THE NATURE OF THE COORDINATION SITES OF TRANSITION METALS IN PROTEINS

C. David Garner, David Collison and Elna Pidcock

Department of Chemistry, The University of Manchester, Manchester M13 9PL, UK

'Inorganic' elements are essential for the normal growth, development and health of all living systems. Although the majority of these elements are usually only present in trace amounts, this does not diminish their significance. A considerable number of these 'trace elements' have been shown to play key roles in biological processes, including many of the d-transition metals such as: vanadium, manganese, iron, cobalt, nickel, copper, zinc, molybdenum and tungsten.

Protein crystallographic studies have revealed vital new information concerning the nature of the d-transition metal centre (or centres) which are often the site of catalytic activity. Furthermore, the structural details provided for many of these metal binding sites are novel and involve some important but unanticipated structural facet. Developments in the techniques of protein crystallization, new and more powerful X-ray sources - including the general availability of synchrotron radiation as an intense and tunable source - improved detectors and tremendous gains in computational power for data collection and analysis, have combined to advance the applicability and precision of protein crystallography.

Despite the significance of this structural information, protein crystallographic results should be taken to represent the beginning of a systematic investigation of the role of the d-transition metal (or metals) in the biological function of the enzyme, not the end of such a study. The advantages of combining protein crystallography with spectroscopic studies will be discussed. The latter are crucial to the assignment of an oxidation state to the metal atom(s) and a determination of the electronic structure of the metal centre which, it is anticipated, will be optimised with respect to the biological function of the macromolecule.

X-ray Absorption Spectroscopy (XAS) at the metal K-(or L-)edge is seen as a particularly useful probe of metal centres in biological systems. Thus, the position of the absorption edge is indicative of the metal's oxidation state and the Extended X-ray Absorption Fine Structure (EXAFS) provides accurate radial information regarding the inner coordination sphere of the metal. The use of Bond Valence Sum Analysis and a distortion theorem to inspect the details of the geometry of a metal binding site in a protein obtained by protein crystallography and/or EXAFS analysis is seen as advantageous.