Fructose sucrose synthesis

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,. 

The partitioning of triose phosphates between sucrose synthesis and starch synthesis is regulated by fructose 2,6-bisphosphate (F2,6BP), an allosteric effector of the enzymes that determine the level of fructose 6-phosphate. F2,6BP concentration varies inversely with the rate of photosynthesis, and F2,6BP inhibits the synthesis of fructose 6-phosphate, the precursor to sucrose. 

The photosynthetic block precedes sucrose synthesis, and sucrose partially reverses the inhibition caused by the lack of fixation of carbon dioxide. D-Fructose disappears first, after treatment, followed by sucrose, and then u-glucose.188 It is likely that other metabolic systems are involved as well, since the amino acid distributions were not identical to the I4C02 dark-fixation products.189 Uptake of sucrose-14C increased some acids (as-... 

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... 

D-fructose, reaction occurred in the same manner as with the sucrose synthesis and the resulting disaccharides, a-D-glucopyranosyl a-L-sorbo-furanoside89 and a-D-glucopyranosyl /3-D-xyloketofuranoside,40 were isolated. 

Stitt, M., and Heldt, H. W. 1985. Control of photosynthetic sucrose synthesis by fructose-2,6 bisphosphate. Planta 164, 179-188. 

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]. 

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. 

The requirement of chloroplast photosynthesis for Pj and the release of Pj by sucrose synthesis in the cytosol require that these two processes be closely coordinated. Part of this coordination, as explained above, lies in the characteristics of the triose phosphate translocator. Results obtained in the last few years have led to the identification of a second component serving this function. Fructose 2,6-bis-phosphate (Fru-2,6-P2) coordinates the metabolism of sucrose, starch and CO2 fixation and, in so doing, links metabolic processes of the chloroplast with those of the cytosol. 

Stereospecificity. Sucrose, a major product of photosynthesis in green leaves, is synthesized by a battery of enzymes. The substrates for sucrose synthesis, d-glucose and d-fructose, are a mixture of a and P anomers as well as acyclic compounds in solution. Nonetheless, sucrose consists of a-d-glucose linked by its carbon-1 atom to the carbon-2 atom of P-d-fructose. How can the specificity of sucrose be explained in light of the potential substrates ... 

The availability of metabolites for sucrose synthesis and the need for products of sucrose degradation regulate gene expression. For respiration, sucrose is hydrolyzed by invertase to free glucose and fructose, which are phosphorylated and undergo glycolysis to pyruvate. The pyruvate is then either metabolized by mitochondrial electron transport to ATP and NADH (respiration), or metabolized to provide starting products for amino acid, lipid, and nucleotide syntheses. 

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...

After Its formation In the chloroplast stroma, glyceraldehyde 3-phosphate Is transported to the cytosol In exchange for phosphate. The final steps of sucrose synthesis occur In the cytosol of leaf cells. In these reactions, one molecule of glyceraldehyde 3-phosphate Is Isomerized to dihydroxyacetone phosphate. This compound condenses with a second molecule of glyceraldehyde 3-phosphate to form fructose 1,6-bIs-phosphate, which Is the reverse of the aldolase reaction In glycolysis (see Figure 8-4, step 0)). Fructose 1,6-bIsphosphate Is converted primarily to sucrose by the reactions shown In the bottom portion of Figure 8-42. 

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. 

This enzyme is very specific it does not react with sorbose, fructose phosphates, glucose, or any other compound tested in place of fructose, and glucose-l-phosphate cannot replace UDPG. Unlike the case of sucrose phosphorylase, the equilibrium of this reversible reaction favors sucrose synthesis, and the presence of this enzyme in the cells of higher plants implicates this reaction in sucrose synthesis. 

---