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 many of them at least, ionisable. As they are weak acids and bases in chemical nature, this ionisatio is infuluenced by pH of the solution. For many enzymes, activity is influenced by surrounding pH. This is depicted in the curve below, explain briefly.

Explanation

Particular enzyme acts on a particular value pH when specific side chains are ionised. Activity of enzyme gets reduced both below and above optimum value.