Fructose sucrose synthesis from

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,. 

Sugars, such as fructose and glucose from honey, have been harvested and processed by humans since the Stone Age [23], The use of sucrose as a sweetener dates back to the eighth century BC and could only be afforded by royalty and the very wealthy [24], More recently, these natural products become critical in a variety of industries focused on the production of paper, pulp, textiles, and pharmaceuticals. As often is the case, industrial applications ultimately provided the economic impetus for investigations into carbohydrate synthesis, purification, and characterization in the late nineteenth century. 

Although sucrose has not been synthesized by strictly chemical means, its synthesis has been accomplished by the use of enzymes from living organisms.86 An enzyme from the bacterium Pseudomonas saccharophila Doudoroff was allowed to act on D-glucose-l-phosphate in the presence of D-fructose. This synthesis gives little information about... 

Although the bundle sheath chloroplasts contain all the enzymes of the RPP cycle, there is now evidence that some of the 3-PGA formed by the activity of rubisco is exported to the mesophyll cells [9]. Bundle sheath chloroplasts of maize are deficient in photosystem II activity and apparently cannot produce sufficient NADPH to reduce all of the 3-PGA formed to triose phosphate. Responsibility for this step is thus shared with the mesophyll chloroplasts which recycle triose phosphate to the bundle sheath as DHAP. This transport of 3-PGA from bundle sheath to mesophyll permits the synthesis of sucrose in the mesophyll cell cytoplasm. The evidence suggests that the mesophyll cells are the major site of sucrose synthesis [10-13]. Sucrose phosphate synthetase, one of the regulatory enzymes of sucrose synthesis and fructose 6-phosphate, 2-kinase (Fru-6-P,2K), the enzyme synthesizing fructose 2,6-bisphosphate — a potent regulator of cytoplasmic sucrose synthesis (see Section 5.4.1) — are both almost completely confined to the mesophyll cells. 

Scheme 2 Synthesis of ot-D-fructofuranose P-D-fructopyranose l,2 2,l -dianhydride (9) and a-D-fructofuranose P-D-fmctofuranose l,2 2,l -dianhydride (10) from o-fructose, sucrose and D-fmc-tose oligosaccharides by the action of pyridinium poly(hydrogenfluOTide) complex...

Subsequently another method was developed for forcing the nucleophile to approach the anomeric center of a fructose oxocarbenium ion from the (3 side, namely to block the furanoside with bridging substituents so as to make a approach impossible. The method is illustrated here in a benchmark synthesis of sucrose 87... 

In sucrose the fructose unit exists as a 5-membered ring, but on hydrolysis to invert sugar the fructose separates as a 6-membered ring. Maltose contains two molecules of J-glucose, as does cellobiose (from the hydrolysis of acetyl cellulose) but in a different configuration. The synthesis of sucrose was claimed by Pictet and H. Vogel, but this was doubted a definite synthesis from glucopyranose and fructofuranose derivatives was achieved by Lemieux (1953-6). 

The fact that sucrose can be synthesized from -D-glucose 1-phosphate and D-fructose is consistent with the evidence presented by Isbell and Pigman 172) and Gottschalk 173) that D-fructose exists as an equilibrium mixture of the pyranose and furanose forms. The total system of sucrose synthesis can be represented by the equation (Fig. 4) which includes the equilibrium reaction of the two ring forms of D-fructose. The occurrence of D-fructose in the sucrose molecule as D-fructofuranose is an indication that sucrose phosphorylase is specific for the furanose configuration of that ketose 17A, 175),... 

When a mixture of UDPG and D-fructose is subjected to the reaction of enz3nne preparations from these plant sources in the presence of inorganic phosphate buffer at pH 7.0, a nonreducing disaccharide, identified as sucrose, is formed. The equilibrium constant, Ky for this reaction at 37°C was found to be between 2 and 8, and the AF was estimated to be approximately —1000 cal. as compared to +1770 cal. for the sucrose phosphorylase reaction. In contrast to the sucrose phosphorylase reaction, the equilibrium of the reaction starting with UDPG and D-fructose is in favor of sucrose synthesis. 

Starch synthesis is closely related to sucrose synthesis (see Chapters IX and XII). Experiments with isotopically labeled sugars have shown that leaves can form starch from externally supplied glucose, fructose, and sucrose, as well as from several other compounds. Starch is also formed from carbon dioxide during photosynthesis. It was found (78) that when labeled carbon dioxide was fed to leaves, the starch became labeled before the free sugars. Labeled glucose and glucose 1-phosphate did not contribute an... 

The enzymatic conversion of 6-azido-6-deoxy-D-glucose into its D-fructose isomer was utilised as a key step in a five-step synthesis from sucrose [80] of the natural product and powerful D-mannosidase inhibitor 1-deoxymannojirimycin (l,5-dideoxy-l,5-imino-D-mannitol, 66) [81]. Subsequently, it was shown that the immobilised enzyme from Streptomyces murinus sp. (Sweetzyme T from Novo A/S) was able to isomerise D-glucose derivatives with modifications at C-3 and C-6 such as 6-azido-6-deoxy-3-0-methyl-D-glucose (67) as well as the corresponding 3-deoxy derivative 69 (Scheme 24) [64,82]. 

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