Branching Enzyme
	Starch branching enzyme (SBE) synthesizes amylopectin (AP) from amylose (AM). It catalyzes the cleaving alpha-1, 4- glucosidic bond and reattaches it to another point with the alpha-1, 6-glucosidic bond. It was formerly believed that the hydrolytic and transfer activities might be separate in different enzymes. However, Borovsky et al. (1975) showed that both activities derived from a single enzyme. SBEs have been roughly divided into two isozymes (Family A and Family B) based on their primary structures. Although the enzymatic characteristics between the isozymes varied in their sources, the chain length transferred by SBEs belonging to family B is commonly longer than that of family A. The APs produced by SBE isozymes are stained as a bluish (Family A) and reddish (Family B) color with iodine, respectively. Moreover, the two isozymes show some different properties, substrate affinity and preference, and influence on effectors. This suggests that the isozymes are individually controlled by distinct factors in vivo.
	Additionally, the activity behavior on temperature is also interesting. The maximum activities of kidney bean isozymes (KBE1 and KBE3) occur at around 25~300C, but they are stable till 400C. These facts suggest that the structural changes of AM with increase in temperature strongly affect the enzyme activity. The transferred chain length decreases at low temperatures and it is responsible for change in the AM helix. The structure of AP produced may also influence temperature, since the activity ratio of both isozymes varies greatly around the optimum temperature. In rice and maize, there is a mutant, called amylose-extender (ae), lacking family A isozymes. Their accumulated starch shows low viscosity. On the other hand, it has never been reported that the mutant deletes either family B or both isozymes. Most plants have both SBE isozymes (Family A and Family B), while organisms accumulating glycogen have a single sort of glycogen branching enzyme. This suggests that the combined action of both isozymes may be essential for the formation of the orderly specific cluster structure of AP. However, the relationship between the two isozymes has not been clarified. By way of additional comment,the wrinkle seeds obtained by Mendel in his famous hereditary experiments may have been the result of enzyme inactivation by a transposon insertion in the gene of SBE (Family A).
	Kouichi Nozaki (Faculty of Engineering, Shinshu University) Shigeki Hamada (Graduate School of Agriculture, Hokkaido University)
	References (1) D, Borvsky et al., Eur. J. Biochem. 59, 615, 1975 (2) K, Mizuno et al., J. Biochem.112, 643-651, 1992 (3) R A, Burton et al., Plant J., 7, 3-15, 1995 (4) M, Shure et al., Cell 35, 225-233, 1983
Dec. 15, 2000