Phytoglycogens branching enzyme

Phytoglycogen-branching enzyme has been found in du however, no phytoglycogen was isolated by Black et al.23 Preiss and Boyer396 reported that the du mutation lowered starch synthase II activity and also lowered branching enzyme Ha activity. Gao et al.397 used a molecular approach to clone the du gene in maize endosperms and, based on amino acid sequence similarity of the predicted protein product with the soluble starch synthase III of potato,398 concluded that du most likely encodes the 180 000 molecular weight, primer-dependent soluble starch synthase described previously.399,400,401... 

The mature kernel phenotype of du su wx is similar to that of su (Table 3.3). The quantity of sugars is similar to that in su.210 212 Addition of du to su wx causes an increase in WSP (Table 3.5) and a decrease in starch content.272 The phytoglycogen from du su wx has a (3-amylolysis limit and chain length similar to that of su. The enhanced phytoglycogen accumulation may result from the additive effect of the branching enzymes present in each of the component single mutants.23... 

Formation of the a-( 1— 6) linkage branch points present in amylopectin and phytoglycogen is catalyzed by the branching enzyme (EC 2.4.1.18 Bourne and Peat, 1945 Hobson et al., 1950), also called the Q enzyme. 

Two types of branching enzymes have been identified in some plants, one that converts amylose into amylopectin, and the other that converts amylopectin into phytoglycogen [104]. The difference between the two enzymes apparently is in the specificity of the number of glucose residues between the branch points that each enzyme permits. 

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