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Carbonic anhydrase, a zinc metalloenzyme, catalyzes the reversible hydration of carbon dioxide to bicarbonate. It is involved in processes connected with acid--base homeostasis, respiration, and photosynthesis. More than 100 distinct human carbonic anhydrase II (HCAII) 3D structures have been generated in last 3 decades [Liljas A, et al. (1972) Nat New Biol 235:131-137], but a structure of an HCAII in complex with C[O.sub.2] or HC[O.sup.-.sub.3] has remained elusive. Here, we report previously undescribed structures of HCAII:C[O.sub.2] and HCAII:HC[O.sup.-.sub.3] complexes, together with a 3D molecular film of the enzymatic reaction observed successively in the same crystal after extended exposure to X-ray. We demonstrate that the unexpected enzyme activation was caused in an X-ray dose-dependent manner. Although X-ray damage to macromolecular samples has long been recognized [Ravelli RB, Garman EF (2006) Curr Opin Struct Biol 16:624-629], the detailed structural analysis reports on X-ray-driven reactions have been very rare in literature to date. Here, we report on enzyme activation and the associated chemical reaction in a crystal at 100 K. We propose mechanisms based on water photoradiolysis and/or electron radiolysis as the main cause of enzyme activation. C[O.sup.2] binding | crystal pressurization | radiation driven reaction | substrate-product complex structure

An investigation of the electronic and geometric structures of CuA centers through nuclear magnetic resonance spectroscopy found electron delocalization onto S atoms and proton shifts of one histidine that vary from one system to another. Data suggest that these shifts depend on the Cu-S-C-H torsion angle. The line widths of histidine proton signals indicate fast electron relaxation at room temperature arising from the close doublet levels. Results indicate that this relaxation is not sensitive to slight inequivalencies in histidine binding.

The effect of ionic strength on the catalytic properties of four Xenopus laevis Cu,ZN superoxide dismutase (SOD) mutants was studied using pulse radiolysis. The charged residues of the mutants nearest to the copper active site were neutralized at Lys120, Asp130, Glu131 and Lys134. Results showed that some residues occupying the same position in the linear sequence of Cu,ZN SODs from various species differ in functional weight during the transport of anionic substrate toward the active site. Mutants with neutralized charged residues were also found to exhibit layer differences in the catalytic rate with respect to the wild type.

A plausible model of the structure of the alpha-actinin rod is presented. The model was derived after the determination of the crystal structure of the two central repeats in the alpha-actinin rod at a resolution of 2.5 Angstroms. This molecular architecture, which places the actin-binding sides at both ends of the rod in an antiparallel fashion, is indicative of a protein that is capable of forming cross-links between actin filaments.