QUANTUM CHEMICAL STUDIES OF THE N-TERMINAL NUCLEOPHILE HYDROLASE CATALYTIC MECHANISM
Modeling of the catalytic mechanism of penicillin acylase (EC 3.5.1.11), a member of the N-terminal nucleophile (Ntn) hydrolase superfamily, was carried out for the first time at ab initio quantum chemistry level. The uniqueness of this family of enzymes is that their active site lacks His and Asp (Glu), residues comprising the classical catalytic triad. Current investigation confirms that in N-terminal hydrolases, the amino group of the N-terminal serine residue is capable of activating its own hydroxyl group. Using the MP2/HF method with 6-31+G** basis set, stationary points on the potential energy surface of the considered molecular system were localized, corresponding to local minima (complexes of reagents, products, intermediate) and to saddle points (transition states). It turned out that the stage of acyl-serine formation proceeds via two transition states. The first one, separating reagents from the so-called tetrahe-dral intermediate, has the highest relative energy (30 kcal/mol). In contrast to recently proposed heuristic suggestions, we have found that participation of a bridging water molecule in proton shuttling is not necessary for the catalysis. Quantum chemical calculations showed the crucial role of specific solvation in decreasing the activation barrier of the reaction at approximately 10 kcal/mol.