an unstable transition state before it can go downhill to the products. The height of that hill is the activation energy. Enzymes lower the activation energy. They do not change the overall energy change (ΔG) or the equilibrium; they only help the reaction reach equilibrium faster.

Michaelis and Menten (1913) described this mathematically. The enzyme (E) binds the substrate (S) to form a complex (ES), which gives product (P): E + S ⇌ ES → E + P. The starting rate is v₀ = Vmax[S] / (Km + [S]).

• Plotting v₀ against [S] gives a curve called a rectangular hyperbola.
• At low [S], the rate rises in step with [S].
• At high [S], the rate levels off at Vmax.
• When [S] = Km, the rate is exactly half of Vmax. So Km is the substrate concentration that gives half the top speed.