ph is a measure of hydrogen ions (H+) in a solution.
all enzymes contain a tertiary protien structure consisting of many important interactions, e.g.
hydrogen bonds, ionic interactions ect; The ph an enzyme works in may either help, or denature this structure,
According to the effect the ph has.
all enzymes work best at a certain ph called its optimum ph. if there is a change too far away from the optimum ph, i.e. it gets too high or too low,) then this could have a significant change on the rate of enzyme activity. most enzymes have an optimum ph of around near 7, (ph in cells most)
The pH affects the charge of the amino acids at the active site, (e.g. carboxyl acid R groups will be uncharged at a low pH (COOH), but charged at high pH (COO-), depending on the nature of that enzyme, and active site, it may need one or the other form of that group for the active site to function,
so active site properties change according to ph.
the shape of the enzyme can also change because of the ph
at too high PH (alkaline), the enzyme is denatured due to the loss of H+ to OH- ions
The same applies for too low a PH( acidic) level, where too many H+ ions would attach to the negative regions of the enzyme.
In both cases the enzyme's shape is changed which causes it to denature and the substrate can no longer bind.