title: Escherichia coli acid resistance: pH-sensing, activation by chloride and autoinhibition in GadB.
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.
authors: Biochemisches Institut der Universität Zürich, Zürich, Switzerland.
authors: Heinz
authors: H
authors: Gut
authors: Eugenia
authors: E
authors: Pennacchietti
authors: Robert A
authors: RA
authors: John
authors: Francesco
authors: F
authors: Bossa
authors: Guido
authors: G
authors: Capitani
authors: Daniela
authors: D
authors: De Biase
authors: Markus G
authors: MG
authors: Grütter