Energetics for displacing a single chain from the surface of microcrystalline cellulose into the active site of Acidothermus cellulolyticus Cel5A.

A series of molecular mechanics calculations were used to analyze the energetics for moving a single polysaccharide chain from the surface of microcrystalline cellulose into the binding cleft of the Cel5A cellulase from Acidothermus cellulolyticus. A build-up procedure was used to model the placement of a 12-residue oligosaccharide chain along the surface of the enzyme, using as a guide the four residues of the tetrasaccharide substrate co-crystallized with the protein in the crystallographic structure determination. The position of this 12-residue oligosaccharide was used to orient the enzyme properly above two different surfaces of cellulose 1beta, the (1,0,0) and the (1,1,0) faces of the crystal. Constrained molecular dynamics simulations were then used to pull a target chain directly below the enzyme up out of the crystal surface and into the binding groove. The energetics for this process were favorable for both faces, with the step face being more favorable than the planar face, implying that this surface could be hydrolyzed more readily.

Reversible dehydration of trehalose and anhydrobiosis: from solution state to an exotic crystal?

Physico-chemical properties of the trehalose-water system are reviewed with special reference to the transformations that may shed light on the mechanism of trehalose bio-protection. Critical analysis of solution thermodynamics is made in order to scrutinize trehalose properties often called 'anomalous' and to check the consistency of literature results. Discussion on the conversion between the solid state polymorphic forms is given, with a special emphasis of the transformations involving the newly identified anhydrous crystalline form of alpha,alpha-trehalose, TRE(alpha). This exotic crystal is almost 'isomorphous' with the dihydrate crystal structure, and possesses the unique feature of reversibly absorbing water to produce the dihydrate, without changing the main structural features. The reversible process could play a functional role in the well-known ability of this sugar to protect biological structures from damage during desiccation. The final aim of the paper is to add some new insights into and to reconcile previous hypotheses for the peculiar 'in vivo' action of trehalose.