Escherichia coli multiple [Ni-Fe]-hydrogenases are sensitive to osmotic stress during glycerol fermentation but at different pHs

Escherichia coli evolves H2 via multiple [Ni-Fe]-hydrogenases (Hyd). This activity under hyper- and hypo-osmotic stress was investigated with mutants lacking different Hyd enzymes during glycerol fermentation. Inhibitory effects of hypo-stress on H2 production was stronger at pH 6.5 in wild type and mutants except fhlA, which encodes a transcriptional activator for Hyd-3, compared with the effects of N,N′-dicyclohexylcarbodiimide. These results indicate that Hyd-3 and Hyd-4 are osmosensitive at pH 7.5. Hyd-4 and FhlA are implicated in osmotic stress response at pH 6.5. Hyd-1 and FhlA might be osmosensitive at pH 5.5. Thus, osmosensitivity of Hyd enzymes is a novel property that depends on pH. This is significant for mechanisms of cell osmoregulation and H2 production biotechnology when glycerol is used as a fermentation substrate.