Metalloproteins, electron transport and EXAFS
Prizes for the best poster at the IUCr Congress in Glasgow sponsored by the Cambridge Crystallographic Data Center were awarded to: Peter Muller, Germany (Holes in Crystals?); Alicia Beatty and Christer Aakeroy, USA (Beyond the first dimension: hybrid materials assembled via H-bonds); C. Baehtz and H. Fuess (Tetrathiafulvalene and tetracyano-p-quinodimethane in faujasite) and S. Leoni & R. Nesper, Switzerland (Tilings on hyperbolic surfaces). Judith Howard and Steve Maginn (front row) presented the prizes to Muller, Leoni, Beatty and Fuess (accepting for Baehtz) (back row).
Presentations ranged from the use of time-resolved methods in single crystal diffraction to the complementary roles played by crystallography and EXAFS in determining the structures of the metal centres in metalloproteins. I. Schlichting (Germany) described the structure of the ternary complex between P450cam/camphor/dioxygen, formed by co-crystallisation with camphor, chemical reduction, and then oxygen diffusion, and a structure of a reaction intermediate. S. Ramaswamy (Sweden) described the structure of naphthalene dioxygenase, where the initial structure surprisingly contained a reaction intermediate; an indole covalently linked to dioxygen bonded to the active site iron. Following refinement of purification protocols, an 'empty' active site was achieved, and the product could be soaked into the crystals, leading to the conclusion that during the reaction indole binds before dioxygen. H. Freeman (Australia) spoke about using polarised Fe K-edge X-ray absorption spectroscopy with oriented carbonmonoxy-myoglobin crystals to obtain accurate angular geometries between the ligand and metal centre that could be used to restrain metal-ligand angles in single crystal X-ray structures. G. George (Stanford, USA) highlighted the difficulties of interpreting X-ray crystal structures with reference to spectroscopic techniques. In the case of formaldehyde ferredoxin oxidoreductase, a tungsten containing enzyme, EXAFS suggested the presence of two W-oxo ligand interactions, whereas the crystal structure only had one in the model. Re-examination of the electron density suggested the presence of the second interaction. S. S. Hasnain (Daresbury, UK) demonstrated the complementarity of EXAFS and single crystal X-ray crystallographic studies of nitrite reductase, Cu,Zn superoxide dismutase (SOD) and rusticyanin. EXAFS and single crystal structures showed that nitrite reductase was incapable of binding substrate or inhibitors, subsequent to an electron transfer between the two copper ion sites in the enzyme. Therefore, substrate nitrite must bind to the enzyme prior to electron transfer.
J. Mitchell Guss