The biosynthesis of methylcitrate.

Citrate synthase catalyses the formation in vitro of two diastereoisomers of methylcitrate from propionyl-CoA and oxaloacetate. The proportion of diastereoisomers produced is temperature-sensitive. In the presence of oxaloacetate and a thiol trap, the enzyme catalyses the exchange of both alpha-protons of propionyl-CoA with 2H2O. The pro-S-proton at the alpha-carbon atom is exchanged 15 times faster with 2H2O than is the pro-R-proton. The exchanges are over 1000 and 100 times faster respectively than Vmax. These results are interpreted in the light of recent reports that 2R,3S- and 2S,3S-methylcitrates are produced by the enzyme-catalysed condensation and also excreted by patients with propionic acidaemia.