This chapter focuses on glycogen and starch debranching enzymes. The debranching enzymes can be divided into two classes—namely, direct debranching enzyme and indirect debranching enzymes. A “direct” debranching enzyme attacks unmodified glycogen or amylopectin, with hydrolysis of the 1→ 6 bond. For “indirect” debranching, the polymer must first be modified by another enzyme or enzymes. The direct debranching enzyme simply hydrolyzes the 1→ 6 bond in the substrate. If it is an A chain that is being split off, the minimum number of glucose units in that chain must be two, and for the best characterized debranching enzyme, Aerobacter aerogenes pullulanase, the (C) chain carrying the A chain must also have a minimum of two 1 → 4-bonded glucose units. Thus, the smallest substrate for pullulanase is the linear tetrasaccharide 62-α-maltosylmaltos. The indirect debranching system is associated with the occurrence of glycogen in mammals and the direct debranching enzyme with starch in plants. Direct debranching enzymes have not been reported in mammals or the indirect variety in plants. Microorganisms collectively contain all types, and yeast seems to have both the indirect system and isoamylase.
ASJC Scopus subject areas
- Molecular Biology