Stereochemistry of the reaction catalyzed by galactonate dehydratase

Galactonate dehydratase catalyzes the β-eliminaüon reaction of water from D-galactonate to yield 2-keto-3-deoxy-galactonate. The enzymatic product exists as a pyranosyl hemiketal as assessed by 'H and "C NMR. Solvent isotope studies of the reaction were performed, and NMR analysis indicated that a deuterium atom was stereospecifically incorporated at the pro-S position of carbon-3 during the tautomerization reaction. This indicates that the tautomerization reaction is enzymecatalyzed, and the deuterium atom replaces the eliminated 3-OH with retention of configuration. The tautomerization reaction is also not a significant rate limiting step as indicated hy a kinetic solvent isotope effect of 2.0.