Enzymes: function and the factors that affect them

Enzymes make life possible by speeding up reactions. Here you see how they act on the substrate, why they are specific and what denatures them.

Direct answer: Enzymes are biocatalysts (almost always proteins) that speed up the cell's chemical reactions by lowering the activation energy. They act on a specific substrate at their active site, and are not consumed in the reaction.

How does an enzyme act?

The enzyme binds the substrate at its active site (induced-fit model).
It forms the enzyme-substrate complex, lowers the activation energy and releases the product.
The enzyme stays intact and is reused: that is why it works in small amounts.

Factors that affect its activity

Factor	Effect
Temperature	Increases activity up to an optimum; if too high, the enzyme is denatured.
pH	Each enzyme has an optimal pH; outside it, activity drops.
Substrate concentration	Increases the rate until the active site is saturated.
Cofactors / coenzymes	Some (ions, vitamins) are required for it to work.

Exam trap: enzymes are not consumed and do not change the equilibrium of the reaction: they only speed it up. And high temperature does not "kill" the enzyme: it denatures it (it loses its shape and active site).

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Frequently asked questions

What are enzymes?

They are biocatalysts, almost always proteins, that speed up reactions by lowering the activation energy without being consumed.

How do they lower activation energy?

By binding the substrate at their active site and stabilizing the transition state, which makes the reaction easier.

What denatures an enzyme?

High temperatures or a pH outside its optimum alter its structure and active site, and it loses activity.