Cytosine coordination sites with

The aldehyde oxidoreductase from Desulfovibrio gigas shows 52% sequence identity with xanthine oxidase (199, 212) and is, so far, the single representative of the xanthine oxidase family. The 3D structure of MOP was analyzed at 1.8 A resolution in several states oxidized, reduced, desulfo and sulfo forms, and alcohol-bound (200), which has allowed more precise definition of the metal coordination site and contributed to the understanding of its role in catalysis. The overall structure, composed of a single polypeptide of 907 amino acid residues, is organized into four domains two N-terminus smaller domains, which bind the two types of [2Fe-2S] centers and two much larger domains, which harbor the molybdopterin cofactor, deeply buried in the molecule (Fig. 10). The pterin cofactor is present as a cytosine dinucleotide (MCD) and is 15 A away from the molecular surface,... 

The coordination properties of pyrimidine bases seem to be less versatile than those of purine derivatives. Various Pt(II) and Pt(IV) compounds, including cis- and rrans-DDP, preferentially bind to the N3 site in N1-substituted cytosine derivatives (Figure 7), as verified by a variety of methods [7]. Simultaneous binding to N3 and to the exocyclic amino group C(4)-NH2 upon loss of a proton has been observed in a bridged Pt(II) system and in a chelated Pt(IV) system [7]. With 1,3-di-methyluracil, Pt(II) coordination to the C5 atom has been ascertained by X-ray crystallography [22]. 

In the stracture of the polymeric complex of Mn + and 5 -CMP, an unusual cytosine inner-sphere coordination complex has been demonstrated involving, rather than N-3, as well as four phosphate oxygen atoms of three different nucleotides and one water oxygen atom. Thus, in the case of CMP binding, it seems clear that can act as a significant metal-binding site alone, or together with N-3 in chelate structures. 

Examination of the crystals of yeast cytosine deaminase shows a second zinc ion coordinated to the active site by a bridging water/hydroxide ion to the catalytic zinc ion. This zinc ion has three other water molecules bound to it some of which have very short ( 1.8A) bonds to the second zinc. This may represent an inhibitory zinc ion that has bound to the protein with more than one hydroxide ion coordinated to the zinc. 

In the DNA double helix, the known sites for the cation coordination are mainly the N7 atoms of purines, while a simultaneous interaction with the 06 atom of guanine is also acknowledged. Some other sites, such as N3 of cytosine and N1 of adenine, are blocked by the hydrogen... 

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