Escherichia coli has four hydrogenases (Hyd), three genes of which are encoded by the hya, hyb, and hyc operons. The proton-reducing and hydrogen-oxidizing activities of Hyd-2 (hyb) were analyzed in whole cells grown to stationary phase and cell extracts, respectively, during glycerol fermentation using novel double mutants. H2 production rate at pH 7.5 was decreased by ~3.5- and ~7-fold in hya and hyc (HDK 103) or hyb and hyc (HDK 203) operon double mutants, respectively, compared with the wild type. At pH 6.5, H2 production decreased by ~2- and ~5-fold in HDK103 and HDK203, respectively, compared with the wild type. At pH 5.5, H2 production was reduced by ~4.5-fold in the mutants compared with the wild type. The total hydrogen-oxidizing activity was shown to depend on the pH of the growth medium in agreement with previous findings and was significantly reduced in the HDK103 or HDK203 mutants. At pH 7.5, Hyd-2 activity was 0.26 U (mg protein)(-1) and Hyd-1 activity was 0.1 U (mg protein)(-1). As the pH of the growth medium decreased to 6.5, Hyd-2 activity was 0.16 U (mg protein)(-1), and Hyd-1 was absent. Surprisingly, at pH 5.5, there was an increase in Hyd-2 activity (0.33 U mg protein)(-1) but not in that of Hyd-1. These findings show a major contribution of Hyd-2 to H2 production during glycerol fermentation that resulted from altered metabolism which surprisingly influenced proton reduction.
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