Fructose synthesis from starch

Another aspect of chloroplast metabolism is synthesis of starch. Formation of ADP-glucose from glucose 1-phosphate is induced by 3-phosphoglycerate, a "feed-ahead" type of regulation (Fig. 23-36). Although fructose 2,6-bisphosphate is absent from chloroplasts, it has an important regulatory function in the cytoplasm of plants as it does in animals.425 430 In the plant cytosol triose phosphates from the chloroplasts are converted to fructose 6-P, glucose 6-P, UDP-... 

Although TA from yeast is commercially available, it has rarely been used in organic synthesis applications, and no detailed study of substrate specificity has yet been performed. This is presumably due to high enzyme cost and also since the reaction equilibrium is near unity, resulting in the formation of a 50 50 mixture of products. In addition the stereochemistry accessible by TA catalysis matches that of FruA DHAP-dependent aldolase and the latter is a more convenient system to work with. In one application, TA was used in the synthesis D-fructose from starch.113 The aldol moiety was transferred from Fru 6-P to D-glyceraldehyde in the final step of this multi-enzyme synthesis of D-fructose (Scheme 5.60). This process was developed because the authors could not identify a phosphatase that was specific for fructose 6-phosphate and TA offered an elegant method to bypass the need for phosphatase treatment. 

Scheme 5.60. Transaldolase catalyzed synthesis of D-fructose from starch. P = PO32. ...

Transaldolase catalysis used for an apparent transphosphorylation strategy, as incorporated in a synthesis of fructose from starch, and deoxysugar synthesis by related FSA using a non-natural donor. 

Scheme 14. Multi-enzyme synthesis of D-fructose from starch.

The pentose phosphate pathway is an alternative route for the metabolism of glucose. It does not generate ATP but has two major functions (1) The formation of NADPH for synthesis of fatty acids and steroids and (2) the synthesis of ribose for nucleotide and nucleic acid formation. Glucose, fructose, and galactose are the main hexoses absorbed from the gastrointestinal tract, derived principally from dietary starch, sucrose, and lactose, respectively. Fructose and galactose are converted to glucose, mainly in the liver. 

In the plant kingdom, the biosynthesis of the starch granule remains the outstanding subject for investigation. This includes the mode of synthesis of the starch components, the variation in their proportion in different plants, and the occurrence of minor structural features (for example, phosphate groups and n-fructose residues). The relative importance of the enzymes catalyzing n-glucosyl transfer from uridine- and adenosine... 

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