Starch synthesis

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],... 

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

The construction of the structural kinetic model proceeds as described in Section VIII.E. Note that in contrast to previous work [84], no simplifying assumptions were used the model is a full implementation of the model described in Refs. [113, 331]. The model consists of m = 18 metabolites and r = 20 reactions. The rank of the stoichiometric matrix is rank (N) = 16, owing to the conservation of ATP and total inorganic phosphate. The steady-state flux distribution is fully characterized by four parameters, chosen to be triosephosphate export reactions and starch synthesis. Following the models of Petterson and Ryde-Petterson [113] and Poolman et al. [124, 125, 331], 11 of the 20 reactions were modeled as rapid equilibrium reactions assuming bilinear mass-action kinetics (see Table VIII) and saturation parameters O1 1. 

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. 

James, M. G., Denyer, K., and Myers, A. M. (2003). Starch synthesis in cereal endosperm. Curr. Opin. Plant Biol. 6, 215-222. 

Schardinger intended to continue his work on these crystalline dextrins in the hope that they might shed some light on the processes of starch synthesis and degradation, or on its composition. However, when, after a long suspension, he again took up work in this direction, the strain used... 

The chloroplast stroma contains all the enzymes necessary to convert the triose phosphates produced by C02 assimilation (glyceraldehyde 3-phosphate and dihydroxyacetone phosphate) to starch, which is temporarily stored in the chloroplast as insoluble granules. Aldolase condenses the trioses to fructose 1,6-bisphos-phate fructose 1,6-bisphosphatase produces fructose 6-phosphate phosphohexose isomerase yields glucose 6-phosphate and phosphoglucomutase produces glucose 1-phosphate, the starting material for starch synthesis (see Section 20.3). 

Sucrose synthesis in the cytosol and starch synthesis in the chloroplast are the major pathways by which the excess triose phosphate from photosynthesis is harvested. Sucrose synthesis (described below) releases four Pi molecules from the four triose phosphates required to make sucrose. For every molecule of triose phosphate removed from the chloroplast, one Pj is transported into the chloroplast, providing the ninth Pj mentioned above, to be used in regenerating ATP. If this exchange were blocked, triose phosphate synthesis would quickly deplete the available Pj in the chloroplast, slowing ATP synthesis and suppressing assimilation of C02 into starch. 

ADP-Glucose Is the Substrate for Starch Synthesis in Plant Plastids and for Glycogen Synthesis in Bacteria... 

The mechanism of glucose activation in starch synthesis is similar to that in glycogen synthesis. An activated nucleotide sugar, in this case ADP-glucose, is... 

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. 

Intermediate-level review of the genes and proteins involved in starch synthesis in plant tubers. 

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 ... 

The elongation step in starch synthesis, (a) The activated form of glucose in starch synthesis is ADP-glucose. This is formed from glucose and ATP as shown. (b) The ADP-glucose does not react... 

Efforts in genomics have also been directed at manipulation of starch synthesis to modify the amylose amylopectin ratio to increase susceptibility to a-amylase digestion (Smallwood, 2006), in this case to improve ethanol production via fermentation. Such developments would also have spin-offs for other industrial uses of starch where there is a preference for amylose or amylopectin forms. 

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

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