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The functional architecture of a cationic pore that modulates citrate synthase quaternary structure and allostery

The active form of the allosteric Type II citrate synthase of E. coli is a hexamer, having as a large central feature an unusual cationic pore lined with 18 arginines. We have investigated this remarkable concentration of positive charge using a variety of techniques. The importance of the combined repulsive effect of these arginines is evident from the extreme conditions necessary to reestablish a dimer‐hexamer equilibrium with their substitution. Structural studies have determined three repulsive arginine based forces act to destabilize the hexamer state, including: interactions between two arginine rings that encircle the cationic pore; a novel arginine cassette structure formed between dimer units; and, repulsions between arginine side chains at the very center of the pore. Notably, kinetic studies suggest these repulsive pore interactions give the hexameric enzyme the energetic flexibility needed to carry out catalysis. Our results emphasize the finely tuned relationship between the dual dimer ‐ hexamer and R (active) ‐ T (inactive) allosteric equilibria present. Indeed it seems the allosteric NADH binding site is a mechanism designed to fine tune the cationic pore ‐ active site energetic relationship and thereby allow for an equilibrium wherein a T‐state enzyme can be induced without irretrievable inactivation of the enzyme. Supported by the Natural Sciences and Engineering Research Council of Canada.