Sucrose 6 -phosphate, synthesis

FIGURE 20-27 Regulation of sucrose phosphate synthase by phosphorylation. A protein kinase (SPS kinase) specific for sucrose phosphate synthase (SPS) phosphorylates a Ser residue in SPS, inactivating it a specific phosphatase (SPS phosphatase) reverses this inhibition. The kinase is inhibited allosterically by glucose 6-phosphate, which also activates SPS allosterically. The phosphatase is inhibited by Pi, which also inhibits SPS directly. Thus when the concentration of glucose 6-phosphate is high as a result of active photosynthesis, SPS is activated and produces sucrose phosphate. A high P, concentration, which occurs when photosynthetic conversion of ADP to ATP is slow, inhibits sucrose phosphate synthesis. 

Sucrose synthesis in the cytosol and starch synthesis in the chloroplast are the major pathways by which the excess triose phosphate from photosynthesis is harvested. Sucrose synthesis (described below) releases four Pi molecules from the four triose phosphates required to make sucrose. For every molecule of triose phosphate removed from the chloroplast, one Pj is transported into the chloroplast, providing the ninth Pj mentioned above, to be used in regenerating ATP. If this exchange were blocked, triose phosphate synthesis would quickly deplete the available Pj in the chloroplast, slowing ATP synthesis and suppressing assimilation of C02 into starch. 

Some of the triose phosphate available for product synthesis will be transported into the cytosol and converted into sucrose (Fig. 2). In both photosynthetic and nonphotosynthetic cells, sucrose is synthesized via sucrose phosphate synthase, which catalyses the reaction... 

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]. 

Sucrose phosphate synthetase catalyzes the reaction of UDP-glucose with fructose-6-P to form sucrose-6-P and UDP. This step is the penultimate step in the synthesis of sucrose in leaves. The chromatographic method can be applied to many UDP-glucose-requiring enzymes. The method eliminates the need for treatment of reaction mixtures with alkaline 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 synthesis of sucrose by Leloir and collaborators has been discussed previously. These workers found that UDP-D-glucose is the D-glucose donor for formation of sucrose. The synthesis takes place by means of two separate enzymes, one utilizing D-fructose as the acceptor, and the other, D-fructose 6-phosphate. The sucrose phosphate formed in the second reaction is hydrolyzed by a phosphatase, resulting in the formation of free sucrose. As the equilibrium of the reaction for the formation of sucrose phosphate lies to the right, and inasmuch as the large accumulation of sucrose in some plants could be better accounted for by hydrolysis of the sucrose phosphate with phosphatase (which is a practically irreversible reaction), it was suggested that sucrose in plants is most likely synthesized by way of the sucrose phosphate intermediate. 

So the CO2 and O2 g insensitivity observed in water stressed plants indicates an effect on the biochemistry of photosynthesis, and from the analytical gas exchange analysis presented above, it is predicted that the effect is on sucrose or starch synthesis, or both. Vassey and Sharkey (11) found that the extractable activity of sucrose-phosphate synthase (SPS) was reduced by nearly 70% by mild water stress (Figure 4). We have made a number of additional tests and have been able to demonstrate the following 1. SPS in water stressed plants recovers when the plant is placed in high CO2 (1 mbar) even though the stress is not relieved. 2. Unstressed plants held in a low CO2 (75 /ibar) environment for one hour exhibit reduced SPS activity. 3. The recovery of SPS activity in water stressed plants requires one to five hours and can be prevented by feeding cycloheximide. 

A ROLE FOR SUCROSE PHOSPHATE SYNTHASE IN INTRAFIBER CELLULOSE SYNTHESIS... 

Babb V.M. and Haigler C.H. 2001. Sucrose phosphate synthase activity rises in correlation with high-rate cellulose synthesis in three heterotrophic systems. Plant Physiol 127 1234-1242. 

An even more favorable mechanism for sucrose synthesis is suggested by the finding of another enzyme in wheat germ. This forms sucrose phosphate in a reaction almost identical to that described above. 

This finding is in accord with the results of experiments on Canna leaves supplied with labeled substrates, indicating that both of the immediate precursors of sucrose are phosphorylated compounds and that no free D-fructose is involved in the process of synthesis 181), Since phosphatase is also present in wheat germ, the sucrose phosphate formed in the plant may be rapidly dephosphorylated, resulting in the accumulation of free sucrose. 

Sucrose can be synthesized from (a) UDPG and fructose and (b) UDPG and fructose-6-phosphate. In the latter case the product is sucrose phosphate which can be cleaved by a phosphatase to sucrose and phosphate. The enzymes catalyzing the synthesis are called sucrose synthetase (1) and sucrose phosphate synthetase (2), depending on the product. 

Sucrose-phosphate synthetase resides exclusively within the chloro-plast and may be concerned with the initial synthesis of sucrose whereas sucrose synthetase is more abundant in non-photosynthetic tissues and may be primarily involved in the metabolism of translocated sucrose rather than its direct synthesis in association with photosynthetic reactions. 

The pentose phosphate pathway is an alternative route for the metabolism of glucose. It does not generate ATP but has two major functions (1) The formation of NADPH for synthesis of fatty acids and steroids and (2) the synthesis of ribose for nucleotide and nucleic acid formation. Glucose, fructose, and galactose are the main hexoses absorbed from the gastrointestinal tract, derived principally from dietary starch, sucrose, and lactose, respectively. Fructose and galactose are converted to glucose, mainly in the liver. 

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