Proteins are long chains of amino acids linked to each other by peptide bonds

Enzymes are proteins. Proteins are long chains of amino acids linked to each other by peptide bonds. Amino acids have many functional groups in their structure.
These functional groups are ionisable as they are weak acids and bases in chemical nature. This ionisation is influenced by pH of the solution. For many enzymes, activity is influenced by surrounding pH.
This is depicted in the curve given explain briefly.

Enzymes, generally function in a narrow range of pH. Each enzyme shows its highest activity at a particular optimum pH. Activity declines both below and above the optimum value. The graph shows maximum enzyme activity at optimum pH. The rate of enzyme activity decreases above and below that optimum pH.