Thesis (M.Sc.)--Chulalongkorn University, 2003

Cassava starch is one of the major raw material used in many industries. Starch is composed of the polysaccharides, amylose and amylopectin, which were synthesized by a group of enzymes including starch branching enzyme (SBE). Starch branching enzyme involved amylopectin synthesis by catalyzing formation of alpha-1, 6-glucosyl-alpha-1,4-glucan. There are two classes of, class A preferentially branches amylopectin whereas class B preferentially branch amylose. In this study, starch branching enzyme was monitored in cassava tubers from two cultivars, Rayong 1 and KU 50. SBE activity rapidly increased around 9 months old. Purification and isoform studies were performed with tubers from 9 months old KU 50. The crude enzyme was purified by precipitation with 10% (w/v) polyethylene glycol, followed by DEAE-Toyopearl column which could separate SBE into 3 isoforms. Hitrap Q-sepharose column was further used to purify each isoform which resulted in purities of 125, 77 and 250. All isoform showed molecular weight of 57 kDa on Sephacryls HR S-200. Isoform 1 separated by SDS- PAGE, into 2 bands with molecular weight 108 and 57 kDa whereas isoform 2 and 3 showed single band with molecular weight of 57 and 60 kDa. The pI of the 3 isoform was 4.9, 4.9 and 5.0, respectively. Optimum pH{7f2019}s were 8.0, 6.5, 7.5 and optimum temperatures were 37, 37 and 30 ํC for isoform 1, 2 and 3, respectively. All isoforms were stable up to 40 ํC. The K[subscript m]'s for amylose were 1.12, 1.37 and 2.17 mg/ml. Isoform 1 and 2 were more active than isoform 3 towards amylose as substrate, whereas isoform 3 was more active than isoforms 1 and 2 with amylopectin as substrate. All isoforms were absolutely inhibited by divalent metal ion.