In this A-Level Biology Lesson "Enzymes: How pH affects Enzyme Activity" you'll learn how changes in pH through the digestive system affect enzyme action.

Enzymes have optimum pH levels at which they work best. For most enzymes this is about pH 7 - 8 (which is pH of most cells). Some enzymes though have their optimal pH in the extreme, like protease enzymes (in the stomach), which have an optimum of pH 1.

pH affects the 'charge' of the amino acids at the active site, e.g. Carboxyl acid R groups are uncharged a low pH (COOH), but are 'charged' at high pH (COO-). This behaviour affects the properties of the active site - altering it which results in the substrate no longer being able to bind.