Second Generation Artificial Hydrogenases Based on the Biotin-Avidin Technology: Improving Activity, Stability and Selectivity by Introduction of Enantiopure Amino Acid Spacers

We report on our efforts to create efficient artificial metalloenzymes for the enantioselective hydrogenation of <i>N</i>-protected dehydroamino acids using either avidin or streptavidin as host proteins. Introduction of chiral amino acid spacers - phenylalanine or proline - between the biotin anchor and the flexible aminodiphosphine moiety <b>1</b>, combined with saturation mutagenesis at position S112X of streptavidin, affords second generation artificial hydrogenases displaying improved organic solvent tolerance, reaction rates (3-fold) and (<i>S</i>)-selectivities (up to 95 % ee for <i>N</i>-acetamidoalanine and <i>N</i>-acetamidophenylalanine). It is shown that these artificial metalloenzymes follow Michaelis-Menten kinetics with an increased affinity for the substrate and a higher <i>k</i>_cat than the protein-free catalyst (compare <i>k</i>_cat 3.06 min^-1 and <i>K</i>_M 7.38 mM for [Rh(COD)<b>Biot-1</b>]⁺ with <i>k</i>_cat 12.30 min^-1 and <i>K</i>_M 4.36 mM for [Rh(COD) <b>Biot-(<i>R</i>)-Pro-1</b>]⁺ WT Sav). Finally, we present a straightforward protocol using Biotin-Sepharose to immobilize artificial metalloenzymes (>92 % ee for <i>N</i>-acetamidoalanine and <i>N</i>-acetamidophenylalanine using [Rh(COD) <b>Biot-(<i>R</i>)-Pro-1</b>]⁺ Sav S112W).