Any enzyme has an optimum pH, at which it shows maximum activity.
Binding of the substrate to the active site of an enzyme involves interaction with reactive groups provided by the side-chains of amino acids at the binding site. The pH of the incubation medium may affect the ionisation of both the substrate and the amino acid side-chains and will therefore this will affect binding. It may also affect the ionisation of reactive groups that catalyse the reaction, although in the micro-environment of the catalytic site, when it is occupied by the substrate, this is less likely. Extreme values of pH may also disrupt the tertiary structure of the enzyme, and so distort the active site, or even denature the enzyme protein. Different proteins will have different sensitivity to extreme values of pH.
When it is mainly uncharged reactive groups that are involved in the interaction between an enzyme and its substrate, there will be a relatively broad range of pH over which the enzyme has activity. This graph shows the pH dependence of an enzyme that has maximum activity at pH 7.1 (i.e. an optimum pH of 7.1), and a relatively broad range of pH around that optimum at which it has activity. There would still be measurable activity at pH 6 or pH 8.
By contrast, when it is mainly charged groups that are involved in the interaction between an enzyme and its substrate, there will be a relatively narrow range of pH over which the enzyme has activity. This graph shows the pH dependence of an enzyme that also has a pH optimum of 7.1, but a narrower range of pH at which it is active. There is negligible activity at pH 6 or pH 8.
The pH optimum of an enzyme may be very different from the normal plasma or intracellular average pH of 7.35 - 7.45; some enzymes have pH optima as low as 2 - 3, or as high as 9 - 10, and therefore little or no activity around pH 7. In vivo, subcellular compartmentation means that enzymes that have pH optima very different from the intracellular average still act at or near their optimum pH.
This graph shows the pH dependence of two different enzymes, both found in blood plasma, which catalyse the same reaction: hydrolysis of phosphate esters. The enzyme shown in red has a pH optimum of about 3.8, and is known as acid phosphatase, while that shown in blue has a pH optimum around 9.5, and is known as alkaline phosphatase. Neither has any significant activity at the pH of plasma (7.35 - 7.45), and indeed neither has any physiological function in plasma. However, measurement of alkaline phosphatase in plasma can give valuable information about liver function and metabolic bone disease, while measurement of acid phosphatase in plasma can be useful in diagnosis of prostate disease.
If your results are to be meaningful, the activity of an enzyme must
be determined at or near its optimum pH. Obviously, in the example
of these phosphatases, you would work at around pH 3.8 to measure acid phosphatase,
and around pH 9.5 to measure alkaline phosphatase. If you worked at pH 7 then
neither enzyme would show any activity.
When you come to vary the incubation
pH in this simulation you will be offered, initially, a broad range of pH, from
1 - 14. Using this range will give you an approximate idea of the optimum for
your chosen enzyme, then you should repeat the experiments with a narrower range
of pH to give a more precise estimate pf the pH optimum. However, do not try
to be too precise. With a computer simulation, it is tempting to attempt to
determine an extremely precise value for the pH optimum, but in the laboratory
you would be unlikely to achieve precision of better than 0.1 pH unit when preparing
the incubation buffer, and an error of ± 0.1 pH unit will not have any
significant effect on the activity of the enzyme.