Amylopectin, branching enzymes

Recently, the in vitro synthesis of amylopectin- or glycogen-like structures via a tandem reaction of phosphorylase and branching enzyme was reported [197-201]. 

The first enzymic studies done on the dul mutant were carried out in 1981, and both SSII and starch-branching enzyme Ha (SBEIIa) were found to have reduced activity in the endosperm compared to normal maize endosperm.204 SSII was shown to be different from SSI.173,205 SSII requires a primer for activity, and could not catalyze an unprimed reaction even in the presence of 0.5 M citrate, it also has less affinity for amylopectin than does SSI. However, 0.5 M citrate lowered the Km for amylopectin 17-fold. The activity with glycogen as a primer is one-half that observed with amylopectin. Therefore, glycogen is not as effective as a primer as is amylopectin, which differs from what was observed for starch synthase I. Both maize endosperm SSI and SSII had a Km for ADPGlc of 0.1 mM.196,205... 

This mutation also presents another approach to obtaining a high-amylose starch. As will be described later, mutations of branching enzyme can lead to production of not only more amylose, but also of amylopectin molecules with fewer and longer chains, and having properties similar to those of amylose. 

Thus, the synthesis of amylopectin or starch is not the sum of the independent actions of the various starch synthase and branching enzyme isoforms, but is due to a complex combination of the activities of the various isoforms that vary during seed and leaf development due to varying expression of the different isozymes. Essentially, the same conclusions were reached previously.218... 

Assay procedures there are three accepted methods in assaying branching enzyme (BE). One measures the decrease in absorbance of the glucan-I3 complex that results from the branching of amylose or amylopectin. The assay mixture containing amylose or amylopectin is incubated and aliquots are taken and iodine reagent is added.35,247 The decrease of absorbance is measured at 660 nm for amylose and at 530 nm for amylopectin. 

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