Biocatalytic stereocontrolled head-to-tail cyclizations of unbiased terpenes as a tool in chemoenzymatic synthesis.

Terpene synthesis stands at the forefront of modern synthetic chemistry and represents the state-of-the-art in the chemist's toolbox. Notwithstanding, these endeavors are inherently tied to the current availability of natural cyclic building blocks. Addressing this limitation, the stereocontrolled cyclization of abundant unbiased linear terpenes emerges as a valuable tool, which is still difficult to achieve with chemical catalysts. In this study, we showcase the remarkable capabilities of squalene-hopene cyclases (SHCs) in the chemoenzymatic synthesis of head-to-tail-fused terpenes. By combining engineered SHCs and a practical reaction setup, we generate ten chiral scaffolds with >99% ee and de, at up to decagram scale. Our mechanistic insights suggest how cyclodextrin encapsulation of terpenes may influence the performance of the membrane-bound enzyme. Moreover, we transform the chiral templates to valuable (mero)-terpenes using interdisciplinary synthetic methods, including a catalytic ring-contraction of enol-ethers facilitated by cooperative iodine/lipase catalysis.

Computational prediction of extracellular loops of the Por39 outer membrane porin of Rhodospirillum rubrum suitable for epitope surface display.

Outer membrane porins from Gram-negative bacteria are established vehicles for the production of vaccines. Typically, one or more of the extracellular loops of a porin are replaced by a peptide encoding a foreign epitope, and recombinant porin is then used as a vaccine. However, many host strains are potentially pathogenic, and also produce toxic lipopolysaccharide (LPS), both of which are undesirable for safety reasons. In contrast, the outer membrane porins from photosynthetic, purple bacteria have no known human pathology and produce only weakly toxic LPS. The purple bacterium Rhodospirillum rubrum is well-suited for large-scale biotechnology, and expresses a major porin, Por39, which is a candidate for a vaccine platform. Unfortunately, the atomic structure of Por39 could not be determined so far, and Por39 shows only a weak homology to other porins of known structure, making the assignment of external loops difficult. Here, we construct a knowledge-based model of Por39 using secondary structure constraints from both the low sequence homology to the 2POR porin from Rhodobacter capsulatus, for which the X-ray structure is known, as well as those obtained using secondary structure prediction packages. The secondary structure predictions were used to constrain a three-dimensional model created using the I-TASSER package. The modelling procedure was validated by predicting the structure of 2POR using the same strategy, but excluding the 2POR X-ray structure from the I-TASSER database. The final Por39 model allows three external loops to be defined precisely, and could also be used to obtain an initial model for the closely related Por41 using molecular modelling. These structures provide a good starting point for the insertion of epitopes with vaccine potential.

Exploring the Effect of Enhanced Sampling on Protein Stability Prediction.

Changes in protein stability due to side-chain mutations are evaluated by alchemical free-energy calculations based on classical molecular dynamics (MD) simulations in explicit solvent using the GROMOS force field. Three proteins of different complexity with a total number of 93 single-point mutations are analyzed, and the relative free-energy differences are discussed with respect to configurational sampling and (dis)agreement with experimental data. For the smallest protein studied, a 34-residue WW domain, the starting structure dependence of the alchemical free-energy changes, is discussed in detail. Deviations from previous simulations for the two other proteins are shown to result from insufficient sampling in the earlier studies. Hamiltonian replica exchange in combination with multiple starting structures and sufficient sampling time of more than 100 ns per intermediate alchemical state is required in some cases to reach convergence.

Binding free energies for the SAMPL8 CB8 "Drugs of Abuse" challenge from umbrella sampling combined with Hamiltonian replica exchange.

Umbrella sampling along a one-dimensional order parameter in combination with Hamiltonian replica exchange was employed to calculate the binding free energy of five guest molecules with known affinity to cucurbit[8]uril. A simple empirical approach correcting for the overestimation of the affinity by the GAFF force field was proposed and subsequently applied to the seven guest molecules of the "Drugs of Abuse" SAMPL8 challenge. Compared to the uncorrected binding free energies, the systematic error decreased but quantitative agreement with experiment was only reached for a few compounds. From a retrospective analysis a weak point of the correction term was identified.

Umbrella sampling and double decoupling data for methanol binding to Candida antarctica lipase B.

The binding free-energy profile of methanol to Candida antarctica lipase B (CALB) was calculated at infinite dilution and at a finite methanol concentration of 6.1 M using umbrella sampling molecular dynamics simulations with the OPLS all-atom force field. An additional validation of the results was performed by employing alchemical double decoupling simulations. The binding free-energy profiles have been used in a related research article to validate free-energy profiles obtained from direct counting simulations with the aim to use the kinetic information encoded in the latter. The data provided in this work will be useful to study concentration effects on binding, to test alternative free energy methods or to use the proposed simulation protocol for related systems.

Overcoming Convergence Issues in Free-Energy Calculations of Amide-to-Ester Mutations in the Pin1-WW Domain.

Relative folding free energies for a series of amide-to-ester mutations in the Pin1-WW domain are calculated using molecular dynamics simulations. Special focus is given to the identification and elimination of a simulation-related bias which was observed in previous work (Eichenberger et al. Biochim. Biophys. Acta 2015, 1850, 983) by comparing simulation results obtained with two different starting structures. Subtle local variations in the protein starting structure may lead to substantial deviations in the calculated free-energy changes as a consequence of differences in the sampled ϕ/ψ-dihedral angle distributions of the mutated residue. It is found that the combination of alchemical transformation with Hamiltonian replica exchange for enhanced sampling reduces the starting structure dependence considerably. Compared to previous work, the improved sampling of both the folded and unfolded states also improves the agreement between simulation and experiment.

Validation of Trimethylamine-N-oxide (TMAO) Force Fields Based on Thermophysical Properties of Aqueous TMAO Solutions.

Five molecular models for trimethylamine N-oxide (TMAO) to be used in conjunction with compatible models for liquid water are evaluated by comparison of molecular dynamics (MD) simulation results to experimental data as functions of TMAO molality. The experimental data comprise thermodynamic properties (density, apparent molar volume, and partial molar volume at infinite dilution), transport properties (self-diffusion and shear viscosity), structural properties (radial distribution functions and degree of hydrogen bonding), and dielectric properties (dielectric spectra and static permittivity). The thermodynamic and transport properties turned out to be useful in TMAO model discrimination while the influence of the water model and the TMAO-water interaction are effectively probed through the calculation of dielectric spectra.