Structural studies examining the substrate specificity profiles of PC-PLC_Bc protein variants

The phosphatidylcholine preferring phospholipase C from Bacillus cereus (PC-PLC_Bc) catalyzes the hydrolysis of phospholipids in the following order of preference: phosphatidylcholine (PC) > phosphatidylethanolamine (PE) > phosphatidylserine (PS). In previous work, mutagenic, kinetic, and crystallographic experiments suggested that varying the amino acids at the 4th, 56th, and 66th positions had a significant influence upon the substrate specificity profile of PC-PLC_Bc. Here, we report the crystal structures of the native form of several PC-PLC_Bc variants that exhibited altered substrate specificities for PC, PE, and PS at maximum resolutions of 1.90-2.05 Å. Comparing the structures of these variants to the structure of the wild-type enzyme reveals only minor differences with respect to the number and location of active site water molecules and the side chain conformations of residues at the 4th and 56th positions. These results suggest that subtle changes in steric and electronic properties in the substrate binding site of PC-PLC_Bc are responsible for the significant changes in substrate selectivity.