We present the newest version of the GROningen MOlecular Simulation program package, GROMOS96. GROMOS96 has been developed for the dynamic modelling of (bio)molecules using the methods of molecular dynamics, stochastic dynamics, and energy minimization as well as the path-integral formalism. An overview of its functionality is given, highlighting methodology not present in the last major release, GROMOS87. The organization of the code is outlined, and reliability, testing, and efficiency issues involved in the design of this large (73 000 lines of FORTRAN77 code) and complex package are discussed. Finally, we present two applications illustrating new functionality: local elevation simulation and molecular dynamics in four spatial dimensions.

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A network of coupled promoting motions in the enzyme dihydrofolate reductase is identified and characterized. The present identification is based on genomic analysis for sequence conservation, kinetic measurements of multiple mutations, and mixed quantum͞ classical molecular dynamics simulations of hydride transfer. The motions in this network span time scales of femtoseconds to milliseconds and are found on the exterior of the enzyme as well as in the active site. This type of network has broad implications for an expanded role of the protein fold in catalysis as well as ancillaries such as the engineering of altered protein function and the action of drugs distal to the active site.A relationship between the motion of protein structural elements and activity has been implicated in enzyme catalysis (1-3). Evidence for the existence of promoting vibrations or modes that augment catalytic activity has been sought for a number of enzymes. At the amino acid level, motions of residues both in and distal to the active site have been proposed to participate in catalysis. The identification, characterization, and clarification of the function of such proximal and distal promoting motions present a challenging task. Recently the importance of coupled motions sampled in differing time domains involving distal residues in the enzyme dihydrofolate reductase (DHFR; EC 1.5.1.3) has been suggested by a combination of NMR experiments (microsecond to picosecond) (4), classical molecular dynamics simulations (nanosecond) (5), and kinetic experiments for site-directed mutants (millisecond to second) (6, 7). Here we report the results of genomic analysis, kinetic measurements of multiple mutations, and mixed quantum͞classical molecular dynamics simulations (8) of the hydride transfer step in DHFR. Based on the crystal structure framework, these results provide a description of specific residue motions and their linkage to enzyme catalysis.DHFR is required for normal folate metabolism in prokaryotes and eukaryotes. It catalyzes the reduction of 7,8-dihydrofolate (DHF) to 5,6,7,8-tetrahydrofolate (THF) by using nicotinamide adenine dinucleotide phosphate (NADPH) as a coenzyme. Specifically, the pro-R hydride of NADPH is transferred to the C6 of the pterin with concurrent protonation at the N5 position. This reaction is essential to maintain necessary levels of THF needed to support the biosynthesis of purines, pyrimidines, and amino acids, fostering DHFR as a pharmacological target. As a result of its importance, DHFR has been studied extensively with a wide range of methodologies.X-ray crystallographic studies indicate that the Escherichia coli DHFR enzyme contains an eight-stranded ␤-sheet and four ␣-helices interspersed with flexible loop regions that connect these structural elements (ref. 9; see Fig. 1). Depending on the nature of the bound ligand, three different conformations have been observed for a surface loop formed by residues 9-24 (denoted the Met-20 loop). When the DHF substrate and NADPH coenzyme are bound, the Met-20...

An algorithm for linear scaling geometry optimisation and transition state search using hybrid delocalised internal coordinates (HDLC) has been developed and implemented in the context of a semiempirical quantum-chemistry program (MNDO) and a modular quantum-mechanical/molecular-mechanical (QM/MM) package (ChemShell). Linear scaling is achieved by a divide-and-conquer approach: the system is partitioned into user-defined fragments, and all coordinate manipulations are performed exclusively within these fragments. The optimiser employs a limited-memory quasi-Newton algorithm (L-BFGS) for energy minimisation, and a microiterative scheme for transition state search using a Hessian eigenmode-following algorithm (P-RFO) for the reaction core and the L-BFGS algorithm for the environment. There are automatic procedures for generating redundant sets of internal coordinates and non-redundant sets of HDLC from Cartesian coordinates. The input to the optimiser consists of the initial Cartesian geometry, the fragmentation of the system, the choice of the working coordinate system, and any constraints to be imposed in Cartesian and/or internal coordinates. The optimiser requires an external function that provides the energy and gradient at a given Cartesian geometry. Systems with thousands of atoms have been optimised, and transition states of a model enzymatic reaction have been determined

Mixed quantum/classical molecular dynamics simulations of the hydride transfer reaction catalyzed by dihydrofolate reductase are presented. The nuclear quantum effects such as zero point energy and hydrogen tunneling, as well as the motion of the entire solvated enzyme, are included during the generation of the free energy profiles and the real-time dynamical trajectories. The calculated deuterium kinetic isotope effect agrees with the experimental value. The simulations elucidate the fundamental nature of the nuclear quantum effects and provide evidence of hydrogen tunneling in the direction along the donor−acceptor axis. The transmission coefficient was found to be 0.80 for hydrogen and 0.85 for deuterium, indicating the significance of dynamical barrier recrossings. Nonadiabatic transitions among the vibrational states were observed but did not strongly affect the transmission coefficient. A study of motions involving residues conserved over 36 diverse species from Escherichia coli to human implies that motions of residues both in the active site and distal to the active site impact the free energy of activation and the degree of barrier recrossing. This analysis resulted in the characterization of a network of coupled promoting motions that extends throughout the protein and involves motions spanning femtosecond to millisecond time scales. This type of network has broad implications for protein engineering and drug design.