Identification of distinct NAD-linked hydrogenase protein species in mutants and nickel-deficient wild-type cells of Alcaligenes eutrophus H16

NAD(H)-coupled hydrogen cycling - structure-function relationships of bidirectional [NiFe] hydrogenases

Hydrogenases catalyze the activation or production of molecular hydrogen. Due to their potential importance for future biotechnological applications, these enzymes have been in the focus of intense research for the past decades. Bidirectional [NiFe] hydrogenases are of particular interest as they couple the reversible cleavage of hydrogen to the redox conversion of NAD(H). In this account, we review the current state of knowledge about mechanistic aspects and structural determinants of these complex multi-cofactor enzymes. Special emphasis is laid on the oxygen-tolerant NAD(H)-linked bidirectional [NiFe] hydrogenase from Ralstonia eutropha.

Use of a reverse micelle system for study of oligomeric structure of NAD+-reducing hydrogenase from Ralstonia eutropha H16

Inclusion of an oligomeric enzyme, NAD+-dependent hydrogenase from the hydrogen-oxidizing bacterium Ralstonia eutropha, into a system of reverse micelles of different sizes resulted in its dissociation into catalytically active heterodimers and subunits, which were characterized in reactions with various substrates. It was found that: 1) the native tetrameric form of this enzyme catalyzes all types of studied reactions; 2) hydrogenase dimer, HoxHY, is a minimal structural unit catalyzing hydrogenase reaction with an artificial electron donor, reduced methyl viologen; 3) all structural fragments containing FMN and NAD+/NADH-binding sites exhibit catalytic activity in diaphorase reactions with one- and two-electron acceptors; 4) small subunits, HoxY and HoxU also exhibit activity in diaphorase reactions with artificial acceptors. These results can be considered as indirect evidence that the second FMN molecule may be associated with one of the small subunits (HoxY or HoxU) of the hydrogenase from R. eutropha.

Genes encoding the NAD-reducing hydrogenase of Rhodococcus opacus MR11

The dissociation of the soluble NAD-reducing hydrogenase of Rhodococcus opacus MR11 into two dimeric proteins with different catalytic activities and cofactor composition is unique among the NAD-reducing hydrogenases studied so far. The genes of the soluble hydrogenase were localized on a 7.4 kbp Asnl fragment of the linear plasmid pHG201 via heterologous hybridization. Analysis of the nucleotide sequence of this fragment revealed the seven open reading frames ORF1, hoxF, -U, -Y, -H, -W and ORF7. The six latter ORFs belong to the gene cluster of the soluble hydrogenase. Their gene products are highly homologous to those of the NAD-reducing enzyme of Alcaligenes eutrophus H16. The genes hoxF, -U, -Y and -H encode the subunits alpha, gamma, delta and beta, respectively. The gene hoxW encodes a putative protease, which may be essential for C-terminal processing of the beta subunit. Finally, ORF7 encodes a protein which has similarities to cAMP- and cGMP-binding protein kinases, but its function is not known. ORF1, which lies upstream of the hydrogenase gene cluster, encodes a putative transposase found in IS elements of other bacteria. Northern hybridizations and primer extensions using total RNA of autotrophically and heterotrophically grown cells of R. opacus MR11 indicated that the hydrogenase genes are under control of a delta 70-like promoter located at the right end of ORF1 and are even transcribed under heterotrophic conditions at a low level. Furthermore, this promoter was shown to be active in the recombinant Escherichia coli strain LHY1 harbouring the 7.4 kbp Asnl fragment, resulting in overexpression of the hydrogenase genes. Although all four subunits of the soluble hydrogenase were shown via Western immunoblots to be synthesized in E. coli, no active enzyme was detectable.

Comparative biochemical characterization of the iron-only nitrogenase and the molybdenum nitrogenase from Rhodobacter capsulatus

The component proteins of the iron-only nitrogenase were isolated from Rhodobacter capsulatus (delta nifHDK, delta modABCD strain) and purified in a one-day procedure that included only one column-chromatography step (DEAE-Sephacel). This procedure yielded component 1 (FeFe protein, Rc1Fe), which was more than 95% pure, and an approximately 80% pure component 2 (Fe protein, Rc2Fe). The highest specific activities, which were achieved at an Rc2Fe/Rc1Fe molar ratio of 40:1, were 260 (C2H4 from C2H2), 350 (NH3 formation), and 2400 (H2 evolution) nmol product formed x min(-1) x mg protein(-1). The purified FeFe protein contained 26 +/- 4 Fe atoms; it did not contain Mo, V, or any other heterometal atom. The most significant catalytic property of the iron-only nitrogenase is its high H2-producing activity, which is much less inhibited by competitive substrates than the activity of the conventional molybdenum nitrogenase. Under optimal conditions for N2 reduction, the activity ratios (mol N2 reduced/mol H2 produced) obtained were 1:1 (molybdenum nitrogenase) and 1:7.5 (iron nitrogenase). The Rc1Fe protein has only a very low affinity for C2H2. The Km value determined (12.5 kPa), was about ninefold higher than the Km for Rc1Mo (1.4 kPa). The proportion of ethane produced from acetylene (catalyzed by the iron nitrogenase), was strictly pH dependent. It corresponded to 5.5% of the amount of ethylene at pH 6.5 and was almost zero at pH values greater than 8.5. In complementation experiments, component 1 proteins coupled very poorly with the 'wrong' component 2. Rc1Fe, if complemented with Rc2Mo, showed only 10-15% of the maximally possible activity. Cross-reaction experiments with isolated polyclonal antibodies revealed that Rc1Fe and Rc1Mo are immunologically not related. The most active Rc1Fe samples appeared to be EPR-silent in the Na2S2O4-reduced state. However, on partial oxidation with K3[Fe(CN)6] or thionine several signals occurred. The most significant signal appears to be the one at g = 2.27 and 2.06 which deviates from all signals so far described for P clusters. It is a transient signal that appears and disappears reversibly in a redox potential region between -100 mV and +150 mV. Another novel EPR signal (g = 1.96, 1.92, 1.77) occurred on further reduction of Rc1Fe by using turnover conditions in the presence of a substrate (N2, C2H2, H+).