Glucose ring formation

Carbohydrates are aldehydes or ketones with multiple hydroxyl groups. Many of the organic chemistry reaction mechanisms of aldehydes and ketones were selected by AAMC for the exam were almost certainly selected because of their relevance to the carbohydrate chemistry.

For example, the reaction through which a straight chain sugar assumes the ring form is a hemi-acetal formation. Opening and closing the ring through the hemiacetal formation path is the mechanism of mutarotation, the change in the optical rotation through equilibrium between two anomers. Additionally, you see the second half of the acetal formation mechanism completed when two ring sugar forms undergo glycosidic bond formation.

Imine formation between an enzyme lysine side-chain and the carbonyl of a sugar is the initiating step in the aldolase mechanism (in the gluconeogenesis direction). In aldolase, the carbonyl under nucleophilic attack belongs to dihydroxyacetone phosphate. In transaldolase, from the pentose phosphate pathway, a similar attack occurs by enzyme lysine upon the carbonyl of sedoheptulose-7-phosphate.

Keto-enol tautomerism is also important in carbohydrate chemistry with even prominent examples too numerous to mention. Passage through a common enol tautomer is the mechanism of isomerization in phosphohexose isomerase, triose phosphate isomerase, and ribulose-5-phosphate isomerase, and it's the mechanism of epimerization in ribulose-5-phosphate epimerase. Aldol addition like mechanisms figure into aldolase, transaldolase, and citrate synthase, among others.