Characterisation of the Citrate Synthase Reaction with Propionyl-CoA.

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Abstract
Experiments with propionyl-CoA stereoselectively deuterated in the propionyl moiety demonstrate that the formation of (2S,3S)-methylcitric acid (1) catalyzed by citrate (si)-synthase occurs with inversion of configuration in the propionyl moiety; the absolute configurations of 1 and (2S,3R)-methylcitronic acid indicate a si attack on oxaloacetate. Deuterium in the pro-S position is exchanged for protium 60 times faster than deuterium in the por-R position. Experiments with (R,S)-(2-2H1)propionyl-CoA allowed the determination of isotope effects. For the enzymatic formation of 1, a primary deuterium isotope effect kH/kD = 1.8 and a secondary .alpha.-deuterium isotope effect kH/KD = 0.99 were calculated; both are effects on Vmax/Km.