Selenocysteine ligand

In addition to the presence of one of five MPT derivatives, cofactor variants differ in metal-ligand stoichiometry, coordination number, and coordination geometry. The cofactor variations include (1) the numbers of bound MPT derivatives, either one or two (2) the presence or absence of oxido and/or sulfido coordination (3) the presence or absence of bound protein ligands (either serine, cysteine, or selenocysteine) and (4) the presence or absence of coordinated water, hydroxido, or hydrosulfido ligands. These distinctions are discussed in detail in Section IV. 

Recent EXAFS [124] studies are in agreement with the crystallography but suggest that the selenocysteine and one of the sulfur atoms of the MPTpG ligand are in bonding contact, with a Se—S bond distance of 2.19 A. 

Arsenite oxidase was solved at higher resolution (1.64 A) offering a more reliable view of the active site (40). Two dithiolene chelates are symmetrically bound at normal distances (2.4 A) and a single oxo ligand is observed at 1.6 A. The absence of any other covalent link from the protein leaves the Mo as five coordinate (alanine replaces the aminoacid position normally occupied by the coordination of serine, cysteine or selenocysteine residues). This Mo environment was interpreted as indicating a reduced Mo site, possibly from photoreduction in the X-ray beam. 

The most appropriate description of the Ni oxidation state in all forms of the enzyme appears to be Ni(II), based on a valence bond sum analysis of the best EXAFS model (Thorp, H. H., University of North Carolina, unpublished data). This model is in agreement with studies of the magnetic properties of the EPR silent intermediate of the H2ase from D. baculatus that demonstrated that the Ni site was diamagnetic, albeit in an enzyme that has a selenocysteinate Ni ligand (29). 

An engineered azurin variant containing a selenocysteine copper ligand. /. Am. Chem. Soc. 124, 2084-2085. 

C/N in higher plants is 7-30 for the entire plant but 20-50 for photosynthetic organs, N/P uses to be 50-80 and C/S about 500 (stoichiometric ratios each). Both S and Se are partly bound into ligands (cysteine, methionine, selenocysteine, dimethyl selenide and many plant components such as those of Allium species including garlic), but likewise partly in weakly complexing anions like SO/ or SeO ", respectively (total S in leaves or needles is about 3 mg/g DM or 90 mmol/kg C/S 450 and N/S < 20). 

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