We have purified the citrate synthase from Azotobacter vinelandii and have determined that the size of the subunit is 48 000 Da and the structure of the holoenzyme is a hexamer. This contrasts with earlier estimates that indicate a 58 000 Da subunit and a tetrameric structure. In addition, the enzyme is allosteric with a Hill coefficient of 1.5 and is inhibited by NADH. The Hill coefficient is changed to about 1 by high ionic strength and AMP. The enzyme is thus similar to the citrate synthases of many other Gram-negative, facultative, anaerobic organisms. In addition, the amino acid sequence of about 100 residues has been determined and found to be highly similar to the sequence of Pseudomonas aeruginosa citrate synthase.
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