Sucrose and Other Disaccharides

Publisher Summary This chapter discusses sugar nucleotide transformations in plant The study of nucleotide sugars and of their central role in metabolism started with the discovery of UDP-Glc UDP-Glc and other sugar nucleotides are ubiquitous in living organisms The amount of nucleotide sugars is usually found to be 10–25 % of the pool of soluble nucleotides extracted from plant tissues Many enzymes directly associated with the metabolism of the nucleotide sugars are known The chapter also reviews the enzymes responsible for the formation of compounds and for inters conversions of the sugar skeleton in the nucleotide sugar molecules The major mechanism by which nucleotide sugars are produced de novo is by the action of a pyrophosphorylase that catalyzes the reaction The pyrophosphorylase reactions are actually nucleotidyl transfer reactions The requirement for nucleotide sugars varies according to the stage of plant growth, and different control mechanisms may contribute to regulating the supply of nucleotide sugar precursors at the site of utilization

Sugar nucleotide transformations in plants

Mitochondrial, plastid and cytosolic isozymes of hexokinase from developing endosperm of Ricinus communis

Some photosynthetic and biochemical properties of the chlorophyl containing layers of the pericarp of developing barley have been investigated. The tissue changes from pale green to bright green early in development, chlorophyll disappearing only at the later stages of maturity. It contains chloroplasts and probably amyloplasts and starch bearing chloroplasts. It is capable of high rates of light dependent oxygen evolution. It has been shown that the enzyme phosphoenol pyruvate carboxylase (EC 4.1.1.31) is present in the pericarp and is 100 times as active in carbon dioxide fixation as ribulose diphosphate carboxylase (EC 4.1.1.39). Other enzymes present in the pericarp are phosphoenol pyruvate synthetase, pyrophosphatase (EC 3.6.1.1), malate NAD and NADP dehydrogenases (EC 1.1.1.37), malic enzyme (EC 1.1.1.40), and fructose 1,6 diphosphatase (EC 3.1.3.11).

Some enzyme activities associated with the chlorophyll containing layers of the immature barley pericarp

The levels of reducing and nonreducing sugars, starch, soluble protein, and selected enzymes involved in the metabolism of sucrose, glucose-1-P, and glucose nucleotides were assayed in dehulled developing rice grains (Oryza sativa L. line IR1541-76-3) during the first 3 weeks after flowering. The level of reducing sugars in the grain was highest 5 to 6 days after flowering. The level of nonreducing sugars and the rate of starch accumulation were maximum 11 to 12 days after flowering, when the level of soluble protein was also the highest. The activities of bound and free invertase, sucrose-UDP and sucrose-ADP glucosyltransferases, hexokinase, phosphoglucomutase, nucleoside diphosphokinase, and UDP-glucose and ADP-glucose pyrophosphorylases were high throughout starch deposition, and were maximum, except for nucleoside diphosphokinase which did not increase in activity, between 8 and 18 days after flowering. Soluble primed phosphorylase and ADP glucose-alpha-glucosyltransferase (starch synthetase) were both present during starch accumulation. Phosphorylase activity was at least 2-fold that of soluble starch synthetase but the synthetase followed more closely the rate of starch accumulation in the grain. The activity of starch synthetase bound to the starch granule also increased progressively with increased starch content of the grain.

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E.Denise Baxter

Papers

Enzymes of carbohydrate metabolism in developing Hordeum distichum grain

Phosphorylase activity in relation to starch synthesis in developing Hordeum distichum grain