On the distinct binding modes of expansin and carbohydrate-binding module proteins on crystalline and nanofibrous cellulose: implications for cellulose degradation by designer cellulosomes.

Transformation of cellulose into monosaccharides can be achieved by hydrolysis of the cellulose chains, carried out by a special group of enzymes known as cellulases. The enzymatic mechanism of cellulases is well described, but the role of non-enzymatic components of the cellulose-degradation machinery is still poorly understood, and difficult to measure using experiments alone. In this study, we use a comprehensive set of atomistic molecular dynamics simulations to probe the molecular details of binding of the family-3a carbohydrate-binding module (CBM3a) and the bacterial expansin protein (EXLX1) to a range of cellulose substrates. Our results suggest that CBM3a behaves in a similar way on both crystalline and amorphous cellulose, whereas binding of the dual-domain expansin protein depends on the substrate crystallinity, and we relate our computed binding modes to the experimentally measured features of CBM and expansin action on cellulose.

Molecular Engineering of Rigid Hydrogels Co-assembled from Collagenous Helical Peptides Based on a Single Triplet Motif.

The potential of ultra-short peptides to self-assemble into well-ordered functional nanostructures makes them promising minimal components for mimicking the basic ingredient of nature and diverse biomaterials. However, selection and modular design of perfect de novo sequences are extremely tricky due to their vast possible combinatorial space. Moreover, a single amino acid substitution can drastically alter the supramolecular packing structure of short peptide assemblies. Here, we report the design of rigid hybrid hydrogels produced by sequence engineering of a new series of ultra-short collagen-mimicking tripeptides. Connecting glycine with different combinations of proline and its post-translational product 4-hydroxyproline, the single triplet motif, displays the natural collagen-helix-like structure. Improved mechanical rigidity is obtained via co-assembly with the non-collagenous hydrogelator, fluorenylmethoxycarbonyl (Fmoc) diphenylalanine. Characterizations of the supramolecular interactions that promote the self-supporting and self-healing properties of the co-assemblies are performed by physicochemical experiments and atomistic models. Our results clearly demonstrate the significance of sequence engineering to design functional peptide motifs with desired physicochemical and electromechanical properties and reveal co-assembly as a promising strategy for the utilization of small, readily accessible biomimetic building blocks to generate hybrid biomolecular assemblies with structural heterogeneity and functionality of natural materials.