Structure and Function of Branching Enzymes in Eukaryotes

Abstract

Brancihing enzymes (BEs) catalyze the transglycosylation reactions to form a new branching point consisting of an α-1,6-glucosidic linkage by cleaving an α-1,4-glucosidic linkage of an α-glucan chain. BEs are ubiquitous throughout the prokaryotes but are only found in limited taxa in the eukaryotes. Prokaryotic and eukaryotic BEs are classified into distinct families based on similarities in their primary structures. Land plants have multiple BE isoforms, whose function in vivo has been identified using mutants. Recent studies have revealed that plant BEs form protein–protein complexes with related enzymes involved in α-glucan metabolism in vivo. Although their biological significance is unclear, it is proposed that they are essential for efficient amylopectin biosynthesis. Crystal structures of BEs from eukaryotes (rice and human) are now available. These two structures have the same domain organization, containing a catalytic domain common to all members of glycoside hydrolase family 13. Recently, the authors have reported the structures of BE from a cyanobacterium (a photosynthetic prokaryote) in complex with maltooligosaccharides at the active site. On the other hand, little is known about the structure–function relationship of eukaryotic BEs. This review describes recent progress in research on the structure and function of eukaryotic BEs.