A molecular orbital theoretical study on the acid-base property and reactivity of the charge relay system in alpha chymotrypsin

The catalytic mechanism of serine proteases. III. An Indo-ISCRF study of the methylacetate docking in alpha-chymotrypsin

Mechanistic steps of the catalytic hydrolysis of methylacetate by alpha-chymotrypsin have been studied with the inhomogeneous selfconsistent reaction field theory of protein core effects. Protein-substrate interactions were optimized at each step of the reaction path by using the REFINE program. The state of charge of the acid and basic side chains together with the amino terminal group have been used to mimic pH effects. The experimental facts concerning the catalytic activity are fairly well reproduced in our simulations. In particular, the trends in activation barrier as a function of pH are good, although the actual energetic barriers are much too large.

The catalytic mechanism of serine proteases II: The effect of the protein environment in the alpha-chymotrypsin proton relay system

The proton relay system of alpha-chymotrypsin is analyzed by the INDOISCRF method. The effects of the protein electric and polarization fields are explicitly introduced in the calculations. It is shown that the multicharged structure Ser-His+Asp- is the most sensitive, from an energetic view-point, towards the protein surrounding effects. Variations in the permanent and polarization fields are discussed, as well as the influence of the substrate and one water molecule localized in the active site of the enzyme. The catalytic role of such changes is conjectured.

The catalytic mechanism of serine proteases: single proton versus double proton transfer

The two possible mechanisms of proton transfer on the catalytic process of serine proteases (single or double proton transfer) have been analysed. Intermediate neglect of differential overlap calculations have been performed in the absence and in the presence of the substrate molecule and one water molecule localized in the active site. It is shown that, in the absence of the substrate and water, double proton transfer seems to be the most feasible mechanism. However, when these molecules are introduced in the calculation, the role played by them is to facilitate the formation of the zwitterionic structure (single proton transfer) and to destabilize the intermediate structure which leads to double proton transfer. All calculations were made in vacuo.

Interactions between Asp, His, Ser residues within models of the active site of serine proteases. A theoretical empirical study

Empirical theoretical calculations have been performed on a simplified model of the active site of two serine proteases: alpha-chymotrypsin and subtilisin Novo. The stability of the catalytic triad and the hydrogen bond formation between the Asp-His and His-Ser pairs have been examined for different protonation states. The results show that the Asp-His interactions prevail upon the His-Ser ones. Agreement between calculated configurations and the crystal structure of the site suggests that the presence of other residues near the functional residues is not determinant for the stability of the triad in alpha-chymotrypsin. In subtilisin Novo, on the contrary, the presence of the neighbouring residues seems to contribute more largely to the stability. Strong hydrogen bond interactions between the His and Ser residues do not exist in the resting enzymes. Any improvement of the His-Ser interactions requires large destabilization of the Asp-His diad. Our results suggest that the mechanism of the proton transfer can occur only from perturbations of the active site structure induced by the presence of the substrate.

Catalytic mechanism of serine proteases: reexamination of the pH dependence of the histidyl 1J13C2-H coupling constant in the catalytic triad of alpha-lytic protease

L-Histidine, 90% 13C enriched at the C2 position, was incorporated into the catalytic triad of alpha-lytic protease (EC 3.4.21.12) with the aid of histidine-requiring mutant of Lysobacter enzymogenes (ATC 29487), and the pH dependence of the coupling constant between this carbon atom and its directly bonded proton was reinvestigated. The high degree of specific 13C isotopic enrichment attainable with the auxotroph permits direct observation and measurement of this coupling constant in proton-coupled 13C NMR spectra at 67.89 MHz and at 15.1 MHz. In contrast to the earlier study, the present study indicate that this coupling constant does respond to a microscopic ionization with pKa near 7.0; moreover, the magnitude of the values of 1JC-H observed are in accord with those expected for titration of the histidyl residue. We conclude that the original measurement must be in error and that this coupling constant now also supports a histidyl residue that titrates more or less normally as a component of the catalytic triad of serine proteases.

Dynamics of proton transfer and enzymatic activity

The rate-determining elementary reaction step, i.e. proton transfer from the chymotrypsin active centre to the scissile substrate bond had been studied in the present work. On the basis of our theoretical results a hypothesis was formulated to explain chymotrypsin enzymatic efficiency. After ES complex formation excited vibrational states are populated in the enzyme molecule. In the rate-determining elementary reaction step, the proton transfer takes place from the first excited vibrational state of the N-H bond in the imidazole group of His57. This proton transfer is realised by quantum mechanical tunneling mechanism.