Other Enzymes Involved in Starch Synthesis

In addition to the three starch biosynthetic enzymes, other enzymes have been shown to have some effect on starch structure. 

As noted in Section 4.3 (see also Chapter 3), a sul mutation in maize that causes a deficiency of isoamylase, an enzyme normally considered to be mainly involved in starch degradation in plants and in C. reinhadtii, results in accumulation of a water-soluble polysaccharide, phytoglycogen, instead of starch.45 19,291 In C. reinhardtii, the mutation results in complete loss of starch, but in higher plants, the lower amount of amylopectin seen in the mutant plant may be related to the severity of the enzyme deficiency.292,293 

Some starch-deficient mutants of Arabidopsis and Chlamydomonas have been shown to be defective in a-l,4-glucanotransferase activity. The enzyme is also known as D-enzyme. The reaction it catalyzes is as follows  

Other oligosaccharides can also act as substrates. The transglucosylase of Arabidopsis leaf disproportionates maltotriose and forms higher maltodextrins of much greater size than maltooctaose.295 

NADPH reduced nicotinamide adenine dinucleotide phosphate 

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Genes encoding enzymes involved in starch biosynthesis or other relevant pathways (e.g., synthesis of sucrose) can be used for the overexpression of enzyme activity, as described in this chapter for ADPGlc PPase. Another approach is the use of antisense (complementary) DNA or RNA to decrease gene expression, a good way to assess the role of an enzyme and whether it limits the rate of the overall pathway. 

The development of amylase activity in extracts of embryo-free and of GA3-treated, embryo-free maize kernels has been determined. The increase in amylase activity was accompanied by the appearance of several starch-degrading enzymes. Actinomycin D and cycloheximide prevented the amylolytic activity from developing. Other results indicated that the development of a-amylase activity in embryo-free maize kernels does not depend on gibberellic acid, but involves the de novo synthesis of protein. 

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