64. The class of enzymes, which catalyze addition of groups to double bonds and non-hydrolytic
removal of chemical groups, is
(A) oxidoreductase
(B) transferase
(C) hydrolase
(D) lyase

The correct answer to the query is (D) lyase. These enzymes specifically handle addition of groups to double bonds and non-hydrolytic removal of chemical groups, distinguishing them from other classes.​

Correct Answer: Lyase

Lyases catalyze the addition of chemical groups to double bonds or the removal of groups from substrates without hydrolysis, often forming double bonds in the process. Examples include aldolase in glycolysis, which cleaves fructose-1,6-bisphosphate into glyceraldehyde-3-phosphate and dihydroxyacetone phosphate, and decarboxylases like histidine decarboxylase that release CO2 to form double bonds. This matches the question’s description precisely, as lyases (EC class 4) break C-C, C-O, or C-N bonds non-hydrolytically.​

Oxidoreductase Explained

Oxidoreductases (EC class 1), or dehydrogenases, facilitate oxidation-reduction reactions by transferring electrons, hydrogen, or oxygen atoms between molecules. Common examples include lactate dehydrogenase, which converts lactate to pyruvate while transferring hydride ions. They do not add groups to double bonds or remove them non-hydrolytically, focusing instead on redox changes.​

Transferase Functions

Transferases (EC class 2) catalyze the transfer of functional groups like phosphate, methyl, or amino groups from a donor to an acceptor molecule. Kinases (a transferase subclass) add phosphate groups using ATP, while aminotransferases shift amino groups in amino acid metabolism. This class involves group movement between molecules but not double bond modifications or non-hydrolytic cleavages.​

Hydrolase Role

Hydrolases (EC class 3) perform hydrolysis reactions, adding water to break bonds and produce smaller molecules, such as esterases cleaving esters or proteases digesting proteins. Their mechanism relies on water addition, directly contrasting the “non-hydrolytic” removal specified in the question.​