Both the phosphatidylinositol-hydrolysing and the phosphatidylcholine-hydrolysing phospholipases C have been implicated in the generation of second messengers in mammalian cells. The phosphatidylcholine-hydrolysing phospholipase C (PLC) from Bacillus cereus, a monomeric protein containing 245 amino-acid residues, is similar to some of the corresponding mammalian proteins. This, together with the fact that the bacterial enzyme can mimic the action of mammalian PLC in causing, for example, enhanced prostaglandin biosynthesis, suggests that B. cereus PLC can be used as a model for the hitherto poorly characterized mammalian PLCs. We report here the three-dimensional structure of B. cereus PLC at 1.5 A resolution. The enzyme is an all-helix protein belonging to a novel structural class and contains, at least in the crystalline state, three Zn2+ in the active site. We also present preliminary results from a study at 1.9 A resolution of the complex between PLC and inorganic phosphate (Pi) which indicate that the substrate binds directly to the metal ions.

By using geometry optimizations with local density functional theory and double-i plus polarization basis sets, an extensive study has been carried out on the molecular structures and stabilities of free-base and metal-complexed corrole isomers. The optimized structures of normal metallocorroles have been found to agree well with crystallographic results. For both free-base and metal-complexed derivatives, the [I .

Using local density functional calculations, we have examined whether tetrapyrrolic porphyrin isomers can accommodate trans −CHCH− double bonds within their frameworks and still exist as stable substances. The molecular total energies of the trans stereoisomers of free-base [3.0.1.0]- and [3.1.0.0]-porphyrin are only 23.7 and 26.6 kcal/mol above porphine and 7.7 and −12.7 kcal/mol relative to their respective cis stereoisomers. Introduction of a trans double bond also results in a dramatic stabilization of the [4.0.0.0]-isomer. At least, the trans isomers of [3.0.1.0]- and [3.1.0.0]-porphyrin are predicted to be stable, chemically isolable compounds. The energetics results are analyzed in terms of the optimized geometries.

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