Escherichia coli acid resistance: pH-sensing, activation by chloride and autoinhibition in GadB

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Abstract
Escherichia coli and other enterobacteria exploit the H+‐consuming reaction catalysed by glutamate decarboxylase to survive the stomach acidity before reaching the intestine. Here we show that chloride, extremely abundant in gastric secretions, is an allosteric activator producing a 10‐fold increase in the decarboxylase activity at pH 5.6. Cooperativity and sensitivity to chloride were lost when the N‐terminal 14 residues, involved in the formation of two triple‐helix bundles, were deleted by mutagenesis. X‐ray structures, obtained in the presence of the substrate analogue acetate, identified halide‐binding sites at the base of each N‐terminal helix, showed how halide binding is responsible for bundle stability and demonstrated that the interconversion between active and inactive forms of the enzyme is a stepwise process. We also discovered an entirely novel structure of the cofactor pyridoxal 5′‐phosphate (aldamine) to be responsible for the reversibly inactivated enzyme. Our results link the entry of chloride ions, via the H+/Cl− exchange activities of ClC‐ec1, to the trigger of the acid stress response in the cell when the intracellular proton concentration has not yet reached fatal values.