Arsenic binding

The x-ray structure of 1,2,3-benzothiadiazole complexed with AsFj (9) shows that the arsenic binds at N3 <86CJC849>. When Fe2(CO)9 reacted with (10) one of the products was (11), for which x-ray diffraction revealed the unusual feature of the nitrogen and sulfur joined by an iron atom (Equation (2)) <890M296l>. The mesoionic structure (13) is formed by methylation of 1,2,3-thiadiazole (12). It can best be described as a resonance hybrid of structures (13a) and (13b) and this was corroborated by the x-ray data (Scheme 1) <91jhC477>. 

Buschmann, J., Kappeler, A., Lindauer, U. et al. (2006) Arsenite and arsenate binding to dissolved humic acids influence of pH, type of humic acid, and aluminum. Environmental Science and Technology, 40(19), 6015-20. 

The structure of the bacterial ArsC from Escherichia coli plasmid R773 has been solved at 1.65 A resolution, and revealed that arsenate reductase (ArsC) has only one cysteine residue (Cys-12) in the active site, surrounded by an arginine triad composed of Arg-60, Arg-94, and Arg-107 (Mukhopadhyay et al, 2002). However, the arsenate reductase from Staphylococcus aureus has three cysteine residues (Cys-10, Cys-82, and Cys-89). The biochemical and mutational studies established that the arsenate binds to the triad of arginine (Arg-60, Arg-94, and Arg-107) residues and forms a covalent bond with the cysteine (Cys-12) residue near the N-terminus at the active site of arsenate reductase and/or participates in catalysis (Rosen, 2002a Silver and Phung, 2005 Shi etal, 2003 Martin et al, 2001). 

Merrifield, M.E., Ngu, T., Stillman, M.J. (2004). Arsenic binding to Fucus vesiculosus metallothionein. Biochem. Biophys. Res. Commun. 324 127-32. 

D. E., Rosen, B.P. (2007). Convergent evolution of a new arsenic binding site in the ArsR/SmtB family of metal-loregulators. J. Biol. Chem. 282 34346-55. 

Trivalent arsenic is less abundant in nature, but considerably more toxic. Detoxification of arsenite results in the same methylated species as those arising from arsenate. However, because trivalent arsenic binds tightly to thiol-containing molecules in tissues, it is much less easily detoxified and excreted. 

Oladimeji, A. A. (1985). An arsenic-binding protein in rainbow trout. Ecotoxicol Environ. Saf., 9,1-5. 

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