Q.Enzymes are proteins. Proteins are long chains of aminoacids 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 pH of the solution. For many enzymes, activity is influenced by surrounding pH. This is depicted in the curve below (a graph of Enzyme activity on the y-axis versus pH on the x-axis, showing a bell-shaped curve that rises to a peak at an intermediate pH and then falls), explain briefly.

• Amino acids carry ionizable functional groups (the amino and carboxyl groups, and sometimes ionizable R groups), which behave chemically as weak acids and weak bases.
• Because these groups are weak acids and bases, how much they are ionized, and in which direction, depends on the pH of the surrounding solution.
• Since an enzyme is a protein and its tertiary structure, including the shape of its active site, depends on the ionization states of the amino acids that build it, changing the pH changes the enzyme's shape and therefore its activity. …

Proteins, including enzymes, are chains of amino acids linked by peptide bonds, and every amino acid carries functional groups - chiefly the amino group and the carboxyl group, plus certain R groups - that behave chemically as weak acids and weak bases. Being weak acids and bases means their degree of ionization is not fixed; it changes with the pH of the surrounding solution.

Because an enzyme's catalytic power depends on its correctly folded tertiary structure, and especially on the precise shape of its active site, the ionization state of the amino acids that make up the enzyme matters enormously. At one particular pH, the enzyme's optimum pH, the ionizable groups are charged in just the pattern needed to hold the active site in its most effective shape, giving the enzyme its peak activity. …

Method: Link Molecular Ionization to Enzyme Shape

Questions built around a pH-vs-activity graph are really testing whether you can connect three separate ideas into one causal chain: chemical group ionization, protein folding, and catalytic function. Start from the smallest scale — amino acids carry functional groups (amino, carboxyl, and some R-groups) that behave as weak acids/bases, so how ionized they are depends on the pH around them. Do not stop there; the question is really about the enzyme's activity, not the amino acid's chemistry in isolation.

The next link in the chain is that an enzyme's active site only works because the protein is folded into a very specific three-dimensional shape, and that folding depends on interactions (ionic bonds, hydrogen bonds) between the very same ionizable groups you just described. So when pH changes, the ionization pattern of the enzyme's amino acids changes, which subtly reshapes the protein, which in turn changes how well the substrate fits and reacts at the active site. …