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Molecular dynamics simulations of the tetradecasaccharide XXXGXXXG in complex with the hybrid aspen xyloglucan endo-transglycosylase PttXET16-34 have been performed and analysed with respect to structure, dynamics, flexibility and ligand interactions. Notably, the charge state of the so-called ‘helper residue’ aspartate 87 (Asp87), which lies between the catalytic nucleophile [glutamate 85 (Glu85)] and general acid/base (Glu89) residues on the same beta strand, had a significant effect on PttXET16-34 active site structure. When Asp87 was deprotonated, electrostatic repulsion forced the nucleophile away from C1 of the sugar ring in subsite − 1 and the proton–donating ability of Glu89 was also weakened due to the formation of a hydrogen bond with Asp87, whereas the protonation of Asp87 resulted in the formation of a hydrogen bond with the catalytic nucleophile and correct positioning of the catalytic machinery. The results suggest that catalysis in glycoside hydrolase family 16, and by extension clan GH-B e...

Molecular dynamics simulations of hexapeptides TFDWMK and LFPWMR; the highly conserved regions of Hox proteins Hox B1 and Hox B8, respectively, are carried out starting from extended structures to investigate their conformational space in water solution. In addition, we have studied TADWMK and TADAMK, where the aromatic residues Phenylalanine and Tryptophan were successively substituted for Alanine to investigate effects from the presence/absence of aromatic amino acids and interactions between them to folding behavior. The backbone of the hexapeptides in all simulations folds to a similar conformation found in experimental studies in solution. Intramolecular, hydrophobically driven interactions between the aromatic residues and internal hydrogen bonds are found to stabilize the conformations.

MD simulations of Hox protein N-terminal hexapeptides TFDWMK (Hox B1) and LFPWMR (Hox B8) are performed in water solution and complemented with simulations where the aromatic residues phenylalanine (F) and tryptophan (W) are successively replaced by alanine (A). Results from this study give support that different hexapeptides can form similar folded structures in water, stabilized mainly by internal hydrogen bonding where the arrangement of the aromatic side chains together with the methionine (M) side chain forming a hydrophobic core covers and protects the internal hydrogen bonds from water. Replacement of the aromatic side chains with Alanine did not lead to unfolding, but rather the hexapeptides were slightly changing their conformations where the Methionine side chain together with the peptide backbone protected the internal hydrogen bonds and the hexapeptides remain folded. Our results give support that these hexapeptides are able to remain folded to some extent even without the aromatic side chains.

Molecular dynamics simulations of a single dipeptide (Ala-Pro) molecule in water were carried out using different water models, the modified TIP3P (transferable intermolecular potential 3P), the refined SPC (simple point charge), and the original SPC/E (extended simple point charge). Both trans and cis isomers of the dipeptide were simulated, and conformations from simulations with the different water models were compared. Both isomers have several conformations, but the total conformational space is limited. Two experimentally obtained subconformations of the cis isomer were found in simulations with the different water models. The bioactive conformation, which is binding to Cyclophilin A, was the major cis conformation, and the conformation with an intramolecular hydrogen bond between the N-terminal and the C-terminal was found to exist as the minor cis conformation. The hydration of the dipeptide was found to depend both on its conformation and on the water model.

Reorganization and degradation of the wall crosslinking and seed storage polysaccharide xyloglucan by glycoside hydrolase family 16 (GH16) endo-transglycosylases and hydrolases are crucial to the growth of the majority of land plants, affecting processes as diverse as germination, morphogenesis, and fruit ripening. A high-resolution, three-dimensional structure of a nasturtium (Tropaeolum majus) endo-xyloglucanase loop mutant, TmNXG1-DeltaYNIIG, with an oligosaccharide product bound in the negative active-site subsites, has been solved by X-ray crystallography. Comparison of this novel complex to that of the strict xyloglucan endo-transglycosylase PttXET16-34 from hybrid aspen (Populus tremula x tremuloides), previously solved with a xylogluco-oligosaccharide bound in the positive subsites, highlighted key protein structures that affect the disparate catalytic activities displayed by these closely related enzymes. Combination of these "partial" active-site complexes through molecular dynamics simulations in water allowed modeling of wild-type TmNXG1, TmNXG1-DeltaYNIIG, and wild-type PttXET16-34 in complex with a xyloglucan octadecasaccharide spanning the entire catalytic cleft. A comprehensive analysis of these full-length complexes underscored the importance of various loops lining the active site. Subtle differences leading to a tighter hydrogen bonding pattern on the negative (glycosyl donor) binding subsites, together with loop flexibility on the positive (glycosyl acceptor) binding subsites appear to favor hydrolysis over transglycosylation in GH16 xyloglucan-active enzymes.

Six different molecular dynamics simulations of Cyclophilin A, three with the protein free in water and three with the Ala-Pro dipeptide bound to the protein, have been performed, and analysed with respect to structure and hydration of the active site. The water structure in the binding pocket of the free Cyclophilin A was found to mimic the experimentally obtained binding cis conformation of the dipeptide. Cyclophilin A is a peptidyl–prolyl cis–trans isomerase (PPIase), but the mechanism of the cis/trans isomerization is not exactly clear. This study was performed to understand better the binding between dipeptide and Cyclophilin A, but also two previously proposed isomerization mechanisms are discussed.