In the same way that every enzyme has an optimum temperature, so each enzyme also has an optimum pH at which it works best. For example, trypsin and pepsin are both enzymes in the digestive system which break protein chains in the food into smaller bits – either into smaller peptide chains or into individual amino acids. Pepsin works in the highly acidic conditions of the stomach. It has an optimum pH of about 1.5. On the other hand, trypsin works in the small intestine, parts of which have a pH of around 7.5. Trypsin’s optimum pH is about 8.
Table (PageIndex{1}): pH for Optimum Activity Enzyme Optimal pH Enzyme Optimal pH Lipase (pancreas) 8.0 Invertase 4.5 Lipase (stomach) 4.0 – 5.0 Maltase 6.1 – 6.8 Lipase (castor oil) 4.7 Amylase (pancreas) 6.7 – 7.0 Pepsin 1.5 – 1.6 Amylase (malt) 4.6 – 5.2 Trypsin 7.8 – 8.7 Catalase 7.0 Urease 7.0
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If you think about the structure of an enzyme molecule, and the sorts of bonds that it may form with its substrate, it isn’t surprising that pH should matter. Suppose an enzyme has an optimum pH around 7. Imagine that at a pH of around 7, a substrate attaches itself to the enzyme via two ionic bonds. In the diagram below, the groups allowing ionic bonding are caused by the transfer of a hydrogen ion from a -COOH group in the side chain of one amino acid residue to an -NH2 group in the side chain of another.
In this simplified example, that is equally true in both the substrate and the enzyme.
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Now think about what happens at a lower pH – in other words under acidic conditions. It won’t affect the -NH3+ group, but the -COO- will pick up a hydrogen ion. What you will have will be this:
You no longer have the ability to form ionic bonds between the substrate and the enzyme. If those bonds were necessary to attach the substrate and activate it in some way, then at this lower pH, the enzyme won’t work. What if you have a pH higher than 7 – in other words under alkaline conditions. This time, the -COO- group won’t be affected, but the -NH3+ group will lose a hydrogen ion. That leaves . . .
Again, there is no possibility of forming ionic bonds, and so the enzyme probably won’t work this time either. At extreme pH’s, something more drastic can happen. Remember that the tertiary structure of the protein is in part held together by ionic bonds just like those we’ve looked at between the enzyme and its substrate. At very high or very low pH’s, these bonds within the enzyme can be disrupted, and it can lose its shape. If it loses its shape, the active site will probably be lost completely. This is essentially the same as denaturing the protein by heating it too much.
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