Hybrid quantum and classical methods for computing kinetic isotope effects of chemical reactions in solutions and in enzymes

CHARMM at 45: Enhancements in Accessibility, Functionality, and Speed

Since its inception nearly a half century ago, CHARMM has been playing a central role in computational biochemistry and biophysics. Commensurate with the developments in experimental research and advances in computer hardware, the range of methods and applicability of CHARMM have also grown. This review summarizes major developments that occurred after 2009 when the last review of CHARMM was published. They include the following: new faster simulation engines, accessible user interfaces for convenient workflows, and a vast array of simulation and analysis methods that encompass quantum mechanical, atomistic, and coarse-grained levels, as well as extensive coverage of force fields. In addition to providing the current snapshot of the CHARMM development, this review may serve as a starting point for exploring relevant theories and computational methods for tackling contemporary and emerging problems in biomolecular systems. CHARMM is freely available for academic and nonprofit research at https://academiccharmm.org/program.

Ab initio path-integral calculations of kinetic and equilibrium isotope effects on base-catalyzed RNA transphosphorylation models

Detailed understandings of the reaction mechanisms of RNA catalysis in various environments can have profound importance for many applications, ranging from the design of new biotechnologies to the unraveling of the evolutionary origin of life. An integral step in the nucleolytic RNA catalysis is self-cleavage of RNA strands by 2'-O-transphosphorylation. Key to elucidating a reaction mechanism is determining the molecular structure and bonding characteristics of transition state. A direct and powerful probe of transition state is measuring isotope effects on biochemical reactions, particularly if we can reproduce isotope effect values from quantum calculations. This article significantly extends the scope of our previous joint experimental and theoretical work in examining isotope effects on enzymatic and nonenzymatic 2'-O-transphosphorylation reaction models that mimic reactions catalyzed by RNA enzymes (ribozymes), and protein enzymes such as ribonuclease A (RNase A). Native reactions are studied, as well as reactions with thio substitutions representing chemical modifications often used in experiments to probe mechanism. Here, we report and compare results from eight levels of electronic-structure calculations for constructing the potential energy surfaces in kinetic and equilibrium isotope effects (KIE and EIE) computations, including a "gold-standard" coupled-cluster level of theory [CCSD(T)]. In addition to the widely used Bigeleisen equation for estimating KIE and EIE values, internuclear anharmonicity and quantum tunneling effects were also computed using our recently developed ab initio path-integral method, that is, automated integration-free path-integral method. The results of this work establish an important set of benchmarks that serve to guide calculations of KIE and EIE for RNA catalysis.

Direct evaluation of the temperature dependence of the rate constant based on the quantum instanton approximation

A general method for the direct evaluation of the temperature dependence of the quantum-mechanical reaction rate constant in many-dimensional systems is described. The method is based on the quantum instanton approximation for the rate constant, thermodynamic integration with respect to the inverse temperature, and the path integral Monte Carlo evaluation. It can describe deviations from the Arrhenius law due to the coupling of rotations and vibrations, zero-point energy, tunneling, corner-cutting, and other nuclear quantum effects. The method is tested on the Eckart barrier and the full-dimensional H+H(2)-->H(2)+H reaction. In the temperature range from 300 to 1500 K, the error of the present method remains within 13% despite the very large deviations from the Arrhenius law. The direct approach makes the calculations much more efficient, and the efficiency is increased even further (by up to two orders of magnitude in the studied reactions) by using optimal estimators for reactant and transition state thermal energies. Which of the estimators is optimal, however, depends on the system and the strength of constraint in a constrained simulation.

Loop-tryptophan human purine nucleoside phosphorylase reveals submillisecond protein dynamics

Human PNP is a homotrimer containing three tryptophan residues at positions 16, 94, and 178, all remote from the catalytic site. The catalytic sites of PNP are located near the subunit-subunit interfaces where F159 is a catalytic site residue donated from an adjacent subunit. F159 covers the top (beta) surface of the ribosyl group at the catalytic site. QM/MM calculations of human PNP have shown that F159 is the center of the most mobile region of the protein providing access to the substrate in the active site. F159 is also the key residue in a cluster of hydrophobic residues that shield catalytic site ligands from bulk solvent. Trp-free human PNP (Leuko-PNP) was previously engineered by replacing the three Trp residues of native PNP with Tyr. From this active construct, a single Trp residue was placed in the catalytic site loop (F159W-Leuko-PNP) as a reporter group for the ribosyl region of the catalytic site. The F159W-Leuko-PNP fluorescence is red shifted compared to native PNP, suggesting a solvent-exposed Trp residue. Upon ligand binding (hypoxanthine), the 3-fold fluorescence quench confirms conformational packing of the catalytic site pocket hydrophobic cluster. F159W-Leuko-PNP has an on-enzyme thermodynamic equilibrium constant (Keq) near unity in the temperature range between 20 and 30 degrees C and nonzero enthalpic components, making it suitable for laser-induced T-jump analyses. T-jump relaxation kinetics of F159W-Leuko-PNP in equilibrium with substrates and/or products indicate the conformational equilibria of at least two ternary complex intermediates in the nano- to millisecond time scale (1000-10000 s-1) that equilibrate prior to the slower chemical step (approximately 200 s-1). F159W-Leuko-PNP provides a novel protein platform to investigate the protein conformational dynamics occurring prior to transition state formation.