Starch synthesis, inhibition

Potassium is required for enzyme activity in a few special cases, the most widely studied example of which is the enzyme pymvate kinase. In plants it is required for protein and starch synthesis. Potassium is also involved in water and nutrient transport within and into the plant, and has a role in photosynthesis. Although sodium and potassium are similar in their inorganic chemical behavior, these ions are different in their physiological activities. In fact, their functions are often mutually antagonistic. For example, increases both the respiration rate in muscle tissue and the rate of protein synthesis, whereas inhibits both processes (42). 

One example of a naturally occurring diazirine, duazomycin A (137 Scheme 11.20), has been reported, isolated in 1985 from a Streptomyces species during a screen for herbicidal compounds [196], It was fotind to inhibit de novo starch synthesis and it was suggested that this is due to direct inhibition of protein synthesis. Duazomycin A is structurally related to 6-diazo-5-oxo-L-norleucine (138), also reported as a natural product from Streptomyces [197], which acts as a glutamine antagonist and inhibits purine biosynthesis [198],... 

FIGURE 20-28 Regulation of ADP-glucose phosphorylase by 3-phosphoglycerate and Pj. This enzyme, which produces the precursor for starch synthesis, is rate-limiting in starch production. The enzyme is stimulated allosterically by 3-phosphoglycerate (3-PGA) and inhibited by P, in effect, the ratio [3-PGA]/[Pi], which rises with increasing rates of photosynthesis, controls starch synthesis at this step. 

The partitioning of triose phosphates between sucrose synthesis and starch synthesis is regulated by fructose 2,6-bisphosphate (F2,6BP), an allosteric effector of the enzymes that determine the level of fructose 6-phosphate. F2,6BP concentration varies inversely with the rate of photosynthesis, and F2,6BP inhibits the synthesis of fructose 6-phosphate, the precursor to sucrose. 

Regulation of starch synthesis occurs at the ADPGlc synthetic step (1). Activation of ADPGlc synthesis is affected by 3 P glycerate (3-PGA) and inhibition is caused by orthophosphate (P ). These effector metabolites interact in that P can reverse the activation of ADPGlc synthesis caused by 3-PGA and increasing concentrations of 3-PGA can overcome P inhibition. 

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. 

It has been shown starch synthesis that occurs in potato tubers from growing plants is inhibited within 24 hours after... 

PGA. At the same time, concentrations of Pi decreases and that of 3-PGA and other glycolytic intermediates increase, thus increasing the activity of the ADP-Glc PPase and starch synthesis. Conversely, in the dark, phosphate concentration increases, while the concentrations of 3-PGA, glycolytic intermediates, and ATP decrease, leading to inhibition of ADP-Glc synthesis and therefore of starch synthesis. 

Kida T, Shibai H. Inhibition by hadacidin. duazomycin A. and other amino acid derivatives of de novo starch synthesis. Agric Biol Chem 1985 49 3231-3237. 

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