Debranching enzyme characterization

Chen YT, He JK, Ding JH, Brown BI (1987) Glycogen debranching enzyme purification, antibody characterization, and immunoblot analysis of type III glycogen storage disease. Am J Hum Genet 41 1002-1015... 

The glycogen debranching enzyme is the first bifunctional eukaryotic enzyme to be reported that consists of a single polypeptide chain. It catalyzes two distinct activities an oligosaccharide frons-glycosylation followed by hydrolysis of an (1—> 6)-linked D-glucosyl unit to liberate free glucose. Physical-chemical and kinetic characterization of this novel bifunctional enzyme is described by Nelson. 

These reactions do not have to occur in perfect sequence, and the phases may have some overlap (e.g., phases 2, 3, and 4 may overlap, and possibly even 5 and 6). However, the present evidence, such as intermediate products formed by starch mutants of C. reinhardtii and of higher plants, supports the sequence of reactions shown in Fig. 1 for amylopectin and amylose biosynthesis. Further experiments are required to test this hypothetic scheme, and attempts to purify and characterize the debranching enzyme, crucial to this hypothesis, are under way. 

A C. reinhardtii mutant lacking D-enzyme activity has been characterized and has been shown to have significantly lower levels of starch. Other enzymes involved in starch metabolism such as ADP-Glc PPase, granule-bound and soluble starch synthase, BE, phosphorylase, a-glucosidase, amylases, and debranching enzyme activities were not affected. The starch content in the mutant was about 6-13% of wild type and there was an excessive accumulation of maltooligosaccharides up to a polymer size of 16 glucose units. 

Li, B, JC Servaites and DR Geiger (1992). Characterization and subcellular localization of debranching enzyme and endoamylase from leaves of sugar beet. Plant Physiology, 98(4), 1277-1284. 

---