When glucose undergoes base-catalyzed isomerization in the absence of the enzyme, mannose is one
of the products that is formed (Section 20.5). Why is mannose not formed in the enzyme-catalyzed reaction?

Base-catalyzed isomerization involves the rearrangement of molecular structures through the action of a base, which facilitates the conversion of one isomer to another. In the case of glucose, the base can deprotonate hydroxyl groups, allowing for the formation of different structural isomers, such as mannose. This reaction typically occurs under non-enzymatic conditions, leading to a mixture of products.

Enzymes are biological catalysts that accelerate chemical reactions with high specificity for their substrates. In the case of glucose isomerization, enzymes like glucose isomerase preferentially convert glucose to fructose rather than mannose. This specificity arises from the enzyme's active site, which is shaped to fit certain substrates, thus preventing the formation of alternative products like mannose.

The reaction mechanism describes the step-by-step process by which reactants are converted into products. In enzyme-catalyzed reactions, the mechanism often involves multiple intermediates and specific transition states that are stabilized by the enzyme. Understanding the mechanism of glucose isomerization helps explain why mannose is not produced when the reaction is catalyzed by an enzyme, as the pathway diverges from that leading to mannose.