Glycogen debranching pathway deduced from substrate specificity of glycogen debranching enzyme

Glycogen debranching enzyme (GDE) is bifunctional in that it exhibits both 4-α-glucanotransferase and amylo-α-1,6-glucosidase activity at two distinct catalytic sites. GDE converts the phosphorylase-limit biantennary branch [G-G-G-G-(G-G-G-G↔)G-G- residue, where G = D-glucose, hyphens represent α-1,4-glycosidic bonds, and the double-headed arrow represents an α-1,6-glycosidic bond] into a linear maltooligosyl residue, which is then subjected to phosphorylase, and glycogen degradation continues. The prevailing hypothesis regarding the glycogen debranching pathway was that 4-α-glucanotransferase converts the phosphorylase-limit biantennary branch into the G-G-G-G-G-G-G-(G↔)G-G- residue and amylo-α-1,6-glucosidase cleaves the remaining α-1,6-linked G residue. In the present study, we analyzed the substrate specificities of 4-α-glucanotransferase and amylo-α-1,6-glucosidase using fluorogenic biantennary dextrins such as G-G-G-G-(G-G-G-G↔)G-G-GPA (F4/4/2; where GPA = 1-deoxy-1-[(2-pyridyl)amino]-D-glucitol), G-(G-G-G-G↔)G-G-GPA (F1/4/2), and G-G-G-G-G-G-G-(G↔)G-G-GPA (F7/1/2). Contrary to the prevailing hypothesis, the main branch of F4/4/2 was an important donor substrate component of 4-α-glucanotransferase and did not serve as an acceptor substrate. However, when G-G-G-G-G-GPA was added to the mixture, it successfully accepted a maltotriosyl (G₃-) residue from F4/4/2. In addition, amylo-α-1,6-glucosidase exhibited strong activity towards G-G-G-G-(G↔)G-G-GPA but weak activity towards F7/1/2. Furthermore, the debranching activity of GDE towards phosphorylase-limit glycogen substantially increased when methyl α-maltooligosides with lengths equal to or greater than that of methyl α-maltopentaoside (G₅-OCH₃) were added to the enzyme reaction mixture. Based on these results, we propose the following macroscopic debranching pathway: Via 4-α-glucanotransferase, the G₃- residue of the donor branch is transferred to a long (n ≥ 5) linear G_n- residue linked to a different branching G residue.