Glycoside hydrolases active on microbial exopolysaccharide α-glucans: structures and function

Abstract

Glucose is the most abundant monosaccharide in nature and is an important energy source for living organisms. Glucose exists primarily as oligomers or polymers and organisms break it down and consume it. Starch is an important plant-derived α-glucan in the human diet. The enzymes that degrade this α-glucan have been well studied as they are ubiquitous throughout nature. Some bacteria and fungi produce α-glucans with different glucosidic linkages compared with that of starch, and their structures are quite complex and not fully understood. Compared with enzymes that degrade the α-(1→4) and α-(1→6) linkages in starch, biochemical and structural studies of the enzymes that catabolize α-glucans from these microorganisms are limited. This review focuses on glycoside hydrolases that act on microbial exopolysaccharide α-glucans containing α-(1→6), α-(1→3), and α-(1→2) linkages. Recently acquired information regarding microbial genomes has contributed to the discovery of enzymes with new substrate specificities compared with that of previously studied enzymes. The discovery of new microbial α-glucan-hydrolyzing enzymes suggests previously unknown carbohydrate utilization pathways and reveals strategies for microorganisms to obtain energy from external sources. In addition, structural analysis of α-glucan degrading enzymes has revealed their substrate recognition mechanisms and expanded their potential use as tools for understanding complex carbohydrate structures. In this review, the author summarizes the recent progress in the structural biology of microbial α-glucan degrading enzymes, touching on previous studies of microbial α-glucan degrading enzymes.

Bacteroidota polysaccharide utilization system for branched dextran exopolysaccharides from lactic acid bacteria

“…According to the CAZy database ( http://www.cazy.org/ ) ( 35 ), dextranases belong to GH families 31, 49, and 66 (GH31, GH49, and GH66) based on amino acid sequence similarity. Exo-acting dextranases are found in GH13, GH15, GH27, and GH49 families and act on the nonreducing end of dextran to generate glucose, isomaltose (IG2), or isomaltotriose (IG3) (IG n , isomaltooligosaccharide with degree of polymerization of n ) (reviewed in ( 36 )). Dextranase treatment is used in the sugar industry to prevent the loss of sucrose yield from harvested sugarcane due to the formation of dextran by microorganisms ( 37 ).…”