Enzymes in starch synthesis

A further ramification of the translocator-mediated exchange of exported triose phosphate and imported Pj pertains to starch synthesis. When cytosolic metabolism and Pj availability are limited, leading to a high 3-PGA/P ratio in the chloroplast, starch synthesis will be stimulated. This occurs because ADP-glucose py-rophosphorylase, the major regulatory enzyme in starch synthesis, is strongly activated by 3-PGA and inhibited by Pj [29]. As mentioned above, starch synthesis from triose phosphate will release Pj, relieving to some extent the Pj limitation of CO2 fixation. 

Which of the enzymatic activities mentioned previously are involved in starch synthesis in vivo To accept that an enzyme is a likely component of the pathway in the plant itself, it must fulfill the following criteria ... 

In view of its possible importance in starch synthesis, surprisingly little is known about the specificity and mechanism of action of Q-enzyme. It has been suggested both that G40 is not a substrate for the enzyme, and that G40 is the minimum length for rapid action. Also, it has been reported that Q-enzyme acts more rapidly on maize amylopectin than on the amylose thereof. ... 

The enzymatic reactions involved in starch synthesis in higher plants and algae and in glycogen synthesis in cyanobacteria are reviewed in this article. Regulation of a-1,4 and a-1,6 glucan synthesis at the enzymic level is discussed, and mutants that cause specific enzymatic deficiencies that affect starch structure are also reviewed. Recent reviews on starch synthesis have been published and are noted (1-6). 

My own personal work in the starch area, has indicated the challenges and the satisfaction - that may be obtained from working in this interdisciplinary field. The expertise in manipulating the different enzymes involved in starch synthesis, now mean that particular ones can be knocked out reasonably straightforwardly there is even some control over the degree of inactivation of each enzyme. However, the end results of these changes are far from obvious at both the molecular and supramolecular levels. In part this is because the interplay between the various enzymes involved in the synthesis of both the... 

The Enzymic Synthesis and Degradation of Starch. Part VIII, The Use of Mixtures of P- and Q-Enzymes in the Synthesis of Starch-type Polysaccharides, S. A. Barker,... 

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