{Moco}n, (n=0-8): A general formalism for describing the highly covalent molybdenum cofactor of sulfite oxidase and related Mo enzymes

Over 50 molybdenum enzymes in three distinct families (sulfite oxidase, xanthine oxidase, DMSO reductase) are known, and representative X-ray crystal structures are available for all families. Structural analogues that replicate the coordination about the Mo atom in the absence of surrounding protein have been synthesized and characterized. The properties of metal complexes of non-innocent dithiolene ligands and their oxidized counter parts, dithiones, are summarized. Pulsed electron paramagnetic resonance (EPR) spectroscopy of the 33S-labeled molybdenum domain of catalytically active bioengineered sulfite oxidase has clearly demonstrated delocalization of electron density from MoV to the dithiolene component of the molybdenum cofactor (Moco) of the enzyme. Moco is highly covalent and has three redox active components: the Mo atom; the dithiolene; and the pterin. In principle, Moco can have a total of eight redox states, making it one of the most redox rich cofactors in biology. The {Moco}n formalism, developed here, gives the total number of electrons (n) associated with a particular redox state of Moco. This flexible notation eliminates the need to assign a specific oxidation state to each of the three components of Moco and allows for internal redistribution of electrons among the components upon substrate binding, changes in the surrounding network of hydrogen bonds, conformational changes, and catalysis. An unexpected result is that sulfite oxidase (an oxotransferase) is predicted to utilize the {Moco}4-6 electron distributions during catalysis, whereas xanthine oxidase (a hydroxylase) is predicted to utilize the {Moco}6-8 electron distributions during catalysis.