Starch synthase characterization

Additional characterization of the two major soluble starch synthases in maize endosperm indicated distinct catalytic properties.402 Soluble starch synthase II has been reported to be encoded by the du gene.397 the initially observed reduction in BEIIa is a secondary effect and may be related to protein-to-protein interactions. 

Shure et al. (1983) prepared cDNA clones homologous to Wx mRNA. In subsequent experiments (Federoff et al., 1983), restriction endonuclease fragments containing part of the Wx locus were cloned from strains carrying the ac wx-M9, wa-M9, and wx-M6 alleles to characterize further the controlling insertion elements activator (ac) and dissociation (ds). Excision of the ds element from the certain wx alleles produces two new alleles (S5 and S9) that are encoding the wx proteins having altered starch synthase activities (Wessler et al., 1986). Two of these, S9 and S5, had 53 and 32% of the starch synthase activity, respectively, seen in the normal endosperm. Mutant... 

Dry, I., Smith, A., Edwards, A., Bhattarcharyya, M., Dunn, P., and Martin, C. 1992. Characterization of cDNAs encoding two isoforms of granule-bound starch synthase which show differential expression in developing storage organs of pea and potato. Plant J. 2,193-202. 

Macdonald, F. D., and Preiss, J. 1985. Partial purification and characterization of granule-bound starch synthases from normal and waxy maize. Plant. Physiol. 78, 849-852. 

Isozymes of plant starch synthases (2, 16-20) and branching enzymes (2, 16, 21-26) have been characterized. Presumably, they play distinct roles in amylopectin and amylose synthesis. Also, plant and Chlamydomonas reinhardtii granule-bound starch synthases (27-33) are responsible for synthesis of amylose. Mutants defective in this enzyme are known as waxy mutants and contain starch granules that have little or no amylose. 

As with starch synthases, many isozymes have been found for the Branching Enzyme, and they have been characterized in a number of plants. 

Harn C, Knight M, Ramakrishnan A, Guan H, Keeling PL, Wasserman BP. Isolation and characterization of the zSSIIa and zSSIIb starch synthase cDNA clones from maize endosperm. Plant Mol. Biol. 1998 37 639-649. 

A C. reinhardtii mutant lacking D-enzyme activity has been characterized and has been shown to have significantly lower levels of starch. Other enzymes involved in starch metabolism such as ADP-Glc PPase, granule-bound and soluble starch synthase, BE, phosphorylase, a-glucosidase, amylases, and debranching enzyme activities were not affected. The starch content in the mutant was about 6-13% of wild type and there was an excessive accumulation of maltooligosaccharides up to a polymer size of 16 glucose units. 

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