4. Effect of changing pH - What is the optimum pH of an enzyme catalysed reaction?

Enzymes perform best in their 'optimum' ambient conditions

When investigating the effect of pH on enzyme activity, three factors must be kept constant, (i) the temperature and the concentrations of both the (ii) substrate and (iii) enzyme must all be kept constant (see experimental methods).

General description of graphs: For any enzyme, initially, as you increase the pH, the rate of reaction increases.

Then as the pH increases, the reaction rate reaches a maximum at the optimum pH and then decreases with further increase in pH.

The optimum pH varies quite a bit from one enzyme to another.

pH effect: The structure of the protein enzyme can depends on how acid or alkaline the reaction medium is, that is, it is pH dependent.

If it is too acid (very low pH) or too alkaline (very high pH), the structure of the protein is changed and it is 'denatured' affecting the shape of the 'active site' which becomes less effective. The acid or alkali may chemically react with the enzyme at or near the active site affecting the shape of the active site or the ability of the substrate molecule to 'dock in' to the active site - a denaturing effect due to interfering with the bonds holding the enzyme together in its unique 3D shape.

The two graphs above illustrate how the rate of reaction varies with pH for many enzymes.

The enzyme catalase breaks down harmful hydrogen peroxide into water and oxygen, with an optimum pH range around pH 7 (top left graph).

Another enzyme may have an optimum pH range of 3.5 to 4.5 (lower-right graph).

Note that an enzyme is active over a pH range of perhaps several pH units, but beyond this range it is relatively ineffective.

In the optimum pH range, the enzyme catalysis is at its most efficient. In the denaturing process the 'active site' (see 'key and lock' mechanism details above) may be damaged by highly acid (low pH) or highly alkaline (high pH) conditions, and changed in such a way that the enzyme cannot perform its catalytic function on the substrate molecules - they don't fit in the active site.

If the enzyme does not have the correct 'lock' structure in the protein (the 'active site'), it cannot function efficiently by accepting the 'key' substrate molecule to form the substrate-enzyme complex

 Most enzymes have an optimum pH of between 4 and 9, and quite frequently near the neutral point of pH 7. Our own body fluids e.g. in blood or cells have a pH of ~pH 7.2 to 7.4, so its no coincidence that many of our enzymes have an optimum operating pH ~7, but it does depend on where you are in your body!

However, the enzyme pepsin has a peak at pH 2 (graph on right) and can operate in the very acid (hydrochloric) conditions of the stomach to help breakdown proteins for complete digestion in the small intestine.

Examples of optimum pH values for enzyme activity

A few examples quoted after the general shape of an enzyme pH optimisation graph.

Increase in acidity or alkalinity creating a pH well away from the optimum, can affect the protein structure of the enzyme and so affecting the active site, and, the substrate molecule can no longer readily lock into place into the active site and cannot be transformed into the product molecules.

The first diagram is typical of many enzymes operating in near neutral solutions (~pH 7)

The other two diagrams shows the wide range of pH that different enzymes can operate in

e.g pepsin breaks down proteins in the very acid conditions of the stomach.

Blood has a pH of ~7.4 and carbonic anhydrase (optimum pH ~7) is found in red blood cells.

This enzyme enables the efficient conversion of carbon dioxide and water into the carbonic acid and the hydrogen carbonate ion ('bicarbonate ion') and operates in near neutral conditions.

Trypsin is a protease enzyme from the pancreas that breaks down proteins (peptides) in the alkaline conditions (~pH 8.5) of the smaller intestine, so its optimum rate of reaction is around that value.