Rotation of cellulose ribbons during degradation with fungal cellulase

Degradation of bacterial cellulose with a commercial cellulase, Celluclast 1.5 L (Novo Nordisk), from the fungus Trichoderma reesei, causes a rotational movement of the cellulo:,c microfibrils. Purified cellulases (CBH I, CBH II, and EG II) do not induce rotation of bacterial cellulose, however, ratios of CBH I and EG II do cause rotation of bacterial cellulose. Equimolar amounts of CBH r or CBH II and EG II do not result in motion during degradation. Based on these observations, we provide fmiher evidence supporting, at least on theoretical grounds, the hypothesis that cellulose chains have intrinsic chirality. As the cellulase enzymes interact with and degrade the cellulose fibrils, the crystalline structure of the cellulose is altered, allowing the linear cellulose polymers to relax into a lower energy state, thus relieving the strain induced by crystallization of the nascent ~-glucan chains during the biogenesis of the microfibril. This conversion of crystalline bacterial ribbons into more relaxed confonnations produces the rOlation observed during the treatment of bacterial cellulose with cellulase. Introduction ates a helical conformation for ~-l, 4 glucans (Ritcey & Gray, 1988). Furthermore, J3C NMR spectroscopy Cellulose has revealed that the ~-l, 4 linkages between glucose residues in cellulose derivatives adopt a 35° conform­ Cellulose, the most abundant macromolecule, is a ho­ ation when in free solution (Buchanan et al., 1989), mopolymer of ~-l, 4-linked glucose molecules. High suggesting that the glucan chains appear to be most resolution electron microscopy of negatively stained relaxed as a 5/4 helix. cellulose in the model system. Acelobacter xyfinum In native crystalline cellulose, the van del' Waals (White & Brown, 1981), has revealed a hierarchical forces and hydrogen bonding between the glucan association of 50-80 microfibrils (3.0-3.5 nm) thal as­ chains may be holding the crystal together despite con­ sociate into flat structures 40-60nm wide known as formational hindrances. Hence, the crystalline native ribbons. The ribbons are highly twisted, with one 180 cellulose I allomorph can be thought of as being under turn every ~700 nm (White & Brown, 1981; Hirai constant intemal stress. el af., 1998). The bacteria rotate during the synthesis Use of Tinopal can cause the formation of a mini­ of cellulose I ribbons. sheet in the form of a closed tube-like structure (Cous­ Careful analysis of shadowed cellulose microllbrils ins & Brown, 1997; Haigler & Chanzy, 1988). When synthesized by Acetobacter xylinum have shown them the dye molecules are subsequently photoisomerized to be right-handed helices (Hirai et at., 2000). Cel­ by intense UV illumination. they lose their afllnily for lulose microllbrils of Micrasterias denticuLata have the cellulose, causing the single glucan chain sheets to also been shown to be right-handed by AFM, TM­ collapse into microfibrils (Cousins & Brown, 1997). AFM, and TEM, with 700 nm intervals between twists During this process, the cellulose undergoes a massive (Hanley el at., 1997) Induced circular dichroism of torsional motion. It is believed that this motion is the methylcellulose chains and cellulose oligomers indicresult of the final crystallization of the mini-sheets