Biosynthesis of Starch and Sucrose:- UDP-Glucose Is the Substrate for Sucrose Synthesis in the Cytosol of Leaf Cells
Most of the triose phosphate generated by CO2 fixation in plants is converted to sucrose (Fig. 20–25) or starch. In the course of evolution, sucrose may have been selected as the transport form of carbon because of its un usual linkage between the anomeric C-1 of glucose and the anomeric C-2 of fructose. This bond is not hydrolyzed by amylases or other common carbohydrate-cleaving enzymes, and the unavailability of the anomeric carbons prevents sucrose from reacting nonenzymatically (as does glucose) with amino acids and proteins. Sucrose is synthesized in the cytosol, beginning with dihydroxyacetone phosphate and glyceraldehyde 3-phosphate exported from the chloroplast. After condensation of two triose phosphates to form fructose 1,6 bisphosphate (catalyzed by aldolase), hydrolysis by fructose 1,6-bisphosphatase yields fructose 6-phosphate. Sucrose 6-phosphate synthase then catalyzes the reaction of fructose 6-phosphate with UDP-glucose to form sucrose 6-phosphate (Fig. 20–25). Finally, sucrose 6-phosphate phosphatase removes the phosphate group, making sucrose available for export to other tissues. The reaction catalyzed by sucrose 6-phosphate synthase is a low-energy process (ΔG0=-16.5 kJ/mol) to make the overall synthesis of sucrose essentially irreversible. Sucrose synthesis is regulated and closely coordinated with starch synthesis, as we shall see. One remarkable difference between the cells of plants and animals is the absence in the plant cell cy tosol of the enzyme inorganic pyrophosphatase, which catalyzes the reaction PPi+H2O→2Pi ΔG0=-19.2 kJ/mol For many biosynthetic reactions that liberate PPi, pyrophosphatase activity makes the process more favor able energetically, tending to make these reactions i reversible. In plants, this enzyme is present in plastids but absent from the cytosol. As a result, the cytosol of leaf cells contains a substantial concentration of PPi— enough (~0.3 mM) to make reactions such as that cat alyzed by UDP-glucose pyrophosphorylase (Fig. 15–7) readily reversible. Recall from Chapter 14 (p. 527) that the cytosolic isozyme of phosphofructokinase in plants uses PPi, not ATP, as the phosphoryl donor.
FIGURE 20–24 Starch synthesis. Starch synthesis proceeds by a two site insertion mechanism, with ADP-glucose as the initial glucosyl donor. The two identical active sites on starch synthase alternate in displacing the growing chain from each other, and new glucosyl units are inserted at the reducing end of the growing chain.
