Sucrose 6-phosphate

Synthesis of sucrose 6 -phosphate by an enzymic method using uridine 5 -(a-D-glucopyranosyl diphosphate) plus D-glucose 6-phosphate has been reported.126-131 The first, unambiguous, chemical synthesis of sucrose 6 -phosphate was achieved by Buchanan and coworkers.18 The reaction of 2,3,4,6,l, 3, 4 -hepta-0-acetylsucrose, prepared by five steps of synthesis, with cyanoethyl phosphate in pyridine gave a crude product from which sucrose 6 -phosphate was isolated as the barium salt. 

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,. 

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 ... 

Hydrolysis of the sucrose 6-phosphate yields sucrose. This is the major sugar that is transported between plant cells, analogous to the supply of glucose via the bloodstream to animal tissues (see Topic J4). 

UDP-glucose + fructose-6-phosphate — sucrose-6-phosphate + UDP. The sucrose phosphate formed is hydrolyzed by a specific phosphatase to... 

After glucose synthesis in photosynthesis, the disaccharide sucrose (a-D-Glc(l —> 2)(3-D-Fru) is used as a readily transportable sugar. Sucrose synthesis successively involves the following UDP-glucose + fructose-6-phosphate —> sucrose-6-phosphate + UDP [via sucrose phosphate synthase] sucrose-6-phosphate + H20 —> sucrose + P [via sucrose-6-phosphatase]. 

Figure 20.13. Synthesis of Sucrose. Sucrose 6-phosphate is formed by the reaction between fructose 6-phosphate and the activated intermediate uridine diphosphate glucose (UDP-glucose).

Sucrose is a highly soluble disaccharide that provides a mobile energy source for all the plant cells. Sugar cane stores large amounts of sucrose in its leaves and stalk, whereas sugar beet stores it in roots. All plants make sucrose from two molecules of fructose 6-phosphate. One molecule is activated with UDP and isomerized to UDP-glucose. Sucrose 6-phosphate synthase reacts with UDP-glucose and fructose 6-phosphate to make sucrose 6-phosphate. The latter then reacts with a phosphatase to produce sucrose (Fig. 2.9). 

The syntheses of sucrose 6 -phosphate, methyl a-mannopyranoside 4- and 6-phosphates, and 3-deoxy-3-fluoro-D-glucose 1- and 6-phosphates [ (57) and (58) ] have been described. Neither (57) nor (58) was a substrate for UDPGlc-pyrophosphorylase or phosphoglucomutase, although (58) was a poor substrate for glucose 6-phosphate dehydrogenase. ... 

In contrast, sucrose (common table sugar), a disaccharide, is synthesized in the cytosol. Plants lack the ability to transport hexose phosphates across the chloroplast membrane, but an abundant phosphate translocator mediates the transport of triose phosphates from chloroplasts to the cytosol in exchange for phosphate. Fructose 6-phosphate formed from triose phosphates joins the glucose unit of UDP-glucose to form sucrose 6-phosphate (Figure 20.13). Hydrolysis of the phosphate ester yields sucrose, a readily... 

In reaction (b), which is catalyzed by a separate enzyme, the acceptor is D-fructose 6-phosphate and the resulting product is a phosphate of sucrose. The structure (8) assigned to this sucrose 6 -phosphate was deduced by chemical and enzymic analysis. ... 

The equilibrium constant of this reaction has been reported to be 3250 at 38° and pH 7.5. If this value is correct, it indicates that the Af ° of hydrolysis of the glycosidic bond of sucrose 6 -phosphate is —2700 cal.,... 

The existence of two separate mechanisms for the synthesis of sucrose raises the question of their respective roles in vivo. Experiments in which C Mabeled n-glucose was supplied to plants have shown that the d-fructofuranosyl moiety of sucrose becomes highly labeled before any label appears in the free D-fructose pool, suggesting that this monosaccharide is not an intermediate in sucrose synthesis. On the other hand, d-fructose 6-phosphate becomes labeled before sucrose, and small proportions of sucrose 6 -phosphate have also been detected among the labeled pro-ducts. These results are compatible with the hypothesis that sucrose 6 -phosphate is synthesized first and is subsequently hydrolyzed to free sucrose. Such a pathway would be clearly irreversible, since it includes a hydrolytic step, and could account for the large accumulation of sucrose in many plants. 

Results obtained by Leloir and Cardini indicated that two separate enzymes are involved in the biosynthesis, in plants, of sucrose and sucrose phosphate from D-fructose and D-fructose 6-phosphate, respectively in the presence of UDP-D-glucose these enzymes have been partially separated. Slabnik and coworkers succeeded in isolating sucrose synthetase and sucrose 6-phosphate synthetase from potato tubers, and determined some of the properties of the partially purified preparations. The sucrose synthetase showed an optimum activity at 45 and was inhibited completely by ADP and some phenolic D-glucosides, whereas these had no effect on sucrose 6-phosphate synthetase. 

ICrapp A. and Stitt M. 1995. An evaluation of direct and indirect mechanisms for the sink regulation of photosynthesis in spinach change in gas exchange, carbohydrates, metabohtes, enzyme activities and steady-state transcript levels after cold-girdling source leaves. Planta 195 313-323. Krause K.P. and Stitt M. 1992. Sucrose-6-phosphate levels in spinach leaves and their effects on sucrose-phosphate synthase. Phytochemistry 31 1143-1146. 

The phosphate group is split off from sucrose-6-phosphate by sucrose phosphatase. 

The disaccharide sucrose is imported by a specific phosphotransferase system PTSjuc responsible for uptake and phosphorylation of sucrose [125], yielding sucrose-6-phosphate. Sucrose-6-phosphate hydrolase then yields glucose-6-phosphate and free fructose [107, 127]. Fructose is exported out of the C. glutamicum cell and re-imported by PTSf, with concomitant phosphorylation to fructose-l-phosphate [128]. Overexpression of the fructose bisphosphatase genefbp improved L-lysine product yields on sucrose [46]. 

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