BIOLOGICAL METAL CLUSTERS--BIOPHYSICAL & MODEL STUDIES

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The active sites of many hydrogenases and CO-dehydrogenases have been shown to contain magnetically interacting Fe, S clusters and paramagnetic Ni centers. These enzymes serve to oxidize molecular hydrogen (or reduce hydrogen ions) and to oxidize carbon monoxide to carbon dioxide and thus serve key roles in the bioenergetics of certain microorganisms. Very little is known about the active site structure or the mechanism by which the metal clusters perform the required redox catalysis. Synthetic models for the hydrogenase and CO dehydrogenase active sites and proposed intermediates are defined, and the study of their spectroscopic and physical properties, particularly epr spectra and redox chemistry, is proposed as a means for elucidating the structure and function of these biological metal clusters. In addition, EXAFS spectral studies on simple hydrogenases from the photosynthetic purple sulfur bacteria Chromatium vinosum and Thiocapsa roseoperscina, in conjunction with EXAFS studies of the active site models, are expected to contribute to the characterization of the protein active sites. Binuclear iron clusters with O-donor bridging groups constitute another class of protein active sites involved in the proposed investigations. Proteins believed to contain active sites featuring this biological metal cluster include: hemerythrin, uteroferrin and other purple acid phosphatases, ribonucleotide reductase, and methane monooxygenase. The proteins serve diverse functions: oxygen transport, regulation of phosphate metabolism, conversion of ribonucleotides to deoxyribonucleotides ( a key step in DNA synthesis), and oxidation of methane to methanol. In each case, mixed-valent forms of the active site metal cluster appear to be involved in the catalysis or in the construction of the active site. Using hemerythrin as a prototype, electronic absorption spectral studies focusing on possible intervalence transitions and on ligand- field transitions in mixed-valent clusters are proposed in order to define the role of electron transport and changes in ligation in the oxygen-binding mechanism, as well as illuminate similar properties in other binuclear active sites. Ultimately, the role of the metal ligands in tailoring the binuclear iron clusters to specific biological functions will be defined.