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Mslsc2001c04 Metabolism MSLSC2001C04 Course Instructor Dr. Gautam Kumar Dr.Gautam Kr. Dept. of Life Sc. 1 Dr.Gautam Kr. Dept. of Life Sc. 2 Dr.Gautam Kr. Dept. of Life Sc. 3 Dr.Gautam Kr. Dept. of Life Sc. 4 • Cellulose is a major constituent of plant cell walls, providing strength and rigidity • Preventing the swelling of the cell and rupture of the plasma membrane • Plants synthesize more than 1011 metric tons of cellulose, making this simple polymer one of the most abundant compounds in the biosphere. • cellulose must be synthesized from intracellular precursors but deposited and assembled outside the plasma membrane. Dr.Gautam Kr. Dept. of Life Sc. 5 • Terminal complexes, also called rosettes, to be composed of six large particles arranged in a regular hexagon. • The outside surface of the plant plasma membrane in a freeze-fractured sample, viewed here with electron microscopy • Enzyme complex includes a catalytic subunit with eight transmembrane segments and several other subunits that are presumed to act in threading cellulose chains through the catalytic site and out of the cell, and in the crystallization of 36 cellulose strands into the paracrystalline microfibrils Rosettes Dr.Gautam Kr. Dept. of Life Sc. 6 • The complex enzymatic machinery that assembles cellulose chains spans the plasma membrane • UDP-glucose, in the cytosol and another part extending to the outside, responsible for elongating and crystallizing cellulose molecules in the extracellular space. • UDP-glucose used for cellulose synthesis is generated from sucrose produced during photosynthesis, by the reaction catalysed by sucrose synthase • Cellulose synthase spans the plasma Model for the structure of membrane and uses cytosolic UDP-glucose as Cellulose synthase. the precursor for extracellular cellulose synthesis. • Membrane-bound form of sucrose synthase forms a complex with cellulose synthase, feeding UDP- glucose from sucrose directly into cell wall synthesis • Glycosyltransferases that invert configuration are generally assumed to use a single-displacement mechanism. Dr.Gautam Kr. Dept. of Life Sc. 7 • New cellulose chains appear to be initiated by the formation of a lipid-linked intermediate unlike anything involved in starch or glycogen synthesis • Glucose is transferred from UDP-glucose to a membrane lipid, probably the plant sterol Sitosterol on the inner face of the plasma membrane • Intracellular cellulose synthase adds several more glucose residues to the first one, in (β14) linkage, forming a short oligosaccharide chain attached to the sitosterol (sitosterol dextrin) • Whole sitosterol dextrin flips across to the outer face of the plasma membrane, where most of the polysaccharide chain is removed by Endo-1,4-β-glucanase. • This entire process occurs in the rosettes. Dr.Gautam Kr. Dept. of Life Sc. 8 • The potent herbicide CGA-325615, which specifically inhibits cellulose synthesis, Dr.Gautam Kr. Dept. of Life Sc. 9 Synthesis of Cell Wall Polysaccharides Dr.Gautam Kr. Dept. of Life Sc. 10 Catabolic pathways Glucose 6-phosphate to pyruvate: Glycolysis Glycogen to glucose 6-phosphate: Glycogenolysis Anabolic pathways Pyruvate to glucose: Gluconeogenesis Glucose to glycogen: Glycogenesis Dr.Gautam Kr. Dept. of Life Sc. 11 • Excess glucose is converted to polymeric forms for storage: Glycogen in vertebrates and many microorganisms, starch in plants. • In vertebrates, glycogen is found primarily in the liver and skeletal muscle 10% of the weight of liver and 1% to 2% of the weight of muscle • Glycogen is stored in large cytosolic granules • Particle of glycogen, the -particle, about 21 nm in diameter, consists of up to 55,000 glucose residues with about 2,000 non-reducing ends. Dr.Gautam Kr. Dept. of Life Sc. 12 Glucose to Glycogen: Glycogenesis • Glycogenesis occurs in the absorptive state, when blood glucose levels are high. It is activated by a high insulin: glycogen ratio and the availability of glucose 6-phosphate. • Glycogen is synthesized in the cytosol in a process that requires ATP and UTP. Two different bonds are formed (α14 and α16). • Glucose 6-phosphate is converted to Glucose 1-phosphate by phosphoglucomutase • Glucose 1-P is attached to a UDP molecule by UDP glucose pyrophosphorylase and bound to a protein glycogen primer, Glycogenin, by glycogen synthase. • Further elongation occurs by glycogen synthase, and a branching enzyme forms alpha 1-6 linkages (instead of alpha 1-4) every 10 residues or so. Dr.Gautam Kr. Dept. of Life Sc. 13 • A glycosidic bond is formed between the anomeric C1 of the glucose moiety derived from UDP-glucose and the hydroxyl oxygen of a tyrosine side-chain of Glycogenin. Dr.Gautam Kr. Dept. of Life Sc. 14 Phosphoglucomutase Dr.Gautam Kr. Dept. of Life Sc. 15 Dr.Gautam Kr. Dept. of Life Sc. 16 • Glycogen synthase cannot make the (α16) bonds found at the branch points of glycogen; these are formed by the glycogen-branching enzyme, also called amylo (14) to (16) transglycosylase or glycosyl- (46)-transferase. • Pyridoxal phosphate (PLP), a derivative of vitamin B6, serves as prosthetic group for Glycogen Phosphorylase. • Pyridoxal phosphate (PLP) is held at the active site of Phosphorylase enzyme Dr.Gautam Kr. Dept. of Life Sc. 17 .
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