Aldolase as a chirality intersection of L-amino acids and D-sugars.

Aldolase plays an important role in glycolysis and gluconeogenesis to produce D-fructose-1,6-bisphosphate (D-FBP) from dihydroxyacetone phosphate (DHP) and D-glyceraldehyde-3-phosphate (D-GAP). This reaction is stereoselective and retains the D-GAP 2R configuration and yields D-FBP (with the configuration: 3S, 4S, 5R). The 3- and 4-position carbons are the newly formed chiral carbons because the 5-position carbon of D-FBP comes from the 2-position of D-GAP. Although four diastereomeric products, (3S, 4R, 5R), (3R, 4R, 5R), (3R, 4S, 5R), (3S, 4S, 5R), are expected in the nonenzymatic reaction, only the (3S, 4S, 5R) diastereomer (D-FBP) is obtained. Therefore, the chirality in the 3- and 4-positions is induced by the chirality of the enzyme composed of L-amino acid residues. D-Glucose-6-phosphate (D-G6P), which is generated from D-FBP in the gluconeogenesis pathway, produces D-ribose-5-phosphate (D-R5P) in the pentose phosphate pathway. D-R5P is converted to PRPP (5-phosphoribosyl-α-pyrophosphate), which is used for the de novo synthesis of nucleotides. Ribonucleic acid (RNA) uses the nucleotides as building blocks. The configurations of the 4R-carbon and of the 3S-carbon are retained. The stereochemical structure of RNA is based on 3S as well as 4R (D). The consideration above suggests that aldolase is a key enzyme that determines the 3S configuration in D-R5P. It is thus a chirality intersection between amino acids and sugars, because the sugar chirality is determined by the chiral environment of an L-amino acid protein, aldolase, to produce D-FBP.