Producing DFT/MM enzyme reaction trajectories from SCC-DFTB/MM driving forces to probe the underlying electronics of a glycosyltransferase reaction.

The SCC-DFTB/MIO/CHARMM free energy surface for a glycosyltransferase, TcTS, is benchmarked against a DFT/MM reaction trajectory using the same CHARMM MM force field ported to the NWChem package. The popular B3LYP functional, against which the MIO parameter set was parameterized is used to optimize TS structures and run DFT reaction dynamics. A novel approach was used to generate reaction forces from a SCC-DFTB/MIO/CHARMM reaction surface to drive B3LYP/6-31G/MM and B3LYP/6-31G(d)/MM reaction trajectories. Although TS structures compare favorably, differences stemming primarily from a minimal basis set approximation prevented a successful 6-31G(d) FEARCF reaction dynamics trajectory. None the less, the dynamic evolution of the B3LYP/6-31G/MM-computed electron density provided an opportunity to perform NBO analysis along the reaction trajectory. Here, we illustrate that a successful ab initio reaction trajectory is computationally accessible when the underlying potential energy function of the semi-empirical method used to produce driving forces is sufficiently close to the ab initio potential. © 2017 Wiley Periodicals, Inc.

Profiling transition-state configurations on the Trypanosoma cruzi trans-sialidase free-energy reaction surfaces.

Enzymatically catalyzed reactions pass from reactants to products via transition states that are short-lived and potentially characterized from free-energy reaction surfaces. We compute the reaction surface for Trypanosoma cruzi trans-sialidase using the Free Energy from Adaptive Reaction Coordinate Forces method. The reaction coordinates are the bonds between the sialic acid and the leaving group (TYR342) and the sialic acid and the nucpleophile (ASP59). We are able to track progress of the reaction trajectories up to (incomplete), about (recrossed), and across (crossed) the col that divides the reactant (covalent intermediate) and product (Michaelis complex) surfaces. More than 40 transition state configurations were isolated from these trajectories, and the sialic acid substrate conformations were analyzed as well as the substrate interactions with the nucleophile and catalytic acid/base. A successful barrier crossing requires that the substrate passes through a family of E5, (4)H5, and (6)H5 pucker conformations. These puckers interact slightly differently with the enzyme. The E5 and (4)H5 conformations have a high-frequency hydrogen bonding with Asp96, while (6)H5 puckers show increased hydrogen bonding between sialic acid O-8-Glu230. Our analysis of Trypanosoma cruzi trans-sialidase configurations that populate the col separating the reactant from product surfaces brings new evidence to the prevailing premise that there are several pathways from reactant to product passing through the saddle and successful product formation is not restricted to the minimum energy path and transition state.

Conformational preferences of plumbagin with phenyl-1-thioglucoside conjugates in solution and bound to MshB determined by aromatic association.

Here we show that a series of inhibitors, constructed from plumbagin conjugated to a phenyl thioglucoside via an alkyl chain of variable length, are bound in solution-favoured ligand conformations to a mycothiol biosynthetic enzyme MshB, a GlcNAc-Ins deacetylase. The kinetic studies of this ligand series show that MshB is more strongly inhibited as a function of increasing alkyl chain length. While docking studies yielded highest ranked conformations in which the ligands extended along the catalytic site, these conformations produced free energy values prone to large errors and which were inconsistent with experimental kinetic measurements. Solution-favoured conformations of the inhibitors feature a preference for intramolecular aromatic association that results in curled conformations. Free energy perturbation calculations of MshB bound to the inhibitors in the preconfigured solution-favoured curled conformations gave the same binding pattern observed in the kinetic experiments. On investigation of these conformations lodged in the catalytic domain, we found that the selective feature determining their relative binding strength was the result of an optimisation of the dispersion interactions between the ligand aromatic groups phenyl and plumbagin, and the enzyme aromatic groups His144 and Tyr142 respectively. These results show that rather than deform the preferred folded ligand solution conformation, such that the hydrophobic C-2 acyl chain is linearly projected into a buried hydrophobic rich binding cavity adjacent to the active site, MshB binds preconfigured solution inhibitor curled conformations with a preference for aromatic association.