Pterin chelating molybdenum

Figure 2.10 Ball-and-stick drawing of a pterin chelating molybdenum complex [Mo205(xanthopterinate)2p". Red, oxygen green, molybdenum blue, nitrogen grey, carbon.

Three human redox enzymes, and a variety of bacterial enzymes, contain molybdenum chelated by two sulfur atoms in a modified pterin molybdopterin (see Figure 10.1). In sulfite oxidase, the other two chelation sites of the molybdenum are occupied by oxygen in xanthine oxidase / dehydrogenase (Section 7.3.7) and aldehyde oxidase, one site is occupied by oxygen and one by sulfur. In some bacterial enzymes, molybdopterin occurs as a guanine dinucleotide rather than free. In others, tungsten rather than molybdopterin is the chelated metal there is no evidence that any mammalian enzymes contain tungsten. 

The molybdenum cofactor (Moco) is an extraordinary molecule in biology. As a small metal-containing compound, it has the unprecedented combination of a dithiolene chelate for metal binding and a pterin appended to a pyran ring. The resultant cofactor is electronically nimble due to the presence of three redox active moieties, ie. the molybdenum atom, the dithiolene and the pterin, which in concert can support a range of redox events. 

The earliest period of work on pterin models for Moco followed the discovery of the pterin unit within Moco, and occurred prior to the confirmation of the dithiolene chelate. These early studies explored the coordination chemistry between molybdenum and pterins or other structurally related molecules such as pteridines (Figure 2.1, top). The resulting themes of this body of work include the favorable coordination by molybdenum in several oxidation states to the 04, N5 chelate site in pterin (see Figure 2.1 for numbering), a variety of reactivities exhibited by Mo -tetrahydropterin systems and the highly delocalized electronic structures in molybdenum-pterin complexes that defy formal oxidation state assignments to Mo and pterin. 

It required a decade of study to develop a clear understanding of the principles and outcome of pterin reactions to molybdenum. The results summarized here tell the story of non-innocent ligand behavior of pterin and pteridine ligands. During this period the first crystal structure of molybdenum enzymes emerged from which the dithiolene chelation of molyb-dopterin was proved. While protein structures unequivocally confirmed the dithiolene coordination to molybdenum, the alternative coordination mode described in this section underscores the electronic flexibility of the pterin system partnered with molybdenum. 

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