Figure 3.

Forms of the molybdenum cofactor present in enzymes of the xanthine oxidase family. Characteristic to enzymes of the xanthine oxidase family is a sulfido ligand at the equatorial position of the molybdenum atom. In eukaryotic enzymes of this family, the basic form of Moco is a pyranopterin, named Mo-MPT, which coordinates the molybdenum atom (colored in red) by the characteristic dithiolene group (colored in green) at the C1′ and C2′ positions of the pyranopterin ring (colored in blue). In bacteria, the MPT core can be modified by an additional CMP nucleotide (colored in orange) at the phosphate group (colored in black), forming MCD.