“…Molybdenum-containing enzymes are responsible for the biological handling of dinitrogen (Nitrogenase, Equation ( 1)), nitrate (Nitrate reductase (NaR), Equation ( 2)), sulfite (Sulfite oxidase (SO), Equation ( 3)), dimethylsulfoxide (dimethylsulfoxide reductase (DMSOR), Equation ( 4)), aldehydes (Aldehyde oxidase (AO), Equation ( 5)), xanthine (Xanthine oxidase (XO), Equation ( 6)) and carbon dioxide (Carbon dioxide dehydrogenase, Equation (7), and Formate dehydrogenase, Equation ( 8 With the single (as far as is presently known) exception of nitrogenase [9][10][11][12], molybdenum is found coordinated by the cis-dithiolene group (-S-C=C-S-) of one or two molecules of a pyranopterin cofactor (Figure 1). In a parallel situation to the haem ring, this unique cofactor is not an "innocent scaffold" and it is considered to be co-responsible to modulate the active site reactivity, besides acting as a "wire" to conduct the electrons to, or from, the other redox-active centres of the enzyme (intramolecular electron transfer, when this is the case) [13][14][15][16][17][18][19][20]. In addition to the pyranopterin cofactor, the molybdenum ion is coordinated by oxygen and/or sulfur and/or selenium terminal atoms and/or by enzyme-derived amino acid residues (Figure 1), which are also expected to have key roles in catalysis (although their individual roles are not yet understood in many molybdoenzymes).…”