Effect of Temperature on Enzyme Activity

The Core pattern

Temperature affects the rate of an enzyme-controlled reaction. At relatively low temperatures, enzyme activity is low. As temperature increases, the reaction rate usually increases too, up to a point called the optimum temperature — the temperature at which the enzyme-controlled reaction proceeds at its highest rate under the conditions being investigated. Beyond that point, activity falls away as the enzyme becomes denatured: its shape, including the shape of its active site, changes so that the substrate no longer fits it properly. The pattern in full is: activity is slow at low temperature, rises as temperature increases, peaks at the optimum, and then falls as excessive temperature causes denaturation.

Don’t assume every enzyme’s optimum temperature is 37°C just because that figure is familiar from human biology. An optimum temperature belongs to a particular enzyme under particular conditions, and different enzymes function best at different temperatures. If a question gives you data or a graph, the optimum is whatever that evidence actually shows — read it off the data rather than supplying a remembered number.

Why activity rises below the optimum

Supplement

Below the optimum, raising the temperature gives both enzyme and substrate particles more kinetic energy, so they move faster and collide more often. What actually drives the increase in rate is that a greater proportion of those collisions become effective collisions — collisions in which the substrate meets the active site in a way that lets it bind. More effective collisions mean more enzyme–substrate complexes forming per unit time, and that’s what shows up as a faster reaction. The chain is: temperature rises, kinetic energy increases, particles move faster, effective collisions become more frequent, more enzyme–substrate complexes form, and the reaction rate increases.

Why activity falls above the optimum

Supplement

Past the optimum, a different mechanism takes over completely. High temperature changes the enzyme’s shape, and that includes the shape of its active site, so the substrate can no longer fit it as it did before. The enzyme has become denatured. Fewer successful enzyme–substrate complexes can form, so the reaction rate falls — not because particles are moving too fast to collide usefully, but because the active site itself no longer has the right shape to bind the substrate at all.

Low temperature is not the same problem as denaturation

These two halves of the graph get blurred together surprisingly often, so it’s worth stating the distinction plainly. A low temperature slows a reaction because particles have less kinetic energy and collide effectively less often — the enzyme itself is still intact and its active site still has its normal shape. An excessively high temperature is a different kind of problem: the enzyme’s structure has actually changed, and that change doesn’t reverse when the temperature drops back down. An answer that explains a slow reaction at low temperature by saying the enzyme is “partially denatured” is describing the wrong mechanism, even though the outcome — a slower reaction — sounds similar.

Reading a temperature–activity graph

On a typical graph of enzyme activity against temperature, the rising section reflects increasing kinetic energy and more frequent effective collisions; the peak marks the optimum temperature for that enzyme under those conditions; and the falling section beyond the peak reflects denaturation, not a slowing of particle movement. When a question asks you to describe or explain such a graph, naming each section by the mechanism behind it — rather than just describing the shape as “going up and then down” — is usually where the marks actually sit.

Where this connects

Both halves of this page ultimately come back to the same relationship covered in enzyme action and specificity: whether a substrate can fit an enzyme’s active site. Temperature is one route to disturbing that fit; the next page, pH, is a different route to the same kind of disturbance — though, as that page explains, without kinetic energy playing any part in it.