We present an analysis of the electrostatic properties in the catalytic site of papain (EC 3.4.22.2), an archetype enzyme of the C1 cysteine proteinase family, and we investigate their possible role in the formation, stabilization and regulation of the Cys25 (؊) …His159 (؉) catalytic ion pair. The electrostatic properties were computed using a reassociation method based in multicentered multipolar expansions obtained from ab initio quantum calculations of overlapping protein fragments. Solvent effects were introduced by coupling the use of multicentered multipolar expansions to two continuum boundary element methods to solve the Poisson and the linearized Poisson-Boltzmann equations. The electrostatic profile found in the proton transfer region of papain showed that this enzyme has a well-defined electrostatic environment to favor the formation and stabilization of the catalytic ion pair. The papain catalytic site electrostatic profile can be considered as an electrostatic fingerprint of the papain family with the following characteristics: (i) the presence of a net electric field highly aligned in the (Cys25)-SG3(His159)-ND1 direction; (ii) the electrostatic profile has a saddlepoint character; (iii) it is basically a local environmental effect. Furthermore, our analysis describes a possible regulatory mechanism (the E SG3 ND1 attenuation effect) controlling the ion pair reactivity and permits to infer the Asp57 acidic residue as the most probable candidate to act as the electrostatic modulator. Proteins 2003;52:236 -253.
ABSTRACT:We report an analysis of three schemes for fragment reassociation using multicentered multipolar expansions derived from ab initio quantum wave functions at the Hartree-Fock/6-31G * LCAO level, two of them involving single-bond partitioning in the peptide bond region, and the third one using a partially overlapping procedure based on a methodology proposed by Vigné-Maeder 21 (OME-overlap of multipolar expansions-reassociation method). The effects of different peptide junction treatments in the derivation of molecular electrostatic potentials and molecular electric fields of three peptide sequences are discussed. The results show that the OME reassociation method gives a better and a more homogeneous description of both the potential and the electric field than the other two treatments. We conclude that the OME method is the most indicated for studies involving electrostatic properties of proteins. Our results also indicate that the use of multicentered multipolar expansions coupled to the OME treatment is the best choice in protein studies including solvent effects using, for example, a continuum boundary method to solve the linearized Poisson-Boltzmann equation.
We developed a methodology to optimize exponential damping functions to account for charge penetration effects when computing molecular electrostatic properties using the multicentered multipolar expansion method (MME). This methodology is based in the optimization of a damping parameter set using a two-step fast local fitting procedure and the ab initio (Hartree-Fock/6-31G** and 6-31G**+) electrostatic potential calculated in a set of concentric grid of points as reference. The principal aspect of the methodology is a first local fitting step which generates a focused initial guess to improve the performance of a simplex method avoiding the use of multiple runs and the choice of initial guesses. Three different strategies for the determination of optimized damping parameters were tested in the following studies: (1) investigation of the error in the calculation of the electrostatic interaction energy for five hydrogen-bonded dimers at standard and nonstandard hydrogen-bonded geometries and at nonequilibrium geometries; (2) calculation of the electrostatic molecular properties (potential and electric field) for eight small molecular systems (methanol, ammonia, water, formamide, dichloromethane, acetone, dimethyl sulfoxide, and acetonitrile) and for the 20 amino acids. Our results show that the methodology performs well not only for small molecules but also for relatively larger molecular systems. The analysis of the distinct parameter sets associated with different optimization strategies show that (i) a specific parameter set is more suitable and more general for electrostatic interaction energy calculations, with an average absolute error of 0.46 kcal/mol at hydrogen-bond geometries; (ii) a second parameter set is more suitable for electrostatic potential and electric field calculations at and outside the van der Waals (vdW) envelope, with an average error decrease >72% at the vdW surface. A more general amino acid damping parameter set was constructed from the original damping parameters derived for the small fragments and for the amino acids. This damping set is more insensitive to protein backbone and residue side-chain conformational changes and can be very useful for future docking and molecular dynamics protein simulations using ab initio based polarizable classical methods.
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