Re -Citrate Synthase from Clostridium kluyveri Is Phylogenetically Related to Homocitrate Synthase and Isopropylmalate Synthase Rather Than to Si -Citrate Synthase

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Abstract
The synthesis of citrate from acetyl-coenzyme A and oxaloacetate is catalyzed in most organisms by a Si -citrate synthase, which is Si -face stereospecific with respect to C-2 of oxaloacetate. However, in Clostridium kluyveri and some other strictly anaerobic bacteria, the reaction is catalyzed by a Re -citrate synthase, whose primary structure has remained elusive. We report here that Re -citrate synthase from C. kluyveri is the product of a gene predicted to encode isopropylmalate synthase. C. kluyveri is also shown to contain a gene for Si -citrate synthase, which explains why cell extracts of the organism always exhibit some Si -citrate synthase activity.