Lactase is an enzyme composed of a single polypeptide that hydrolyzes the disaccharide lactose to produce monosaccharides. The optimal pH for lactase activity is 6. Transfer of lactase to pH 5 results in a substantial decrease in enzyme activity, likely due to the disruption of __________ ?
(a) Only the primary structure of the enzyme. the primary and secondary structure of the enzyme.
(b) The secondary and tertiary structure of the enzyme
(c) The secondary, tertiary, and quaternary structure of the enzyme.
(d) None of the above

The amino acid sequence of a polypeptide chain makes the primary structure of the protein while the arrangement of particular amino acid residues of a chain makes its secondary structure. Folding of protein in three-dimensional space makes its tertiary structure. Enzymes are proteins and can work in properly folded form only. Any factor that disrupts the protein folding or affects its secondary and tertiary structure makes it non-functional. The process is called denaturation.

Enzymes can work under optimum cellular conditions such as temperature, pH, etc. Any deviations from these conditions lead to loss of function of enzymes due to denaturation. When the lactase enzyme is exposed to the pH conditions below its optimum pH range, the net charge of its polypeptide chain is altered leading to electrostatic repulsion and may also break some hydrogen bonds.

Secondary and tertiary structures of enzymes are stabilized by these weak interactions. Lactase at pH 5 will be non-functional due to the loss of its secondary and tertiary structures.