Cutinase belongs to a group of enzymes that catalyze the hydrolysis of esters and triglycerides. Structural studies on the enzyme from Fusarium solani have revealed the presence of a classic catalytic triad that has been implicated in the enzyme's mechanism. We have solved the crystal structure of Glomerella cingulata cutinase in the absence and in the presence of the inhibitors E600 (diethyl p-nitrophenyl phosphate) and PETFP (3-phenethylthio-1,1,1-trifluoropropan-2-one) to resolutions between 2.6 and 1.9 A. Analysis of these structures reveals that the catalytic triad (Ser136, Asp191, and His204) adopts an unusual configuration with the putative essential histidine His204 swung out of the active site into a position where it is unable to participate in catalysis, with the imidazole ring 11 A away from its expected position. Solution-state NMR experiments are consistent with the disrupted configuration of the triad observed crystallographically. H204N, a site-directed mutant, was shown to be catalytically inactive, confirming the importance of this residue in the enzyme mechanism. These findings suggest that, during its catalytic cycle, cutinase undergoes a significant conformational rearrangement converting the loop bearing the histidine from an inactive conformation, in which the histidine of the triad is solvent exposed, to an active conformation, in which the triad assumes a classic configuration.
Cutinase catalyzes the hydrolysis of water-soluble esters and long-chain triglycerides and belongs to the family of serine hydrolases. The enzyme is thought to represent an evolutionary link between the esterase and lipase families and has potential applications in a wide range of industrial hydrolytic processes, for which an understanding of the molecular basis of its substrate specificity is critical. Glomerella cingulata cutinase has been cloned and the protein has been overexpressed in Escherichia coli, purified and subsequently crystallized in a wide range of different crystal forms in the presence and absence of inhibitors. The best crystals are those of the apo cutinase, which diffract to beyond 1.6 A resolution and belong to space group P4(1)2(1)2 or P4(3)2(1)2. Crystals of cutinase with the inhibitors PETFP or E600 belong to space groups P2(1)2(1)2(1) and P2(1), respectively, and diffract to approximately 2.5 A resolution. All of the crystals are suitable for structural studies, which are currently ongoing.
