Nature provides key components for generating fuels from renewable resources in the form of enzymatic nanomachines which catalyze crucial steps in biological energy conversion, for example, the photosynthetic apparatus, which transforms solar power into chemical energy, and hydrogenases, capable of generating molecular hydrogen. As sunlight is usually used to synthesize carbohydrates, direct generation of hydrogen from light represents an exception in nature. On the molecular level, the crucial step for conversion of solar energy into H(2) lies in the efficient electronic coupling of photosystem I and hydrogenase. Here we show the stepwise assembly of a hybrid complex consisting of photosystem I and hydrogenase on a solid gold surface. This device gave rise to light-induced H(2) evolution. Hydrogen production is possible at far higher potential and thus lower energy compared to those of previously described (bio)nanoelectronic devices that did not employ the photosynthesis apparatus. The successful demonstration of efficient solar-to-hydrogen conversion may serve as a blueprint for the establishment of this system in a living organism with the paramount advantage of self-replication.

[NiFe]-hydrogenases catalyze the oxidation of H(2) to protons and electrons. This reversible reaction is based on a complex interplay of metal cofactors including the Ni-Fe active site and several [Fe-S] clusters. H(2) catalysis of most [NiFe]-hydrogenases is sensitive to dioxygen. However, some bacteria contain hydrogenases that activate H(2) even in the presence of O(2). There is now compelling evidence that O(2) affects hydrogenase on three levels: 1) H(2) catalysis, 2) hydrogenase maturation, and 3) H(2)-mediated signal transduction. Herein, we summarize the genetic, biochemical, electrochemical, and spectroscopic properties related to the O(2) tolerance of hydrogenases resident in the facultative chemolithoautotroph Ralstonia eutropha H16. A focus is given to the membrane-bound [NiFe]-hydogenase, which currently represents the best-characterized member of O(2)-tolerant hydrogenases.

The O 2 -tolerant, NAD + -reducing soluble [NiFe] hydrogenase (SH) from Ralstonia eutropha H16, HoxHYFUI 2 , is a complex enzyme, harboring multiple redox cofactors: a [NiFe] active site, an electron relay of iron-sulfur clusters, and two noncovalently bound flavin mononucleotides (FMN). The interplay and functional role of these cofactors is so far not understood in detail. In the present study, the isolated HoxHY module was investigated, which represents the smallest active subcomplex of a [NiFe] hydrogenase. Direct electrochemical studies and solution assays showed that the as-isolated HoxHY is initially catalytically inactive, but after reductive activation at low potentials, exhibits both H 2 oxidation and H + reduction, consistent with the role of the SH in bidirectional catalysis. The overpotential relative to E(2H + /H 2 ) is minimal, facilitating coupling of the closely spaced 2H + /H 2 and NAD + /NADH half reactions in the SH. Methyl viologen reduction assays revealed that H 2 oxidation by HoxHY is enhanced on addition of excess FMN, in line with results from optical spectroscopy which indicate that FMN is present at substoichiometric levels in as-isolated HoxHY. X-ray absorp-

Ralstonia eutropha H16 is an H(2)-oxidizing, facultative chemolithoautotroph. Using 2-DE in conjunction with peptide mass spectrometry we have cataloged the soluble proteins of this bacterium during growth on different substrates: (i) H(2) and CO(2), (ii) succinate and (iii) glycerol. The first and second conditions represent purely lithoautotrophic and purely organoheterotrophic nutrition, respectively. The third growth regime permits formation of the H(2)-oxidizing and CO(2)-fixing systems concomitant to utilization of an organic substrate, thus enabling mixotrophic growth. The latter type of nutrition is probably the relevant one with respect to the situation faced by the organism in its natural habitats, i.e. soil and mud. Aside from the hydrogenase and Calvin-cycle enzymes, the protein inventories of the H(2)-CO(2)- and succinate-grown cells did not reveal major qualitative differences. The protein complement of the glycerol-grown cells resembled that of the lithoautotrophic cells. Phosphoenolpyruvate (PEP) carboxykinase was present under all three growth conditions, whereas PEP carboxylase was not detectable, supporting earlier findings that PEP carboxykinase is alone responsible for the anaplerotic production of oxaloacetate from PEP. The elevated levels of oxidative stress proteins in the glycerol-grown cells point to a significant challenge by ROS under these conditions. The results reported here are in agreement with earlier physiological and enzymological studies indicating that R. eutropha H16 has a heterotrophic core metabolism onto which the functions of lithoautotrophy have been grafted.

The development of cellular systems in which the enzyme hydrogenase is efficiently coupled to the oxygenic photosynthesis apparatus represents an attractive avenue to produce H 2 sustainably from light and water. Here we describe the molecular design of the individual components required for the direct coupling of the O 2 -tolerant membrane-bound hydrogenase (MBH) from Ralstonia eutropha H16 to the acceptor site of photosystem I (PS I) from Synechocystis sp. PCC 6803. By genetic engineering, the peripheral subunit PsaE of PS I was fused to the MBH, and the resulting hybrid protein was purified from R. eutropha to apparent homogeneity via two independent affinity chromatographical steps. The catalytically active MBH-PsaE (MBH PsaE ) hybrid protein could be isolated only from the cytoplasmic fraction. This was surprising, since the MBH is a substrate of the twin-arginine translocation system and was expected to reside in the periplasm. We conclude that the attachment of the additional PsaE domain to the small, electron-transferring subunit of the MBH completely abolished the export competence of the protein. Activity measurements revealed that the H 2 production capacity of the purified MBH PsaE fusion protein was very similar to that of wild-type MBH. In order to analyze the specific interaction of MBH PsaE with PS I, His-tagged PS I lacking the PsaE subunit was purified via Ni-nitrilotriacetic acid affinity and subsequent hydrophobic interaction chromatography. Formation of PS Ihydrogenase supercomplexes was demonstrated by blue native gel electrophoresis. The results indicate a vital prerequisite for the quantitative analysis of the MBH PsaE -PS I complex formation and its light-driven H 2 production capacity by means of spectroelectrochemistry.