Enzymes: Temperature and pH Effects

0:24Rising temperature: more collisions

Start with the rising part of the graph. Heat gives molecules kinetic energy, so enzyme and substrate move faster. They collide more often, and more of the collisions have enough energy to react. So more enzyme-substrate complexes form each second, and the rate goes up.

0:43The optimum and the fall

But the rate does not keep rising. There is an optimum temperature, where the rate is highest. Go above it, and the graph drops off a cliff. Notice the two effects are happening together. Heat keeps speeding up collisions, but at the same time it starts to damage the enzyme.

Past the optimum, the damage wins.

1:03Enzyme tertiary structure

To see why, look at the enzyme itself. An enzyme is a protein, folded into a tertiary structure. That fold is held in place by bonds, mainly hydrogen bonds and ionic bonds. The fold creates the active site, and the substrate fits it because the shapes are complementary.

1:21Denaturation: bonds break

Now add heat. The enzyme molecule gains kinetic energy and vibrates more and more, until the hydrogen bonds and ionic bonds break. The tertiary structure changes, and so the shape of the active site changes. The substrate is no longer complementary, so it cannot bind to the active site, and fewer enzyme-substrate complexes form.

The enzyme is denatured. And that is usually permanent.

Cooling it back down does not repair the shape. The amino acid chain is still there, so the enzyme is not destroyed. But the active site has changed, so it no longer works. This is why we say denatured, not dead. An enzyme was never alive in the first place.

2:07What pH measures

Now pH. This measures hydrogen ion concentration. The formula is, pH equals minus log ten of the hydrogen ion concentration. So a concentration of zero point zero zero one moles per decimetre cubed gives a pH of three. More hydrogen ions means a lower pH.

2:28The pH rate curve

The graph for pH is a bell shape, with an optimum pH. Move away from the optimum, and the hydrogen ion concentration changes. Hydrogen ions, or hydroxide ions on the alkaline side, interact with charged R groups, disrupting the ionic and hydrogen bonds that hold the tertiary structure. The bonds break, the active site changes shape, the substrate cannot fit, and the enzyme is denatured.

Notice the difference from temperature. A small change in pH either side of the optimum slows the reaction, and a large change denatures the enzyme. Different enzymes have different optimum pH values. Pepsin works in the acid of the stomach, while many enzymes in cells work near neutral.

3:55Examiner trap and recap

The trap. Never say an enzyme is killed, or dies, or is used up. Say it is denatured, and always say the active site changes shape, so the substrate cannot fit. And a cold enzyme is not denatured. It just has less kinetic energy, so fewer successful collisions and fewer enzyme-substrate complexes per second. Recap. Heat speeds collisions, then breaks bonds. Wrong pH disrupts the same bonds too.

Next, what happens when we change how much enzyme or substrate there is, and why the rate levels off.