Enzymes can only work in certain conditions. Most enzymes in the human body work best at around 37°C – body temperature. At lower temperatures, they will still work but much more slowly.
Similarly, enzymes can only function in a certain pH range (acidic/alkaline). Their preference depends on where they are found in the body. For instance, enzymes in the intestines work best at 7.5 pH, whereas enzymes in the stomach work best at pH 2 because the stomach is much more acidic.
If the temperature is too high or if the environment is too acidic or alkaline, the enzyme changes shape; this alters the shape of the active site so that substrates cannot bind to it – the enzyme has become denatured.
Some enzymes cannot function unless they have a specific non-protein molecule attached to them. These are called cofactors. For instance, carbonic anhydrase, an enzyme that helps maintain the pH of the body, cannot function unless it is attached to a zinc ion.