Challenging level of rigid-body approach involving numerical elements (CHLORAINE) applied to repeated elastin peptides.

Elastic proteins and derived biomaterials contain numerous tandemly repeated peptides along their sequences, ranging from a few copies to hundreds. These repetitions are responsible for their biochemical, biological and biomechanical properties. These sequences are considered to be intrinsically disordered, and the variations in their behavior are actually mainly due to their high flexibility and lack of stable secondary structures originating from their unique amino acid sequences. Consequently, the simulation of elastic proteins and large elastomeric biomaterials using classical molecular dynamics is an important challenge. Here, we propose a novel approach that allows the application of the DURABIN protocol to repeated elastin-like peptides (r-ELPs) in a simple way. Four large r-ELPs were studied to evaluate our method, which was developed for simulating extracellular matrix proteins at the mesoscopic scale. After structure clustering applied on molecular dynamic trajectories of constitutive peptides (5-mers and 6-mers), the main conformations were used as starting points to define the corresponding primitives, further used as rigid body fragments in our program. Contributions derived from electrostatic and molecular hydrophobicity potentials were tested to evaluate their influence on the interactions during simple mesoscopic simulations. The CHLORAINE approach, despite the thinner granularity due to the size of the patterns used, was included in the DURABIN protocol and emerges as a promising way to simulate elastic macromolecular systems.

Exploring the aglycone subsite of a GH11 xylanase for the synthesis of xylosides by transglycosylation reactions.

Xylanases Tx-xyn10 and Tx-xyn11 were compared for their transxylosylation abilities in the presence of various acceptors. Tx-xyn10 exhibited a broad specificity for various acceptors, whereas xylanase Tx-xyn11 catalysed transxylosylation reactions only in presence of polyphenolic acceptors. A modelling approach was developed to study the molecular bottlenecks into the active site of the enzyme that could be responsible for this restricted specificity. The glycosyl-enzyme intermediate of Tx-xyn11 was modelled, and a rotamer of the Y78 residue was integrated. In silico mutations of some residues from the (+1) and (+2) subsites were tested for the deglycosylation step in the presence of non-polyphenolic acceptors. The results indicated that the mutant W126A was able to use aliphatic alcohols and benzyl alcohol as acceptors for transxylosylation. Experimental validation was tested by mutating the xylanase Tx-xyn11 at position W126 into alanine. The specific activity and catalytic efficiency of the W126A mutant during the hydrolysis of xylans decreased by 2-fold and 4-fold, respectively, compared to wild-type xylanase. Among tested acceptors, transxylosylation catalysed by mutant W126A was improved with benzyl alcohol leading to a 2-fold higher concentration of benzyl xylobioside, as predicted by in silico mutation. This improved transxylosylation in the presence of benzyl alcohol leading to higher synthesis of benzyl xylobioside could likely be explained by lowest steric hindrance in the aglycone subsite of the mutated xylanase. No secondary hydrolysis of benzyl xylobioside occurred for both wild-type and mutant xylanases. Finally, our results demonstrated that the modelling approach was limited and that accounting for protein dynamics can lead to improved models.

Structure and dynamics of the peptide strand KRFK from the thrombospondin TSP-1 in water.

Theoretical investigations of a solute in liquid water at normal temperature and pressure can be performed at different levels of theory. Static quantum calculations as well as classical and ab initio molecular dynamics are used to completely explore the conformational space for large solvated molecular systems. In the classical approach, it is essential to describe all of the interactions of the solute and the solvent in detail. Water molecules are very often described as rigid bodies when the most commonly used interaction potentials, such as the SPCE and the TIP4P models, are employed. Recently, a physical model based upon a cluster of rigid water molecules with a tetrahedral architecture (AB4) was proposed that describes liquid water as a mixture of both TIP4P and SPCE molecular species that occur in the proportions implied by the tetrahedral architecture (one central molecule versus four outer molecules; i.e., 20% TIP4P versus 80% SPCE molecules). In this work, theoretical spectroscopic data for a peptide strand were correlated with the structural properties of the peptide strand solvated in water, based on data calculated using different theoretical approaches and physical models. We focused on a particular peptide strand, KRFK (lysine-arginine-phenylalanine-lysine), found in the thrombospondin TSP-1, due to its interesting properties. As the activity and electronic structure of this system is strongly linked to its structure, we correlated its structure with charge-density maps obtained using different semi-empirical charge Qeq equations. The structural and thermodynamic properties obtained from classical simulations were correlated with ab initio molecular dynamics (AIMD) data. Structural changes in the peptide strand were rationalized in terms of the motions of atoms and groups of atoms. To achieve this, conformational changes were investigated using calculated infrared spectra for the peptide in the gas phase and in water solvent. The calculated AIMD infrared spectrum for the peptide was correlated with static quantum calculations of the molecular system based on a harmonic approach as well as the VDOS (vibrational density of states) spectra obtained using various classical solvent models (SPCE, TIP4P, and AB4) and charge maps.

Molecular dynamics of the salt dependence of a cold-adapted enzyme: endonuclease I.

The effects of salt on the stability of globular proteins have been known for a long time. In the present investigations, we shall focus on the effect of the salt ions upon the structure and the activity of the endonuclease I enzyme. In the present work, we shall focus on the relationship between ion position and the structural features of the Vibrio salmonicida (VsEndA) enzyme. We will concentrate on major questions such as: how can salt ions affect the molecular structure? What is the activity of the enzyme and which specific regions are directly involved? For that purpose, we will study the behaviour of the VsEndA over different salt concentrations using molecular dynamics (MD) simulations. We report the results of MD simulations of the endonuclease I enzyme at five different salt concentrations. Analysis of trajectories in terms of the root mean square fluctuation (RMSF), radial distribution function, contact numbers and hydrogen bonding lifetimes, indicate distinct differences when changing the concentration of NaCl. Results are found to be in good agreement with experimental data, where we have noted an optimum salt concentration for activity equal to 425 mM. Under this salt concentration, the VsEndA exhibits two more flexible loop regions, compared to the other salt concentrations. When analysing the RMSF of these two specific regions, three residues were selected for their higher mobility. We find a correlation between the structural properties studied here such as the radial distribution function, the contact numbers and the hydrogen bonding lifetimes, and the structural flexibility of only two polar residues. Finally, in the light of the present work, the molecular basis of the salt adaptation of VsEndA enzyme has been explored by mean of explicit solvent and salt treatment. Our results reveal that modulation of the sodium/chloride ions interaction with some specific loop regions of the protein is the strategy followed by this type of psychrophilic enzyme to enhance catalytic activity at the physiological conditions.

Analysis of genetic diversity at the iron-containing superoxide dismutase locus in Plasmodium falciparum wild isolates.

In order to investigate the genetic diversity of iron-containing superoxide dismutase (FeSOD) from Plasmodium falciparum, a potential anti-malarial therapeutic target, we cloned and sequenced Plasmodium FeSOD from 26 blood samples from non-infected patients. Fifteen clones had the same nucleotide sequence as that of the FeSOD gene of the P. falciparum strain HB3 cultivated in vitro. The other 11 clones presented mutations responsible for punctual amino acid changes which did not modify key residues for the function or the structure of the enzyme. The high sequence conservation between FeSOD from the isolates confirms that this enzyme could represent a therapeutic target.

Cloning and expression of an iron-containing superoxide dismutase in the parasitic protist, Trichomonas vaginalis.

A superoxide dismutase (SOD) gene of the parasitic protist Trichomonas vaginalis was cloned, sequenced, expressed in Escherichia coli, and its gene product characterized. It is an iron-containing dimeric protein with a monomeric mass of 22,067 Da. Southern blots analyses suggested the presence of seven iron-containing (FeSOD) gene copies. Hydrophobic cluster analysis revealed some peculiarities in the 2D structure of the FeSOD from T. vaginalis and a strong structural conservation between prokaryotic and eukaryotic FeSODs. Phylogenetic reconstruction of the SOD sequences confirmed the dichotomy between FeSODs and manganese-containing SODs. FeSODs of protists appeared to group together with homologous proteobacterial enzymes suggesting a possible origin of eukaryotic FeSODs through an endosymbiotic event.

Solution structure of R-elafin, a specific inhibitor of elastase.

The solution structure of r-elafin, a specific elastase inhibitor, has been determined using NMR spectroscopy. Characterized by a flat core and a flexible N-terminal extremity, the three-dimensional structure is formed by a central twisted beta-hairpin accompanied by two external segments linked by the proteinase binding loop. A cluster of three disulfide bridges connects the external segments to the central beta-sheet and a single fourth disulfide bridge links the binding loop to the central beta-turn. The same spatial distribution of disulfide bridges can be observed in both domains of the secretory leukocyte protease inhibitor (SLPI), another elastase inhibitor. The structural homology between r-elafin and the C-terminal domain of SLPI confirms the former as a second member of the chelonianin family of proteinase inhibitors. Based on the homology between the two proteins and recent results obtained for elastase binding mutants of the bovine pancreatic trypsin inhibitor (BPTI), we define the segment 22 to 27 as the binding loop of elafin, with the scissile peptide bond between Ala24 and Met25. In our solution structures, this loop is extended and solvent-exposed, and exhibits a large degree of flexibility. This mobility, already observed for the binding loop in other protease inhibitors in solution, might be an important feature for the interaction with the corresponding protease.

Base sequence criteria and Cartesian coordinates for stable B/Z and B/Z/B junctions in relaxed DNA.

It seems increasingly evident that if the Z form of DNA exists in the genome it must exist as short sections of alternating pyrimidine-purine sequences in the midst of very long sections of B-form DNA. We have determined the minimum length of a string of alternating CG base pairs that can go into the Z form in the middle of a long B form. Self-complimentary oligomers of the form T(M)(CG)(N)A(M) were synthesized. The conformation of the resulting duplex was determined in 6M aqueous NaCl solution by Raman scattering. We have found that 12 alternating CG base pairs is the minimum length required to form a stable Z form of DNA inside of a long B form section. Only the 4 center CG base pairs go into the Z form. These 4 CG base pairs in the Z form are flanked on each side by 4 CG base pairs in a non-Z (probably B) form as well as the ..TT.. ..AA.. sequences in the B form. We propose a model of the B/Z junction in which the double helix flips directly from the B form to the Z form so that there are no base pairs in the junction. In this model the B form is nucleated in the AT base pairs on each end and is propagated into the CG base pairs in the center. This model is supported by isotopic H/D exchange experiments that shows that the H/D exchange of the non-Z form CG base pairs is highly retarded and indicates that they remain in the B form. A Thermodynamic analysis of the concentration dependence of the melting point of the duplexes in both low and high salt, supports our model and rules out the possibility of hairpin formation. The enthalpy for the formation of a B/Z junction is determined to be about +16 kcal/junction. A comparison of these results with recent results on B/Z junctions in super-coiled DNA is given. Molecular modeling calculations permit us to obtain values for the coordinates and torsional angles of the oligomers showing both B/Z and B/Z/B junctions. The Cartesian coordinates for these oligomers as well as stereo figures of these models in color are available from the authors.

Molecular dynamics simulations of a monofucosylated biantennary glycan of the N-acetyllactosamine type: the human lactotransferrin glycan.

Molecular dynamics simulations were carried out to explore the conformational flexibility of the antennae of N-linked glycans. They were performed over 200 ps in vacuo on the complete disialylated monofucosylated biantennary glycan of the N-acetyllactosaminic type. Starting from a bird-conformation, the 3-D-structure evolved through 9 successive transitional states to a new, compact and energetically favorable conformation which had never been previously described. In this conformation, both antennae are organized in two coplanar loops rolled in a contrary direction and oriented perpendicularly to the plane of the di-N-acetyl chitobiose residue leading to a 'lobster conformation'. All the glycosidic linkages of the disialylated monofucosylated biantennary glycan, except the Fuc(alpha 1-6)GlcNAc(beta 1-), were modified by a phase transition. Particularly, the Man(beta 1-4) GlcNAc(beta 1-) linkage, which was previously described by NMR and X-ray diffraction as a rigid one, was involved in numerous conformational changes during 83 ps, even before the first transition phase. The freedom of mobility of the torsional angles of the Man(alpha 1-6)Man(beta 1-) linkage was limited, under these simulation conditions, to the angle psi which took three values: 30 degrees, 90 degrees and 180 degrees. Moreover, from 150 ps up to the end of the simulation, the value of the torsional angle omega of the NeuAc(alpha 2-6)Gal(beta 1-) linkage of the alpha-1,6-antenna continuously swung between 60 degrees and -60 degrees. Finally, we observed that the values of the torsional angles of the three linkages: NeuAc(alpha 2-6)Gal(beta 1-), Gal(beta 1-4)GlcNAc(beta 1-) and GlcNAc(beta 1-2)Man(beta 1-) of each of the two antennae were different, demonstrating their asymmetric conformation.

Molecular modeling of a disialylated monofucosylated biantennary glycan of the N-acetyllactosamine type.

The conformations of a disialylated monofucosylated biantennary glycan of the N-acetyllactosamine type were analysed using the Tripos 5.3 force field from the Sybyl software currently used for molecular modelling. The conformation of each glycosidic linkage was calculated when included in oligosaccharide structures of up to 5 units and the influence of the glycosidic environment on the overall structure was measured. The study clearly shows that the conformation of a branched glycan cannot result from the simple addition of the different low energy conformers of each of the glycosidic linkages constituting the glycan structure. The asymmetrical conformation of the two antennae was demonstrated. The lowest energy conformations of the overall glycan structure were built and classified into 5 main models: the Y, T, bird and broken wing conformations already described and a new one called the 'back folded wing conformation.'

[Molecular modelling of glycans: three-dimensional structure and protein fraction interaction. The example of rabbit sero-transferrin]. / Modélisation moléculaire des glycannes: structure tridimensionnelle et interaction avec la fraction protéique. L'exemple de la sérotransferrine de lapin.

On the basis of experimental data and of computer calculations using the Tripos 5.3 force field in order to examine the three-dimensional structures which are sterically feasible and the conformations which are energetically the most favourable, we have designed a program of molecular modelling of biantennary glycans of the N-acetyllactosaminic type (complex type). We demonstrate that, in absence of any interaction with the protein, a high number of glycan conformations exists which can be classified into five basic conformations, four of which have already been described. In fact, in addition to the Y-, T-, "bird" and "broken-wing" conformations, a "back-folded wing" conformation is energetically feasible. In contrast, the glycan linked to the protein is immobilized into only one conformation: the "broken-wing" conformation. Forming a bridge between the two lobes of the peptide chain, it probably contributes to the maintenance of the protein in a biologically active conformation.