Hydrophilic aromatic residue and in silico structure for carbohydrate binding module

The Importance of Surface-Binding Site towards Starch-Adsorptivity Level in α-Amylase: A Review on Structural Point of View

Starch is a polymeric carbohydrate composed of glucose. As a source of energy, starch can be degraded by various amylolytic enzymes, including α-amylase. In a large-scale industry, starch processing cost is still expensive due to the requirement of high temperature during the gelatinization step. Therefore, α-amylase with raw starch digesting ability could decrease the energy cost by avoiding the high gelatinization temperature. It is known that the carbohydrate-binding module (CBM) and the surface-binding site (SBS) of α-amylase could facilitate the substrate binding to the enzyme's active site to enhance the starch digestion. These sites are a noncatalytic module, which could interact with a lengthy substrate such as insoluble starch. The major interaction between these sites and the substrate is the CH/pi-stacking interaction with the glucose ring. Several mutation studies on the Halothermothrix orenii, SusG Bacteroides thetaiotamicron, Barley, Aspergillus niger, and Saccharomycopsis fibuligera α-amylases have revealed that the stacking interaction through the aromatic residues at the SBS is essential to the starch adsorption. In this review, the SBS in various α-amylases is also presented. Therefore, based on the structural point of view, SBS is suggested as an essential site in α-amylase to increase its catalytic activity, especially towards the insoluble starch.

Carbohydrate-aromatic interactions

The recognition of saccharides by proteins has far reaching implications in biology, technology, and drug design. Within the past two decades, researchers have directed considerable effort toward a detailed understanding of these processes. Early crystallographic studies revealed, not surprisingly, that hydrogen-bonding interactions are usually involved in carbohydrate recognition. But less expectedly, researchers observed that despite the highly hydrophilic character of most sugars, aromatic rings of the receptor often play an important role in carbohydrate recognition. With further research, scientists now accept that noncovalent interactions mediated by aromatic rings are pivotal to sugar binding. For example, aromatic residues often stack against the faces of sugar pyranose rings in complexes between proteins and carbohydrates. Such contacts typically involve two or three CH groups of the pyranoses and the π electron density of the aromatic ring (called CH/π bonds), and these interactions can exhibit a variety of geometries, with either parallel or nonparallel arrangements of the aromatic and sugar units. In this Account, we provide an overview of the structural and thermodynamic features of protein-carbohydrate interactions, theoretical and experimental efforts to understand stacking in these complexes, and the implications of this understanding for chemical biology. The interaction energy between different aromatic rings and simple monosaccharides based on quantum mechanical calculations in the gas phase ranges from 3 to 6 kcal/mol range. Experimental values measured in water are somewhat smaller, approximately 1.5 kcal/mol for each interaction between a monosaccharide and an aromatic ring. This difference illustrates the dependence of these intermolecular interactions on their context and shows that this stacking can be modulated by entropic and solvent effects. Despite their relatively modest influence on the stability of carbohydrate/protein complexes, the aromatic platforms play a major role in determining the specificity of the molecular recognition process. The recognition of carbohydrate/aromatic interactions has prompted further analysis of the properties that influence them. Using a variety of experimental and theoretical methods, researchers have worked to quantify carbohydrate/aromatic stacking and identify the features that stabilize these complexes. Researchers have used site-directed mutagenesis, organic synthesis, or both to incorporate modifications in the receptor or ligand and then quantitatively analyzed the structural and thermodynamic features of these interactions. Researchers have also synthesized and characterized artificial receptors and simple model systems, employing a reductionistic chemistry-based strategy. Finally, using quantum mechanics calculations, researchers have examined the magnitude of each property's contribution to the interaction energy.

Comparative genomic analysis of the microbiome [corrected] of herbivorous insects reveals eco-environmental adaptations: biotechnology applications

Metagenome analysis of the gut symbionts of three different insects was conducted as a means of comparing taxonomic and metabolic diversity of gut microbiomes to diet and life history of the insect hosts. A second goal was the discovery of novel biocatalysts for biorefinery applications. Grasshopper and cutworm gut symbionts were sequenced and compared with the previously identified metagenome of termite gut microbiota. These insect hosts represent three different insect orders and specialize on different food types. The comparative analysis revealed dramatic differences among the three insect species in the abundance and taxonomic composition of the symbiont populations present in the gut. The composition and abundance of symbionts was correlated with their previously identified capacity to degrade and utilize the different types of food consumed by their hosts. The metabolic reconstruction revealed that the gut metabolome of cutworms and grasshoppers was more enriched for genes involved in carbohydrate metabolism and transport than wood-feeding termite, whereas the termite gut metabolome was enriched for glycosyl hydrolase (GH) enzymes relevant to lignocellulosic biomass degradation. Moreover, termite gut metabolome was more enriched with nitrogen fixation genes than those of grasshopper and cutworm gut, presumably due to the termite's adaptation to the high fiber and less nutritious food types. In order to evaluate and exploit the insect symbionts for biotechnology applications, we cloned and further characterized four biomass-degrading enzymes including one endoglucanase and one xylanase from both the grasshopper and cutworm gut symbionts. The results indicated that the grasshopper symbiont enzymes were generally more efficient in biomass degradation than the homologous enzymes from cutworm symbionts. Together, these results demonstrated a correlation between the composition and putative metabolic functionality of the gut microbiome and host diet, and suggested that this relationship could be exploited for the discovery of symbionts and biocatalysts useful for biorefinery applications.

Two unique ligand-binding clamps of Rhizopus oryzae starch binding domain for helical structure disruption of amylose

The N-terminal starch binding domain of Rhizopus oryzae glucoamylase (RoSBD) has a high binding affinity for raw starch. RoSBD has two ligand-binding sites, each containing a ligand-binding clamp: a polyN clamp residing near binding site I is unique in that it is expressed in only three members of carbohydrate binding module family 21 (CBM21) members, and a Y32/F58 clamp located at binding site II is conserved in several CBMs. Here we characterized different roles of these sites in the binding of insoluble and soluble starches using an amylose-iodine complex assay, atomic force microscopy, isothermal titration calorimetry, site-directed mutagenesis, and structural bioinformatics. RoSBD induced the release of iodine from the amylose helical cavity and disrupted the helical structure of amylose type III, thereby significantly diminishing the thickness and length of the amylose type III fibrils. A point mutation in the critical ligand-binding residues of sites I and II, however, reduced both the binding affinity and amylose helix disruption. This is the first molecular model for structure disruption of the amylose helix by a non-hydrolytic CBM21 member. RoSBD apparently twists the helical amylose strands apart to expose more ligand surface for further SBD binding. Repeating the process triggers the relaxation and unwinding of amylose helices to generate thinner and shorter amylose fibrils, which are more susceptible to hydrolysis by glucoamylase. This model aids in understanding the natural roles of CBMs in protein-glycan interactions and contributes to potential molecular engineering of CBMs.