How enzymes work

Enzymes are the cell's helpers that speed up chemical reactions. Almost all are proteins; a few are nucleic acids, called ribozymes. When an enzyme protein folds (tertiary structure), it forms pockets on its surface. The pocket that fits the reacting chemical (the substrate) is the active site. Most enzymes are damaged above about 40°C, but enzymes of microbes living in hot springs (thermophiles) still work at 80 to 90°C.

A chemical reaction breaks old bonds and forms new ones. Its rate is the amount of product formed per unit of time. Without an enzyme, the rate roughly doubles for every 10°C rise and halves for every 10°C fall.

Enzymes speed reactions enormously. Carbon dioxide and water form carbonic acid at about 200 molecules an hour on their own. With the enzyme carbonic anhydrase, it is about 600,000 a second: some 10 million times faster. Reactions are often linked in a series, a metabolic pathway. For example, glucose is broken into two pyruvic acid molecules in ten enzyme steps. Then, in working muscle without enough oxygen, pyruvic acid becomes lactic acid; in yeast during fermentation it becomes ethanol.

How does an enzyme do it? The substrate fits into the active site to form an enzyme-substrate (ES) complex, which lasts only a moment. On the way to becoming product, the substrate must pass through a high-energy, unstable transition state. The energy needed to reach it is the activation energy, like a hill to be pushed over. Enzymes lower the activation energy, so the reaction goes much faster.

The catalytic cycle: (1) the substrate binds the active site; (2) the enzyme changes shape slightly to grip it more tightly (induced fit); (3) bonds in the substrate break and an enzyme-product (EP) complex forms; (4) the product is released, and the enzyme is free to work again. It is not used up. In short: E + S ⇌ ES → EP → E + P.