The [NiFe] hydrogenases of Methanococcus voltae: genes, enzymes and regulation
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© Springer-Verlag 1997
MINI-REVIEW
Oliver Sorgenfrei · Steffen Müller · Matthias Pfeiffer · Izabela Sniezko · Albrecht Klein
The [NiFe] hydrogenases of Methanococcus voltae : genes, enzymes and regulation
Received: 17 October 1996 / Accepted: 30 December 1996
Abstract Methanococcus voltae carries genetic information for four [NiFe] hydrogenases. Two of the hydrogenases are predicted to contain selenocysteine on the basis of in-frame TGA codons, while the genes encoding the two other enzymes contain cysteine codons at homologous positions. Their predicted subunit compositions and their electron acceptor specificities are similar to those of the respective selenium-containing enzymes. The selenium-containing hydrogenases have been purified and characterized. Only one of them reduces the deazaflavin F420. The activity of the F420-nonreducing enzyme is exceptionally high. The selenium atom has been shown by EPR spectroscopy to be a ligand to the Ni atom in the primary reaction centers in both enzymes. The spectroscopic analyses also yielded a description of the electronic configuration around the NiFe center at different oxidation states and in the presence of the competitive inhibitor, CO. The genes encoding the selenium-free hydrogenases are expressed only in the absence of selenium. They are linked by an intergenic region in which regulatory cis elements were defined by employing reporter gene constructs and site-directed mutagenesis. Key words [NiFe] Hydrogenases · Methanococcus voltae · Archaea · Selenocysteine · Selenium-dependent gene regulation · Operator · Silencer · EPR Hydrogenases catalyze the heterolytic splitting of dihydrogen molecules into two electrons and two protons (Krasna and Rittenberg 1954). The reaction is principally reversible. Different enzymes preferentially catalyze hydrogen formation or cleavage. Bidirectional enzymes catalyze both reactions equally well. These specific enzyme
O. Sorgenfrei · S. Müller · M. Pfeiffer · I. Sniezko · A. Klein (Y) Molekulargenetik, Fachberich Biologie, Philipps-Universität, D-35032 Marburg, Germany Tel. +49-6421-283014; Fax +49-6421-286812 e-mail: [email protected]
properties are correlated with their biological functions. Extreme cases are the hydrogenases of fermenting organisms, which dispose of electrons by hydrogen formation, and the hydrogenases of hydrogen-oxidizing bacteria, which gain their energy from the reducing power of the electrons formed during hydrogen cleavage. The reader is referred to earlier reviews (Przybyla et al. 1992; Friedrich and Schwartz 1993; Vignais and Toussaint 1994; Wu and Mandrand 1993) for a survey of the different hydrogenases. Most hydrogenases found in prokaryotes are [NiFe] hydrogenases containing the two metals in a bimetallic reaction center involved in the cleavage reaction (Surerus et al. 1994; Volbeda et al. 1995). This review considers only enzymes of this type. The reaction mechanism of a metalfree hydrogenase that catalyzes the reversible reduction of N5N10-methenyltetrahydromethanopterin i
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