Artificial Hydrogenases

Having synthesized a library of biotinylated diphosphine ligands, these were reacted with [Rh(COD)] and combined with a Sav isoform. The resulting artificial metalloenzymes were screened for the reduction of iV-acetamidodehydroalanine and A-acetamidodehydrophenylalanine to afford A-acetamidoalanine (A-AcAla) and 

Entry Protein Ligand ee% (Conv.%) N-AcAla ee% (Conv.%) N-AcPhe 

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Further studies showed that introduction of chiral amino acid spacers - proline I or phenylalanine HI - between the biotin anchor and the flexible aminodiphos-phine moiety 1, combined with satmation mutagenesis at position SI 12 of strepta-vidin, affords second generation artificial hydrogenases displaying improved organic solvent tolerance, reaction rates, and selectivities (>95% ee for both enantiomers) (Scheme 2) [36, 39],... 

The kinetic parameters for selected artificial hydrogenases were determined and the Michaelis-Menten parameters are listed in Table 3. The reactions were carried out at 2 bars hydrogen pressure and room temperature, with the more soluble substrate A -acetamidoacryhc acid. 

All catalytic systems studied display Michaelis-Menten behavior. Compared to the protein-free catalyst (Table 3, entry 1) all artificial hydrogenases display higher affinity for the substrate (i.e., smaller K, entries 2-4) and increased turnover frequencies (i.e., larger k ). It thus appears that incorporation of a biotinylated catalyst within streptavidin contributes to improve both its selectivity and its activity. We hypothesize that this latter feature may be caused by the accumulation of the hydrophobic substrate and in the vicinity of the active site, which bears hydrophobic amino acid residues. 

In summary, relying on a chemogenetic optimization procedure, we have produced artificial hydrogenases based on the biotin-avidin technology for the enantioselective reduction of N-protected dehydroaminoacids [up to 96% ee (R) and 95% ee (5)] [36, 39]. Next, we outhne our recent findings in artificial aUylic... 

In contrast to artificial hydrogenases, only selected biotinylated catalystyielded active catalysts. In this context, rigid spacers bearing... 

U. E. Rusbandi, C. Lo, M. Skander, A. Ivanova, M. Creus, N. Humbert, T. R. Ward, Second generation artificial hydrogenases based on the biotin-avidin technology improving activity, stability and selectivity by introduction of enantiopure amino acid spacers, Adv. Synth. Catal., 2007, 349, 1923-1930. 

Cobalt complexes have been also extensively studied as artificial hydrogenases for the reductive side of water splitting.It has been recently demonstrated that the cobalt(ii) complex of the pentadentate ligand l,4-di(picolyl)-7-(p-toluenesulfonyl)-l,4,7-triazacyclononane (iy2T stacn) displays excellent H2 photoproduction catalytic activity, in the presence of [Ir(ppy)2(bpy)]PF6 as photosensitiser, and EtgN as electron donor. Under the same conditions, the corresponding complex of nick-el(ii) presents low photochemical activity, while the iron(n) analogue is inactive. 

Caserta G, Roy S, Atta M, Artero V, Fontecave M (2015) Artificial hydrogenases biohybrid and supramolecular systems for catalytic hydrogen production or uptake. Curr Opin Chem Biol 25 36-47. doi 10.1016/j.cbpa.2014.12.018... 

In Chapter 2 we saw that hydrogenases of the three basic types are made by organisms that have existed over billions of years. In Chapter 6, the strnctnres of the proteins were laid ont in three dimensions. In Chapter 7 we saw that the metal centres of the protein could exist in particnlar chemical states. We can now begin to understand how the hydrogenases catalyse their reactions with snch extraordinary efficiency. Furthermore we ask, can similar catalysts be constrncted artificially ... 

Artificial cell-free systems have been investigated, to test models of photosynthetic production of H2. Benemann et al. (1973) demonstrated that it was possible to produce H2 and O2 by combining chloroplasts from green plants and bacterial hydrogenase, with ferredoxin as the intermediate electron carrier ... 

Letondor, C., Humbert, N. and Ward, TR. (2005) Artificial metaUoenzymes based on biotin-avidin technology for the enantioselective reduction of ketones by transfer hydrogenation. Proc. Natl. Acad. Sci. U.S.A., 102, 4683-4687 Letondor, C., Pordea, A., Humbert, N., Ivanova, A., Mazurek, S., Novic, M. and Ward, TR. (2006) Artificial transfer hydrogenases based on the biotin-(strept)avidin technology Fine tuning the selectivity by saturation mutagenesis of the host protein. J. Am. Chem. Soc., 128, 8320-8328. 

The three-pulse electron spin-echo envelope modulation (ESEEM) technique is particularly sensitive for detecting hyperfine couplings to nuclei with a weak nuclear moment, such as 14N. It has been used to probe the coordination state of nickel in two hydrogenases from M. tkermoautotrophicum, strain AH (56). One of these enzymes contains FAD and catalyzes the reduction of F420 (7,8-dimethyl-8-hydroxy-5-deazaflavin), while the other contains no FAD and has so far only been shown to reduce artificial redox agents such as methyl viologen. 

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