Combining crystallography and molecular dynamics: the case of Schistosoma mansoni phospholipid glutathione peroxidase.

Oxidative stress is a widespread challenge for living organisms, and especially so for parasitic ones, given the fact that their hosts can produce reactive oxygen species (ROS) as a mechanism of defense. Thus, long lived parasites, such as the flatworm Schistosomes, have evolved refined enzymatic systems capable of detoxifying ROS. Among these, glutathione peroxidases (Gpx) are a family of sulfur or selenium-dependent isozymes sharing the ability to reduce peroxides using the reducing equivalents provided by glutathione or possibly small proteins such as thioredoxin. As for other frontline antioxidant enzymatic systems, Gpxs are localized in the tegument of the Schistosomes, the outermost defense layer. In this article, we present the first crystal structure at 1.0 and 1.7 A resolution of two recombinant SmGpxs, carrying the active site mutations Sec43Cys and Sec43Ser, respectively. The structures confirm that this enzyme belongs to the monomeric class 4 (phospholipid hydroperoxide) Gpx. In the case of the Sec to Cys mutant, the catalytic Cys residue is oxidized to sulfonic acid. By combining static crystallography with molecular dynamics simulations, we obtained insight into the substrate binding sites and the conformational changes relevant to catalysis, proposing a role for the unusual reactivity of the catalytic residue.

Molecular dynamics simulation of protein folding by essential dynamics sampling: folding landscape of horse heart cytochrome c.

A new method for simulating the folding process of a protein is reported. The method is based on the essential dynamics sampling technique. In essential dynamics sampling, a usual molecular dynamics simulation is performed, but only those steps, not increasing the distance from a target structure, are accepted. The distance is calculated in a configurational subspace defined by a set of generalized coordinates obtained by an essential dynamics analysis of an equilibrated trajectory. The method was applied to the folding process of horse heart cytochrome c, a protein with approximately 3000 degrees of freedom. Starting from structures, with a root-mean-square deviation of approximately 20 A from the crystal structure, the correct folding was obtained, by utilizing only 106 generalized degrees of freedom, chosen among those accounting for the backbone carbon atoms motions, hence not containing any information on the side chains. The folding pathways found are in agreement with experimental data on the same molecule.

The effect of protein conformational flexibility on the electronic properties of a chromophore.

In this paper we address the question of how a protein environment can modulate the absorption spectrum of a chromophore during a molecular dynamics simulation. The effect of the protein is modeled as an external field acting on the unperturbed eigenstates of the chromophore. Using a first-principles method recently developed in our group, we calculated the perturbed electronic energies for each frame and the corresponding wavelength absorption during the simulation. We apply this method to a nanosencond timescale molecular dynamics simulation of the light-harvesting peridinin-chlorophyll-protein complex from Amphidinium carterae, where chlorophyll was selected among the chromophores of the complex for the calculation. The combination of this quantum-classical calculation with the analysis of the large amplitude motions of the protein makes it possible to point out the relationship between the conformational flexibility of the environment and the excitation wavelength of the chromophore. Results support the idea of the existence of a correlation between protein conformational flexibility and chlorophyll electronic transitions induced by light.

Selective excitation of native fluctuations during thermal unfolding simulations: horse heart cytochrome c as a case study.

The effect of temperature on the activation of native fluctuation motions during molecular dynamics unfolding simulations of horse heart cytochrome c has been studied. Essential dynamics analysis has been used to analyze the preferred directions of motion along the unfolding trajectories obtained by high temperature simulations. The results of this study have evidenced a clear correlation between the directions of the deformation motions that occur in the first stage of the unfolding process and few specific essential motions characterizing the 300 K dynamics of the protein. In particular, one of those collective motions, involved in the fluctuation of a loop region, is specifically excited in the thermal denaturation process, becoming progressively dominant during the first 500 ps of the unfolding simulations. As further evidence, the essential dynamics sampling performed along this collective motion has shown a tendency of the protein to promptly unfold. According to these results, the mechanism of thermal induced denaturation process involves the selective excitation of one or few specific equilibrium collective motions.