3-Deuterio-l-malic acid obtained from the fumarase catalyzed hydration of fumaric acid in D2O is shown to have the erythro configuration by n.m.r. comparison with stereospecifically synthesized threo-3-deuterio-dl-malic acid. The fumarase and aspartase systems thus operate by a trans mechanism as do the β-methylaspartase and cis-aconitase systems. Brewster's rules are applied to the problem of the stereochemistry of citric acid synthesized in the Krebs cycle and a configuration related to d-malic acid is arrived at. This configuration plus the 2 configuration of d-isocitric acid permits depiction of the stereochemical pathway of the cis-aconitase system and of the reactions of the Krebs cycle. The possibility that cis-aconitase exhibits a preferred direction for addition of OH, with concomitant trans addition of H, is discussed.
ASJC Scopus subject areas
- Colloid and Surface Chemistry