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