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.

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X-ray crystallographic analyses of two sterically unhindered copper meso-triarylcorroles, Cu[5,15-P(2)-10-(4-MeOP)C] and Cu[5,15-(4-CF(3)P)(2)-10-(4-MeOP)C] (P = phenyl and C = corrole), revealed substantially saddled corrole rings. These results are in marked contrast to those on highly sterically hindered cobalt(III) and iridium(III) corroles, which exhibit planar corrole macrocycles. The solution to this conundrum is that copper corroles are inherently saddled, as a result of a specific copper(d)-corrole(pi) orbital interaction. This orbital interaction results in a noninnocent corrole ligand, and the overall electronic structure may thus be described as Cu(II)-corrole(*2-). While many specific metal(d)-macrocycle(pi) orbital interactions are known for nonplanar metalloporphyrins, this work provides a rare example of such an orbital interaction providing the actual driving force for a significant nonplanar distortion. Our findings on copper corroles, along with those of others on cobalt and iridium corroles, thus constitute an intriguing and somewhat counterintuitive chapter in the structural chemistry of metallocorroles.

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The first crystal structure of a copper beta-octabromo-meso-triarylcorrole exhibits a uniquely saddled corrole macrocycle, where adjacent pyrrole rings are tilted relative to each other by 60-80 degrees. Such strong nonplanarity may be contrasted with the essentially planar macrocycle conformations observed in the vast majority of metallocorrole crystal structures. Density functional theory calculations suggest that two effects, ligand noninnocence and peripheral overcrowding, acting in concert, are responsible for the unique, observed conformation.