Several factors affect how well an enzyme works. Temperature and pH are the two main factors you need to consider at GCSE.
Both temperature and pH affect the shape of the active site and this is what alters the activity of the enzyme. Both have similar effects for the same reasons.
Every enzyme has an optimum temperature and an optimum pH that they work best at.
At their optimum temperature and pH, the enzyme works fastest, making them break down their into product quickly.
If the pH and temperature go above their optimum, the rate of reaction decreases and eventually drops down to zero.
When it reacts zero, the enzyme is inactive. This change in rate of reaction is irreversible as the active site is permanently changed by the change in pH and the change in temperature.
Think of the enzyme like an egg. If you boil the egg, the yolk will change from liquid to solid. There is no way to get the runny egg yolk back. The change is permanent. The same thing can be said for enzymes at high temperatures or at a high pH.
If you lower the pH or the temperature, the rate of reaction drops as well. Lowering the pH changes the active site of the enzyme permanently denaturing the enzyme.
Lowering the temperature lowers the rate of reaction for a different reason. The of both the enzyme and the substrate are tied to the temperature.
The higher the temperature, the more kinetic energy the enzyme and substrate have. This means that they are more likely to collide with each other causing a reaction, this stays true until the temperature gets too high and the enzyme is denatured.
At lower temperatures, the enzymes and substrate move slowly, resulting in less collisions which means less reactions happen.
The active site isn’t changed at low temperatures. This means that you can reverse the effect of low temperature but not low pH or high temperatures or high pH.