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The preparation, crystallization and low-resolution structure determination of beta-lactamase (EC 3.5.2.6, ‘penicillinase’) from Staphylococcus aureus is described. The enzyme crystallizes in space group I222 with 1 molecule per asymmetric unit and cell dimensions a = 5.45(1), b = 9.39(1) and c = 13.87(2) nm. The structure was determined at 0.5 nm resolution by using phases calculated from (NH4)2Pt(CN)4 and KAu(CN)2 derivatives. The mean figure of merit mean value of m, for the 1106 reflexions used was 0.70. Difference Fourier syntheses for data collected from crystals soaked in platinum D-methionine and in 6-(4-hydroxy-3,5-di-iodobenzamido)penicilloic acid revealed the likely position of the active site of the enzyme.

It has been shown previously that the solvation energies of small molecules can be computed to useful accuracy with continuum models of the solvent that separate the solvation energy into electrostatic and nonpolar parts. The electrostatic part of the solvation energy can be computed with detailed solutions of the Poisson equation, but such calculations are time-consuming. This paper examines the reliability of an approximate method for computing electrostatic solvation energies within the continuum model. The central approximation is that, although the electrostatic field is weakened by the high-dielectric solvent, the shape of the field is not influenced by the solvent. This “dielectric screening” approximation appears to work well when used with a nonlinear, lattice-based model of the solvent. Here, it is developed in the context of linear, continuum electrostatics. The optimal screening function is found to depend upon the distribution of charge within the solute. The present dielectric screening method performs well in comparisons with finite-difference solutions of the Poisson equation and with experimental data, for a range of molecular systems.

The initial contractile event in muscle is the binding of Ca2+ ions to troponin C of the troponin complex, leading to a series of conformational changes in the members of the thin and thick filaments. Knowledge of the crystal structure of turkey skeletal muscle troponin C has provided a structural basis for the modeling of the first stage of this process in atomic detail. This crystal structure probably represents the molecule in the relaxed state of muscle, with two of the maximum of 4 Ca2+ ions bound. The basis for the model presented here is that upon binding of the additional two Ca2+ ions, the regulatory domain of the molecule undergoes a conformational transition to become closely similar in structure to the domain which always binds Ca2+ or Mg2+ under physiological conditions. The root mean square discrepancy in atomic coordinates between the apo and the modeled Ca2+-bound states of the regulatory domain is 4.8 A, with some shifts as large as 10-15 A in the region near the linker between the two Ca2+ binding sites. It is demonstrated that this Ca2+-bound conformation of the regulatory domain conforms to accepted protein structure rules and that the change in conformation can be accomplished without encountering any barriers too high to be surmounted on the physiological time scale.