ELSEVlER Process Uiorhewnsny 29 11994) 633-644 8 1994 Elswier Science Limited Printed in Great Britain. All rights reserved 0032-9592/94/$7.00 Review Biochemical and Molecular Genetic Basis of Hydrogenases D. Hahn & U. Kick Lehrstuhl fiir Allgemeine Botanik, Ruhr-UniversitPt Bochum. D-44780 Bochum, Germany (Received 4 June 1993; revised version received and accepted 2 October 1993) Hydrogenuses catalyse the reversible reduction ofprotons to molecular hydrogen. Applied research is focused on structure and catalytic function under the aspect of hydrogen formation. In this review we summarize the current knowledge about properties and physiological roles of hydrogenases in pro- and eukaryotes and compile molecular genetical data about structural features ofprokatyotic hydrogenases. Finally, prospects are given for the possible application of hydrogenases or ‘hydrogenase-like catalysts in energy production. INTRODUCTION For about two decades, hydrogen has been dis- cussed as a future alternative source of energy. Since water is the only end-product when hydro- gen is oxidized, hydrogen is a non-polluting fuel. Besides electrolytic techniques and solar batte- ries, biological systems could also be used to produce hydrogen. Numerous bacteria and several algae are able to produce H, using hydro- genases. These hydrogenases are enzymes that catalyse the reversible reduction of protons to molecular hydrogen, as represented by: 2H+ + 2e- *H2 (1) to right in the equation) in vitro when appropriate electron mediators are available; alternatively, hydrogen can be oxidized to protons (right to left).’ However, in vivo a distinction can be made between unidirectional uptake and reversible hydrogenases. Uptake hydrogenases only catalyse the oxidation of molecular hydrogen (right to left) in viva, whereas reversible hydrogenases are also able to reduce protons to molecular hydrogen (left to right). Furthermore, differences between the hydrogenases can be observed with regard to their sensitivity to oxygen. While reversible hydro- genases are inactivated when exposed to oxygen, uptake hydrogenases are not as sensitive to oxy- gen. All hydrogenases that have been isolated so far Uptake hydrogenases have been found in a have the ability to evolve molecular hydrogen (left variety of bacteria and lower plants, ranging from archaebacteria to mosses. In contrast, reversible Corresponding author: U. Kiick. Tel: 234 700 6212; Fax: hydrogenases are found only in anaerobic fer- 234 7094 I X4. mentative microorganisms, cyanobacteria and 633