The Synthesis of Starch

Whether it is transported unmodified into the endosperm, or resynthesized there from its component monosaccharides, sucrose is the starting point for starch formation. It is converted to fructose and uridine diphosphoglucose (UDPG) by sucrose synthetase (sucrose-UDP glucosyl transferase)  

Fructose is converted to glucose-1-phosphate (G-l-P), as is the UDPG, the latter by UDPG pyrophosphorylase  

This G-l-P must now be combined to form another sugar nucleotide, ADPG, by ADPG pyrophosphorylase  

The glucose from the ADPG is then added on to a small glucose primer to increase its chain length by one glucose unit this is repeated until the starch 

Another complement of enzymes capable of catalyzing starch synthesis are the phosphorylases, four of which are present at different stages of development of the maize endosperm [182]. These enzymes utilize G-l-P as starting substrate  

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The synthesis of starch as described in the chapter, The Biosynthetic Reactions of Starch Synthesis, requires the presence of a glucan primer, that is, a glucan chain that is then elongated by the addition of a glucosyl group in the following reaction ... 

Erlander, S. 1958. Proposed mechanism for the synthesis of starch from glycogen. Enzymol 19, 273-283. 

During photosynthesis, chloroplasts convert CO2, water and Pj to triose phosphates that are exported to the cytosol. Phosphate is therefore a substrate of this process and the continued operation of the RPP cycle is dependent on the utilization of triose phosphate for the synthesis of starch (in the chloroplast) and sucrose (in the cytosol). These synthetic processes release Pj, preventing the level of free Pj in the cell from falling to a concentration where photosynthesis may be limited by its availability. Such a limitation of photosynthesis is observed during O2-insensitive CO2 assimilation [56] and is suggested by the increase in CO2 fixation detected on feeding P via the transpiration stream to a cut leaf [57]. It has long been known that isolated chloroplasts require a continuous supply of P in order to sustain photosynthesis. 

The utilization of carbon dioxide by ATP and NADPH occurs in the chloroplast matrix, (outside the thylakoid lumen). A series of reactions assimilates carbon dioxide (Fig. 2.6), the Calvin cycle or dark reaction, and generates fructose 6-phosphate. Fructose 6-phosphate is the immediate precursor of glucose 6-phosphate for the synthesis of starch in 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,... 

Because glyceraldehyde-3-phosphate and dihydroxyacetone phosphate are readily interconverted, these two molecules (referred to the triose phosphates) are both considered to be Calvin cycle products. The synthesis of triose phosphate is sometimes referred to as the C3 pathway. Plants that produce triose phosphates during photosynthesis are called C3 plants. Triose phosphate molecules are used by plant cells in such biosynthetic processes as the formation of polysaccharides, fatty acids, and amino acids. Initially, most triose phosphate is used in the synthesis of starch and sucrose (Figure 13A). The metabolism of each of these molecules is briefly discussed below. 

In fruits, storage organs, or seeds, the synthesis of starch usually occurs during the development and mamration of the tissue. Starch degradation in these tissues occurs at the time of sprouting or germination of... 

Transfer of D-glucopyranosyl residues is ten times faster from adenosine 5-(a-D-glucopyranosyl pyrophosphate) than from the uridine derivative. The D-glucopyranosyl pyrophosphates of inosine, cytidine, and guanosine are not active as substrates for the synthesis of starch. Since adenosine 5-(a-D-glucopyranosyl pyrophosphate) and a pyrophosphorylase catalyzing its synthesis have been found in plant material,it seems likely that this a-D-glucosyl nucleotide is a precursor of starch in vivo. 

The synthesis of starch acetate from starch and acetic anhydride. 

Fischer pol3onerized monosaccharides to disacdiarides an dextrin-t3T)e substances. He recognized the synthesis of starch and cellulose would be difficult. He was confident, however, that such syntheses are possible. 

The enzyme-catalysed reactions that living organisms use to generate the energy they need and to prepare the molecules they require, are collectively termed metabolism . In anabolism, enzyme-catalysed reactions are used to make large complex molecules, for example the synthesis of starch, cellulose and proteins. Photosynthesis is an example of an anabolic reaction. These processes require an input of energy from ATP. In catabolism, nutrient molecules are hydrolysed under enzyme control to provide smaller and simpler molecules for synthesis and also to supply energy (to convert ADP back to ATP). 

The free energy change in phosphorolysis compared to hydrolysis is much closer to zero, and thus the known phosphorolytic enzymes catalyze biosynthetic reactions as well as catabolic changes. They are implicated chiefly in the synthesis of starch by plants and glycogen by animals. Recent work by Kornberg and his associates has pointed to a pyro-phosphorylase in yeast and hog liver that catalyzes the following general reaction ... 

Inorganic carbohydrate hybrid systems - Limited information is available on the use of saccharides in nanostructured inorganic hybrids. Some papers describe the incorporation of nanosized clays in thermoplastic starch by exfoliation 21-22), Recently the synthesis of starch/clay superabsorbents was reported (23). Several studies concern the preparation and properties of organo-silicon derivatives and cellulose-based hybrids (24-25). Inorganic starch hybrids with electrorheological activity were prepared using in situ sol-gel synthesis... 

Our data show that several cytosolic metabolites could be utilized by the plastids for the synthesis of starch and/or fatty acids. The presence of specific translocators for some of these metabolites is also implied by the intactness-dependence of their incorporation. Which of the metabolites are used in vivo during embryo development is likely to be determined by the concentrations of the metabolites, those of other metabolites which compete for the same translocators, and the fine and coarse control of the enzymes in the plastids which convert them into the immediate precursors for starch and fatty acid synthesis. 

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