Starch synthesis, regulation

OPARKA, K.J., WRIGHT, K.M., Osmotic regulation of starch synthesis in potato-tubers, Plant a, 1988, 174, 123-126. 

For the irreversible reactions, we assume Michaelis Menten kinetics, giving rise to 15 saturation parameters O1. C [0, 1] for substrates and products, respectively. In addition, the triosephospate translocator is modeled with four saturation parameters, corresponding to the model of Petterson and Ryde-Petterson [113]. Furthermore, allosteric regulation gives rise to 10 additional parameters 7 parameters 9" e [0, — n for inhibitory interactions and 3 parameters 0" [0, n] for the activation of starch synthesis by the metabolites PGA, F6P, and FBP. We assume n = 4 as an upper bound for the Hill coefficient. 

Starch synthesis is regulated at the level of ADP-glucose formation, as discussed below. 

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 and Sucrose Synthesis Sucrose synthesis occurs in the cytosol and starch synthesis in the chloroplast stroma, yet the two processes are intricately balanced. What factors shift the reactions in favor of (a) starch synthesis and (b) sucrose synthesis ... 

Compartmentation and Regulation of Starch Synthesis and Degradation in Chloroplasts... 

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. 

Analyses of the starch biosynthetic system in a number of plants and green algae indicate that an important site of regulation of starch synthesis is at the ADPGlc PPase and that 3PGA and Pi are important regulatory metabolites of that enzyme. 

Thus, regulation of ADPGlc PPase, at both the transcriptional level and by allosteric control of the enzyme, modulates the rate of ADPglucose synthesis and starch synthesis. 

REGULATION OF THE STARCH SYNTHESIS PATHWAY TARGETS FOR BIOTECHNOLOGY... 

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