Sucrose formation

That the same enzyme,28 sucrose phosphorylase, is involved in the reaction of L-arabinose is indicated by the following observation. When L-arabinose is added to a mixture containing the enzyme, D-glucose-1-phosphate and D-fructose, of which the last is present in insufficient concentration to give the maximum rate of sucrose formation, an increase... 

Sucrose formation by the sucrose phosphorylase reversible reaction. 

Treatment of silyl ethers of sucrose with the Vilsmeier reagent gave various sucrose formates. When only one silyl protecting group was present, the reaction afforded the corresponding monoesters in high yields. However, when di- or tri-silylated sucroses were used as substrates, a mixture of mono- and di-formates were obtained (Scheme 26).301... 

Fig. 21. Relationship of average Chi content and average sucrose formation rate per dry cell weight against length of a single root...

The accumulated fructose 2,6-P2 inhibits fructose-1,6-bisphosphatase and slows the conversion of triose phosphates to sucrose (Eq. 23-34). Accumulation of fructose 6-P due to decreased utilization for sucrose formation will have a similar effect. However, both 3-phosphoglycerate and dihydroxyacetone phosphate have opposite effects and will act to remove the inhibition by lowering the fructose 2,6-P2 level and to promote rapid sucrose formahon (Eq. 23-34). °... 

The activities of FBPase and of SPS appear to control sucrose formation, as in spinach leaves Cl23. These enzymes, as well as UDPG pyrophosphorylase and SS, have lower activities at lO C. The activity of FBPase decreases about 6 times whereas the activity of UDPG pyrophosphorylase is about 4 times lower (Table 1). These changes cannot explain the increase in F6P and G6P at low temperatures. Also, the small drecrease in the activities of SPS and SS, observed at 10°C, do not explain the decrease on sucrose, previously reported C6, Si. 

A second, probably minor site of sucrose synthesis in the germinated barley grain is the aleurone layer [42]. The mechanism of synthesis is unknown, but presumably as starch hydrolysis occurs glucose is absorbed by the aleurone cells from the endosperm, converted therein to sucrose, and then released back into the endosperm (the release, but not the synthesis, being enhanced two-fold by GA). Sucrose may then be absorbed by the scutellum. The importance of this mode of sucrose formation is unknown because the relative contribution of the aleurone layer and the scutellum to the total production of sucrose for the growing embryo has yet to be fully assessed. 

Analysis of the infrared spectra is consistent with a product containing units such as 5 Z with the formation of the Sn-O-R ether linkage with hydroxyl groups contained on the sucrose. Formation of the ether litjkage is indicated by the presence of bands about 660 to 690 cm (Figure 1) characteristic of the asynjpietric stretch of tin ethers and a doublet about 550 to 600 cm attributed to the symmetric stretch of tin ethers. 

The high energy bond in a substituted glucosyl phosphate might be important for sucrose synthesis. The equilibrium constant for reaction (i) was shown to be 2 to 8 at 37° and pH 7.4, indicating that this reaction favors sucrose formation. As for sucrose phosphorylase, L-sorbose, D-xylulose and D-rhamnulose acted as acceptors to give the corresponding disaccharides °. 

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