Over the last 15 years, a plethora of research has provided major insights into the structure and function of hydrogenase enzymes. This has led to the important development of chemical models that mimic the inorganic enzymatic co-factors, which in turn has further contributed to the understanding of the specific molecular features of these natural systems that facilitate such large and robust enzyme activities. More recently, efforts have been made to generate guest-host models and artificial hydrogenases, through the incorporation of transition metal-catalysts (guests) into various hosts. This adds a new layer of complexity to hydrogenase-like catalytic systems that allows for better tuning of their activity through manipulation of both the first (the guest) and the second (the host) coordination sphere. Herein we review the aforementioned advances achieved during the last 15 years, in the field of inorganic biomimetic hydrogenase chemistry. After a brief presentation of the enzymes themselves, as well as the early bioinspired catalysts, we review the more recent systems constructed as models for the hydrogenase enzymes, with a specific focus on the various strategies employed for incorporating of synthetic models into supramolecular frameworks and polypeptidic/protein scaffolds, and critically discuss the advantages of such an elaborate approach, with regard to the catalytic performances. evolving systems with enhanced activity, it has also recently provided a novel and exciting route for the direct and facile activation of native [FeFe] hydrogenases [10,11]. HydrogenasesCharacterization of certain living organisms, such as archaea, bacteria, cyanobacteria and algae, has led to the exciting discovery that hydrogen can be either produced or utilised as a source of low-potential electrons within living cells participating in a global H 2 cycle [12].Bacteria such as Ralstonia eutropha (a facultative chemolithoautotrophic organism) provide a good example of this as they are able to use hydrogen as their sole source of energy [13].Another example comes from micro-algaea such as Chlamydomonas reinhardtii, which under certain conditions is able to use sunlight to transiently drive the reverse reaction, i.e. extracting electrons from water and using them to reduce protons into hydrogen [14]. Finally, methanogens such as Methanobacterium thermoautotrophicum are able to exploit the reducing power of H 2 to produce CH 4 from CO 2 [15]. This chemical activity is made possible through the expression of fascinating metalloenzymes called hydrogenases [13,16,17]. There are two classes of hydrogenmetabolizing enzymes, the [NiFe]-and [FeFe]-hydrogenases, which catalyse these reactions without any overpotential [18] and at very high rates (one molecule of hydrogenase produces between 1500 to 20000 molecules of H 2 per second at pH 7 and 37 °C in water) [3,19,20]. A third class, [Fe]-hydrogenase or Hmd (Hydrogen-forming methylene-tetrahydromethanopterin dehydrogenase), is only found in archaea methanogens and requires the use of a...

Cobaloximes are popular H 2 evolution molecular catalysts, but have so far mainly been studied in non-aqueous conditions. We show here that they are also valuable for the design of artificial hydrogenases for application in neutral aqueous solutions and report on the preparation of two well-defined biohybrid species via the binding of two cobaloxime moieties {Co(dmgH) 2 } and {Co(dmgBF 2 ) 2 }(dmgH 2 = dimethylglyoxime) to apo Sperm-whale myoglobin (SwMb). All spectroscopic data confirm that the cobaloxime moieties are inserted within the binding pocket of the SwMb protein and are coordinated to a histidine residue in axial position of the cobalt complex, resulting in thermodynamically stable complexes. QC/MM docking calculations indicated coordination preference for His93 over the other histidine residue (His64) present in the vicinity. Interestingly, the redox activity of the cobalt centers is retained in both biohybrids which provides them with catalytic activity for H 2 evolution in near neutral aqueous conditions.

We describe here a systematic, reliable, and fast screening method that allows the comparison of H2-forming catalysts that work under aqueous conditions with two readily prepared chemical reductants and two commonly used photosensitizers. This method uses a Clark-type microsensor for H2 detection and complements previous methods based on rotating disk electrode measurements. The efficiencies of a series of H2 -producing catalysts based on Co, Ni, Fe, and Pt were investigated in aqueous solutions under thermal conditions with europium(II) reductants and under photochemical conditions in the presence of two different photosensitizers {[Ru(bipy)3]Cl2(bipy=2,2-bipyridine) and eosin-Y} and sacrificial electron donors (ascorbate and triethanolamine, respectively). The majority of catalysts tested were active only under specific conditions. However, our results also demonstrate the impressive versatility of a group of Co catalysts, which were able to produce H2 under different reducing conditions and at various pH values. In particular, a cobaloxime, [Co(dmgH)2(H2O)2] (dmgH2 =dimethylglyoxime), and a cobalt tetraazamacrocyclic complex, {Co(CR)Cl2}(+) [CR=2,12-dimethyl-3,7,11,17-tetraazabicylo(11.3.1)heptadeca-1(17),2,11,13,15-pentaene], displayed excellent catalytic rates under the studied conditions, and the best rates were observed under thermal conditions.

International audienceThe insertion of cobaloxime catalysts in the heme-binding pocket of heme oxygenase (HO) yields artificial hydrogenases active for H-2 evolution in neutral aqueous solutions. These novel biohybrids have been purified and characterized by using UV/visible and EPR spectroscopy. These analyses revealed the presence of two distinct binding conformations, thereby providing the cobaloxime with hydrophobic and hydrophilic environments, respectively. Quantum chemical/molecular mechanical docking calculations found open and closed conformations of the binding pocket owing to mobile amino acid residues. HO-based biohybrids incorporating a {Co(dmgH)(2)} (dmgH(2)=dimethylglyoxime) catalytic center displayed up to threefold increased turnover numbers with respect to the cobaloxime alone or to analogous sperm whale myoglobin adducts. This study thus provides a strong basis for further improvement of such biohybrids, using well-designed modifications of the second and outer coordination spheres, through site-directed mutagenesis of the host protein

Synthetic biology (or chemical biology) is a growing field to which the chemical synthesis of proteins, particularly enzymes, makes a fundamental contribution. However, the chemical synthesis of catalytically active proteins (enzymes) remains poorly documented because it is difficult to obtain enough material for biochemical experiments. We chose calstabin, a 107-amino-acid proline isomerase, as a model. We synthesized the enzyme using the native chemical ligation approach and obtained several tens of milligrams. The polypeptide was refolded properly, and we characterized its biophysical properties, measured its catalytic activity, and then crystallized it in order to obtain its tridimensional structure after X-ray diffraction. The refolded enzyme was compared to the recombinant, wild-type enzyme. In addition, as a first step of validating the whole process, we incorporated exotic amino acids into the N-terminus. Surprisingly, none of the changes altered the catalytic activities of the corresponding mutants. Using this body of techniques, avenues are now open to further obtain enzymes modified with exotic amino acids in a way that is only barely accessible by molecular biology, obtaining detailed information on the structurefunction relationship of enzymes reachable by complete chemical synthesis.

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