Borovik, A.S.; Hendrich, M.P.; Moënne-Loccoz, P.

J. Am. Chem. Soc. 2021, 143, 2384-2393, 10.1021/jacs.0c12564

Dinuclear iron centers with a bridging hydroxido or oxido ligand form active sites within a variety of metalloproteins. A key feature of these sites is the ability of the protein to control the structures around the Fe centers, which leads to entatic states that are essential for function. To simulate this controlled environment, artificial proteins have been engineered using biotin–streptavidin (Sav) technology in which Fe complexes from adjacent subunits can assemble to form [FeIII–(μ-OH)–FeIII] cores. The assembly process is promoted by the site-specific localization of the Fe complexes within a subunit through the designed mutation of a tyrosinate side chain to coordinate the Fe centers. An important outcome is that the Sav host can regulate the Fe···Fe separation, which is known to be important for function in natural metalloproteins. Spectroscopic and structural studies from X-ray diffraction methods revealed uncommonly long Fe···Fe separations that change by less than 0.3 Å upon the binding of additional bridging ligands. The structural constraints imposed by the protein host on the di-Fe cores are unique and create examples of active sites having entatic states within engineered artificial metalloproteins.

Mangiatordi, G.F.; Salmain, M.

J. Mol. Catal. B: Enzym. 2015, 122, 314-322, 10.1016/j.molcatb.2015.10.007

Two half-sandwich d6-rhodium(III) complexes of the general formula [(η5-Cp*)Rh(N^N)Cl]Cl where N^N is a phenanthroline or a bispyridine methane derivative carrying a thiol-targeting maleimide or chloroacetamide function were synthesized and characterized. Both complexes were able to catalyse the transfer hydrogenation of 2,2,2-trifluoroacetophenone in aqueous medium using formate or phosphite as hydrogen donor. Covalent anchoring of these complexes to the cysteine endoproteinase papain yielded hybrid metalloproteins with transfer hydrogenase properties. Under optimized conditions of pH, hydrogen donor concentration and catalyst load, conversion of substrate was nearly quantitative within 24 h at 40 °C and the (S)-enantiomer was obtained preferably albeit with a modest enantiomeric excess of 7–10%. Covalent docking simulations complemented the experimental findings suggesting a molecular rationale for the observed low enantioselectivity. The harmonious use of experimental and theoretical approaches represents an unprecedented starting point for driving the rational design of artificial metalloenzymes built up from papain with higher catalytic efficiency.