Outer-sphere effects on the O 2 sensitivity, catalytic bias and catalytic reversibility of hydrogenases.

The comparison of homologous metalloenzymes, in which the same inorganic active site is surrounded by a variable protein matrix, has demonstrated that residues that are remote from the active site may have a great influence on catalytic properties. In this review, we summarise recent findings on the diverse molecular mechanisms by which the protein matrix may define the oxygen tolerance, catalytic directionality and catalytic reversibility of hydrogenases, enzymes that catalyse the oxidation and evolution of H 2 . These mechanisms involve residues in the second coordination sphere of the active site metal ion, more distant residues affecting protein flexibility through their side chains, residues lining the gas channel and even accessory subunits. Such long-distance effects, which contribute to making enzymes efficient, robust and different from one another, are a source of wonder for biochemists and a challenge for synthetic bioinorganic chemists.