Preliminary X-ray crystallographic data on phospholipase C from Bacillus cereus

Crystal structures of phosphate, iodide and iodate-inhibited phospholipase C from Bacillus cereus and structural investigations of the binding of reaction products and a substrate analogue

The crystal structure of the complex formed between phospholipase C (PLC) from Bacillus cereus and inorganic phosphate (Pi), which is an inhibitor, has been determined and refined to 2.1 A resolution. The final R-factor is 19.7%. We have also studied the binding of two other inhibitors, iodide and iodate, to PLC. X-ray data for these two complexes were collected to 2.8 A resolution during the search for heavy-atom derivatives. A series of screening experiments where PLC crystals have been treated with several reaction products and a substrate analogue were carried out to clarify the question of substrate binding. The results have so far been ambiguous but are discussed briefly. Phosphate and iodate are both found to bind to the three metal ions in the protein molecule, suggesting that these ions are involved directly in the catalytic process and thereby identifying the active site. PLC also binds nine iodide ions, eight of which are on the surface of the molecule and of lower occupancy. The ninth blocks the entrance to the active site cleft and is of higher occupancy. Altogether, these results suggest that the substrate, a phospholipid, is associated directly with the metal ions during catalysis.

High-resolution (1.5 A) crystal structure of phospholipase C from Bacillus cereus

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.

Identification of the apparently essential lysine residues in phospholipase C (Bacillus cereus)

Phospholipase C (Bacillus cereus) contains two apparently essential and very reactive lysine residues that may be labelled selectively by pyridoxal 5'-phosphate [Aurebekk & Little (1977) Biochem, J. 161, 159--165]. One of these lysine residues was found in the 25-amino acid N-terminal fragment liberated by CNBr digestion of the pyridoxal-labelled enzyme and identified as lysine-6. Two of the labelled peptides isolated from the chymotryptic digest of pyridoxal-labelled enzyme contained proline, suggesting that the other labelled lysine residue is situated in the same region of the primary structure as the single proline residue of the enzyme.

Lysis of erythrocytes from stored human blood by phospholipase C (Bacillus cereus)

The ability of phospholipase C (Bacillus cereus) to lyse erythrocytes from human blood that had been stored under Transfusion Service conditions for up to 16 weeks has been examined. When incubated at 20 degrees C with enzyme (0.03 mg/ml, 55 units/ml) for up to 1 h fresh erythrocytes were not lysed. After about 4 weeks of storage a population of very readily lysed erythrocytes appeared. The morphological changes in erythrocytes from blood stored up to 16 weeks were examined by scanning electron microscopy. The proportion of very readily lysed erythrocytes correlated well with the proportion of spheroechinocytes I. This morphological form was shown to be preferentially removed by phospholipase C and before lysis a transient appearance of smooth spheres occurred. The decrease in blood ATP concentrations on storage was measured and found to correlate with the disappearance of discoid erythrocyte forms, but not directly with the increased susceptibility of the erythrocytes to lysis by the enzyme. However, erythrocytes of up to at least 15 weeks of age could be made less susceptible to lysis by pre-incubation in a medium designed to cause intracellular regeneration of ATP. During the lysis of spheroechinocytes I by electrophoretically pure recrystallized phospholipase C a rapid degradation of phosphatidylcholine, phosphatidylethanolamine and phosphatidylserine + phosphatidylinositol) occurred together with a slower degradation of sphingomyelin.