Multiple and reversible hydrogenases for hydrogen production by Escherichia coli: dependence on fermentation substrate, pH and the F₀F₁-ATPase

Abstract

Molecular hydrogen (H₂) 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 H₂-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 F₀F₁-ATPase by supplying reducing equivalents and depending on proton-motive force. Two other hydrogenases, 1 (Hyd-1) and 2 (Hyd-2), are H₂-oxidizing enzymes during glucose fermentation at neutral and low [pH]_out. They operate in a reverse, H₂-producing mode during glycerol fermentation at neutral [pH]_out. Hyd-1 and Hyd-2 activity depends on F₀F₁. Moreover, Hyd-3 can also work in a reverse mode. Therefore, the operation direction and activity of all Hyd enzymes might determine H₂ production; some metabolic cross-talk between Hyd enzymes is proposed. Manipulating of different Hyd enzymes activity is an effective way to enhance H₂ production by bacteria in biotechnology. Moreover, a novel approach would be the use of glycerol as feedstock in fermentation processes leading to H₂ production, reduced fuels and other chemicals with higher yields than those obtained by common sugars.

Keywords::

• Bacteria
• glucose and glycerol fermentation
• formate
• hydrogenases
• gene expression and regulation
• pH
• F₀F₁-ATPase
• bioenergy

Acknowledgements

Thanks to all people in our lab who contributed to this study. Cordial thanks to Prof. August Bock (Munich University, Munich, Germany), who has pushed on hydrogenase study in our lab. Thanks to Prof. Simon S. Andrews (The University of Reading, Reading, UK), Prof. Thomas K. Wood (Texas A & M University, College Station, TX, USA) and Prof. R. Gary Sawers (University of Halle, Halle-Wittenberg, Germany) for collaboration, mutants supplied and many valuable comments and advices.

Declaration of interest

The authors report no declarations of interest. The study in our lab was done within the framework of research grants financed by the Ministry of Education and Science of the Republic of Armenia (0167–2005; 1012–2008 and 11F-202–2011) and by grants from the US Civilian Research & Development Foundation (AB1-2307-YE2) and the US Armenian National Science and Education Fund (05-NS-microbio-724-10).

Editor: Michael M. Cox