Quantum chemical conformational analysis of the catalytic triad in α-chymotrypsin

Abstract

We have applied a recently developed method for the conformational analysis of the Asp-102, His-57 and Ser-195 catalytic triad of α-chymotrypsin. The method is based on the semiempirical CNDO/2 parametrization and makes use of a bond orbital framework allowing a considerable reduction in computing time. The enzyme active site is modelled by the HCONHCH(CH 2COO −)CONH 2 ⋯ HCONHCH(CH 2Im)CONH 2 ⋯ HCONHCH(CH 2OH)CONH 2 system containing 58 atoms and 157 valence orbitals (Im stands for imidazole). We also studied a model where the imidazole of His-57 is protonated. In this form, which can be compared to experiment, our method predicts conformations for the His-57 and Asp-102 side chains close to those obtained by X-ray diffraction studies. In the neutral form which exists under physiological conditions two stable conformations (A: χ 1 = 84°, χ 2 = 90° and A′: χ 1 = 168°, χ 2 = −81°) were located for the Ser-195 side chain. A very shallow minimum, B (χ 1 = −66°, χ 2 = 0°) was also detected. Ab initio minimal basis set calculations on suitable small models indicate that, in agreement with the CNDO/2 results, conformations A and A′ have about the same energy. The effect of hydration was modelled by one and two attached water molecules. Conformation A which is close to the one supposed to exist at physiological pH values, is stronger stabilized by hydration than conformation A′. We compare our results to other experimental and theoretical data. Possible consequences of the conformational properties on the enzymatic mechanism are also discussed.