Multiple and reversible hydrogenases for hydrogen production by Escherichia coli: dependence on fermentation substrate, pH and the F(0)F(1)-ATPase

Molecular hydrogen (H(2)) can be produced via hydrogenases during mixed-acid fermentation by bacteria. Escherichia coli possesses multiple (four) hydrogenases. Hydrogenase 3 (Hyd-3) and probably 4 (Hyd-4) with formate dehydrogenase H (Fdh-H) form two different H(2)-evolving formate hydrogen lyase (FHL) pathways during glucose fermentation. For both FHL forms, the hycB gene coding small subunit of Hyd-3 is required. Formation and activity of FHL also depends on the external pH ([pH](out)) and the presence of formate. FHL is related with the F(0)F(1)-ATPase by supplying reducing equivalents and depending on proton-motive force. Two other hydrogenases, 1 (Hyd-1) and 2 (Hyd-2), are H(2)-oxidizing enzymes during glucose fermentation at neutral and low [pH](out). They operate in a reverse, H(2)-producing mode during glycerol fermentation at neutral [pH](out). Hyd-1 and Hyd-2 activity depends on F(0)F(1). Moreover, Hyd-3 can also work in a reverse mode. Therefore, the operation direction and activity of all Hyd enzymes might determine H(2) production; some metabolic cross-talk between Hyd enzymes is proposed. Manipulating of different Hyd enzymes activity is an effective way to enhance H(2) production by bacteria in biotechnology. Moreover, a novel approach would be the use of glycerol as feedstock in fermentation processes leading to H(2) production, reduced fuels and other chemicals with higher yields than those obtained by common sugars.