New insights into the membrane-binding and activation mechanism of pyruvate oxidase from Escherichia coli

Pyruvate oxidase from Escherichia coli (EcPOX) is a thiamin diphosphate- (ThDP) and FAD-dependent peripheral membrane protein that carries out the irreversible oxidative decarboxylation of pyruvate to acetate and carbon dioxide. Concomitant two-electron reduction of the flavin cofactor was suggested to induce a structural rearrangement of the C-terminus triggering recruitment of the protein from the cytosol to the cell membrane, where the electrons are eventually transferred to final electron acceptor ubiquinone 8. Binding to the membrane, or alternatively, mild proteolytic digestion leads to a multifold enhancement of both the catalytic activity and substrate affinity. Recent X-ray crystallographic studies on EcPOX in the resting state and on a C-terminal truncation variant mimicking the membrane-bound activated form have fueled our understanding of the membrane-binding mechanism and concomitant catalytic activation. In the resting state, the auto-inhibitory C-terminal membrane anchor adopts a half-barrel/helix fold that occludes the active site. Upon activation, the C-terminus is expelled and becomes structurally flexible thereby freeing the active site. Circular dichroism spectroscopic analysis revealed the isolated C-terminus to be disordered, however, formation of a helical structure was observed in the presence of micelles. Limited proteolysis experiments indicate that activation of EcPOX involves at least two sequential structural transitions: the first occurring after binding of pyruvate to ThDP and the second after two-electron reduction of the flavin.