X-Ray Absorption Spectroscopic Studies of Metal Coordination in Zinc and Copper Proteins

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Abstract
X-ray absorption spectroscopic studies have contributed to the elucidation of the metal coordination in zinc and copper proteins, and to the refinement of structures of such proteins already known from crystallographic studies. Zinc proteins can be classified according to their X-ray absorption spectroscopic characteristics, and it is possible to discriminate between ligand environments with two sulfur ligands, as in the transcription factor, and with one sulfur ligand, as in the sorbitol dihydrogenase, in a comparative study. For phospholipase C, it has been shown in a substitution study that cobalt has a higher coordination number than the native zinc in the same size. Comparisons of the copper—ligand interactions of various electron transfer or “blue-copper” proteins do not show a clear correlation with redox potential except perhaps for the relative rigidity of the Cu—S distances in that with the highest redox potential, rusticyanin. EXAFS (extended X-ray absorption fine structure) studies of type-2 copper enzymes are critically reexamined in view of the possibility that coordination by the recently discovered o-quinoline cofactors has been mistaken for imidazole coordination. Changes in the copper coordination of the dinuclear copper site in hemocyanin upon oxygen binding are interpreted in terms of the coordination of one additional imidazole ligand per copper.

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