Temperature, pH and enzyme activity

Last time

Before we start: last time, in Enzymes, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

Too hot, too acid

Boil an egg and the clear white turns solid and white for good. Its proteins have changed shape permanently. Enzymes are proteins too, so heat and the wrong p H can wreck them in the same way. Today: why, and how to read the graphs.

Temperature: the rising part

As the temperature rises, the enzyme and substrate molecules gain kinetic energy and move faster. They collide more often, so more enzyme-substrate complexes form each second. So the rate of reaction increases.

The optimum

The optimum temperature is the temperature at which the enzyme works fastest. For most human enzymes it is about thirty-seven degrees, body temperature. Enzymes from bacteria in hot springs can have an optimum of seventy degrees or more.

Temperature: the falling part

Above the optimum, the heat makes the enzyme vibrate so much that bonds holding its shape break. The shape of the active site changes, so the substrate no longer fits. The enzyme is denatured. This is permanent, so the rate falls quickly to zero.

Watch it denature

Here the active site has changed shape. The substrate is the same, but it no longer fits, so no enzyme-substrate complex can form.

The temperature graph

Read the graph in three parts. The rate rises to the optimum, because of more collisions. It peaks at the optimum. Then it falls steeply, because the enzyme is denatured.

Cold is different

At low temperatures enzymes work slowly, because the molecules have little kinetic energy. But the enzyme is NOT denatured. Its shape is unchanged. Warm it up again and it works normally. That is why food keeps in a fridge.

pH

Each enzyme also has an optimum p H, where it works fastest. Pepsin in the stomach works best at about p H two, which is very acidic. Amylase in the mouth and small intestine works best near p H seven. Above or below the optimum p H, the shape of the active site changes and the enzyme is denatured.

The pH graph

The p H graph is a hill. The top is the optimum p H. On both sides the active site changes shape, so fewer substrates fit, and the rate falls. Note: p H does NOT change kinetic energy.