UDP-Fructose

The most important compound is inulin, which occurs as reserve material, particularly in Compositae. It consists of a chain of 32-34 fructose units which are linked to each other through jS-glycosidic 1-2 linkages. Fructose is present in the form of a fructofuranose. At one end of the molecule there is a sucrose unit. The biosynthesis of inulin proceeds by the attachment of fructose units, one after the other, to the sucrose molecule. The fructose donor is presumably a sugar nucleotide, probably UDP-fructose. The presence of UDP-fructose in the inulin-con-taining bulbs of dahlias is consistent with this supposition. Thus, our... 

Scheme 2.2.6.1 The reversible reaction of the Leloir glycosyltransferase sucrose synthase (EC 2.4.1.13) 1 sucrose 2 UDP-Gic 3 D-fructose.

FIGURE 20-25 Sucrose synthesis. Sucrose is synthesized from UDP-glucose and fructose 6-phosphate, which are synthesized from triose phosphates in the plant cell cytosol by pathways shown in Figures 15-7 and 20-9. The sucrose 6-phosphate synthase of most plant species is allosterically regulated by glucose 6-phosphate and P,. 

Sucrose is synthesized in the cytosol in two steps from UDP-glucose and fructose... 

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-... 

UDP-GlcNAc represents the branching point for the bacterial biosynthesis of murein and lipopolysaccharides where secondary nucleotide sugars are involved [329, 358], Starting from fructose-6-P, all enzymes are available as recombinant proteins and enzymatic synthesis of important intermediates seems to be possible. 

Much of the glyceraldehyde 3-phosphate produced by the Calvin cycle in chloroplasts is exported to the cytosol and used to produce the disaccharide, sucrose. First the glyceraldehyde 3-phosphate is converted to fructose 6-phos-phate and glucose 1-phosphate. The chemical reactions involved are essentially a reversal of glycolysis (see Topic J3). The glucose 1-phosphate is then converted to UDP-glucose and this reacts with fructose 6-phosphate to synthesize sucrose 6-phosphate ... 

FIGURE 10.20 The action of fructan 1-exohydrolase and fructan fructan 1-fructosyltransferase in dormant and sprouting tubers on inulin depolymerization. Abbreviations 1-feh = fructan 1-exohydrolase 1-fft = fructan fructan 1-fructosyltransferase hk = hexokinase spp = sucrose phosphate phosphatase sps = sucrose phosphate synthase GF = sucrose G = glucose F = fructose GFF = 1-ketose GF-P = sucrose phosphate UDP-G = uridine diphosphate glucose F-6-P = fructose-6-phosphate. 

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