pH
Most enzymes work inside cells that make them. Since the pH inside cells is normally between 7.2 - 7.4, these are the best conditions for most enzymes. However enzymes in the gut (eg protease) work in acidic conditions so loose their shape and do not function in neutral or basic conditions.
For example, in the diagram below, the enzyme pepsin works in a pH range of around 1-3 If it is put in a environment of pH 5 or above it will be denatured. The enzyme amylase works in a pH range of around 5-9. If it is put in an acidic environment of pH 3 or below it will be denatured.
