Exploring the functional versatility of the catalytic reaction of molybdoenzymes: a critical view of formate dehydrogenase

Molybdoenzymes are widespread in all domains of life and catalyze key steps in carbon, sulfur and nitrogen metabolism. In the DMSO reductase family of molybdenum enzymes present only in prokaryotes, the molybdenum coordination sphere generally is composed of two dithiolene groups from two molybdopterin (MPT) guanine dinucleotide (MGD) molecules, one amino acid ligand from the protein backbone and a Mo=O or =S group as sixth ligand. Overall Mo-containing enzymes were shown by Holm and coworkers in the 1980ies to catalyze classical oxygen-atom transfer (OAT) reactions. In OATs the oxygen atom from water is transferred to the substrate which is oxidized, or in the opposite direction, from the substrate to yield water; these reactions are coupled to the reversible transfer of two electrons and two protons in the course of the transformation cycles. The electrons are directly transferred to the Mo metal ion of the cofactor and the metals cycle between the MoIV and MoVI oxidation states with MoV as intermediate state. However, some enzymes have been proposed to present exceptions from this reaction to catalyse substrate reduction or oxidation without classical oxygen atom transfer. We critically want to discuss these exceptions and present a model for the reaction mechanism possibly catalysed by molybdoenzymes of the DMSO reductase family with the example of formate dehydrogenase.