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Submitted to: Carbohydrate Polymers
Publication Type: Peer Reviewed Journal Publication Acceptance Date: 3/25/2021 Publication Date: N/A Citation: N/A Interpretive Summary: This study is based on experimental 3-dimensional atomistic structural data of molecules that are shorter versions of cellulose, the main molecule of cotton fibers as well as all plant cell walls. Usually, well-ordered cellulose structures of associated cellulose molecules (crystals) are studied. In the present work, they were observed in complexes with proteins, such as enzymes that synthesize cellulose or hydrolyze it. There are many more independent, detailed experimental observations of the structure in these protein complexes than in pure cellulose, although the protein complex structures are of lower quality. The variety of the different shapes gives insight into the possible structures of amorphous (non-crystalline) cotton and other cellulosic materials. It has long been said that cotton has both crystalline and amorphous regions. The structures in amorphous regions are not well understood, despite their critical importance as the most reactive phase of cellulose structure. The observed structures were compared with theoretical models to assess the ability of the models to predict such structures, as well as to provide guidance to the experimentalists who determine the structures of the protein complexes. Technical Abstract: Glycosidic linkage torsion angles phi and psi were assessed for cellooligosaccharides in complexes with proteins in the Protein Data Bank (PDB). These torsions were compared with Ramachandran phi/psi maps for cellobiose based on quantum mechanics (QM) energies, and maps for analogs without hydroxyl groups. While less accurately determined, it was expected that most PDB conformations would fall into the lowest-energy regions of the maps. An approximately exponential decline in the energies was observed, with a high temperature for the distribution, 812 K. Lactose moieties from the PDB were less well predicted by the cellobiose map. The minimum on a map based on the frequency of observations was slightly shifted from that of the QM map. The map for the analog with methyl groups was predictive. These maps show which structures might be of special interest or an experimental error. Also, the shapes of cellulose in amorphous regions are mostly extended. |
