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Bacterial protein disulfide isomerase: efficient catalysis of oxidative protein folding at acidic pH.

Periplasmic protein disulfide isomerase (DsbA) is essential for disulfide formation in Escherichia coli. The tryptophan fluorescence of DsbA measures the redox state of the enzyme during catalysis of the oxidative folding of hirudin, a thrombin inhibitor containing three disulfide bonds and lacking tryptophan. With stoichiometric amounts of DsbA, reduced hirudin is rapidly oxidized in a process initially leading to random disulfides. Disulfide reshuffling involving reduced DsbA yields completely native inhibitor within 1 h, even at pH 4. Catalytic amounts of DsbA become essential for hirudin folding in the presence of redox buffers at acidic pH. The second-order rate constants of disulfide exchange reactions involving DsbA are several orders of magnitude above the known values for alkyl dithiols and disulfide oxidoreductases. DsbA preferably reacts with reduced, unfolded polypeptides. The reduction of DsbA by hirudin is faster by 1 order of magnitude than its reduction by the strong reductant dithiothreitol. Together, unusually fast disulfide interchange reactions and a preference for folding polypeptides appear to be responsible for the catalytic efficiency of DsbA and for disulfide formation in vivo at acidic pH.