Arginine kinase belongs to the family of enzymes, including creatine kinase, that catalyze the buffering of ATP in cells with f luctuating energy requirements and that has been a paradigm for classical enzymological studies. The 1.86-Å resolution structure of its transition-state analog complex, reported here, reveals its active site and offers direct evidence for the importance of precise substrate alignment in the catalysis of bimolecular reactions, in contrast to the unimolecular reactions studied previously. In the transitionstate analog complex studied here, a nitrate mimics the planar ␥-phosphoryl during associative in-line transfer between ATP and arginine. The active site is unperturbed, and the reactants are not constrained covalently as in a bisubstrate complex, so it is possible to measure how precisely they are pre-aligned by the enzyme. Alignment is exquisite. Entropic effects may contribute to catalysis, but the lone-pair orbitals are also aligned close enough to their optimal trajectories for orbital steering to be a factor during nucleophilic attack. The structure suggests that polarization, strain toward the transition state, and acid-base catalysis also contribute, but, in contrast to unimolecular enzyme reactions, their role appears to be secondary to substrate alignment in this bimolecular reaction.

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Phosphagen kinases catalyze the reversible transfer of a phosphoryl group between guanidino phosphate compounds and ADP, thereby regenerating ATP during bursts of cellular activity. Large quantities of highly pure arginine kinase (EC 2.7.3.3), the phosphagen kinase present in arthropods, have been isolated from E. coli, into which the cDNA for the horseshoe crab enzyme had been cloned. Purification involves size exclusion and anion exchange chromatographies applied in the denatured and refolded states. The recombinant enzyme has been crystallized as a transition state analog complex. Near complete native diffraction data have been collected to 1.86 A resolution. Substitution of a recombinant source for a natural one, improvement in the purification, and data collection at cry0 temperatures have all yielded significant improvements in diffraction. This reaction and those of the rest of the guanidino (or phosphagen) kinase family, including creatine kinase (CK), enable ATP to be quickly regenerated from storage phosphagens during bursts of cellular activity. In addition to this "temporal" buffering, it has been proposed that these enzymes also function in "spatial" buff-

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The ZnO thin films are used in the field of pharmaceutical sciences and it has more number of applications which plays an important role on antibacterial activity. In modern era, the ZnO thin films pepared by biological and chemical methods using microorganisms, enzymes, and plant extracts has been suggested as ecofriendly to environment. The potential toxicity of ZnO thin films were investigated using both Gram positive and Gram negative bacteria as test organisms. The results showed that ZnO thin film nanoparticles enhanced the good antibacterial activity.