Dependence of enzyme on pH value is due to the presence of charge particle of amino acid at its active site, variation causes the changes in charge of amino acid. The enzymatic activity decreases when pH is less than the optimum pH value.
Enzymes are proteins. Proteins are long chains of amino acids linked to each other by peptide bonds. Aminoacids have many functional groups in their structure. These functional groups are, many of them at least, ionisable. As they are weak acids and bases in chemical nature, this ionization is influenced by the pH of the solution. For many enzymes, activity is influenced by the surrounding pH. This is depicted in the curve below,explain briefly.