Mechanism of Enzyme Action

Only a small amount of enzyme is needed

Because an enzyme acts as a catalyst and is not itself consumed in the reaction it speeds up, a reaction can be driven to completion using only a tiny quantity of the enzyme relative to the amount of substrate being converted. The same enzyme molecule can be used over and over again as more substrate becomes available.

Enzymes work by lowering the activation energy

Like ordinary chemical catalysts, enzymes speed up a reaction by bringing down the activation energy barrier that must be crossed for the reaction to proceed — but they do this far more effectively, letting the reaction run under the mild conditions found inside a living cell.

The hydrolysis of sucrose illustrates the difference in scale:

When sucrose is hydrolysed by simple acid catalysis, the activation energy required is about $6.22\ \text{kJ mol}^{-1}$.

When the same hydrolysis is carried out by the enzyme sucrase, the activation energy drops to only about $2.15\ \text{kJ mol}^{-1}$.

This large drop in activation energy is what allows the enzyme-catalysed reaction to proceed rapidly at body temperature, where the acid-catalysed version would be comparatively very slow.

The lock-and-key model of enzyme action

The specificity of an enzyme for its substrate is explained by picturing the enzyme molecule as having a precisely shaped pocket on its surface, called the active site. The active site's shape and chemical environment match only one substrate molecule (or a narrow family of closely related substrates) — much like a lock that will only turn with its own matching key.

The sequence of events can be summarised as follows:

1. The substrate molecule approaches the enzyme and fits snugly into the active site, because the two are geometrically and chemically complementary.
2. This binding produces a temporary enzyme–substrate complex.
3. Held in this complex, the substrate is converted into product(s) far more readily than it would be on its own — the enzyme provides the favourable environment that lowers the activation energy for the reaction step.
4. Once the reaction is complete, the product(s) are released from the active site, and the enzyme itself is left chemically unchanged, free to bind another substrate molecule and repeat the cycle.

This lock-and-key picture accounts for two of the central features of enzyme behaviour discussed above: …