Carbohydrate Metabolism Lecture 3 PDF

Carbohydrate Metabolism Lecture 3 GLYCOGEN & Hexose MonoPhosphate Shunt By: Dr. Asmaa Mostafa Bayoumi Associate Professor, Department of Biochemistry, Faculty of Pharmacy, Minia University Reference Book: Biochemistry: The Molecular Basis o...

Carbohydrate Metabolism Lecture 3 GLYCOGEN & Hexose MonoPhosphate Shunt By: Dr. Asmaa Mostafa Bayoumi Associate Professor, Department of Biochemistry, Faculty of Pharmacy, Minia University Reference Book: Biochemistry: The Molecular Basis of Life, (McKee&McKee) Third Edition Glycogen Metabolism The synthesis and degradation of glycogen are carefully regulated so that sufficient glucose is available for the body's energy needs. Both glycogenesis and glycogenolysis are controlled primarily by three hormones; insulin, glucagon, and epinephrine. Glycogenesis Glycogen synthesis occurs in liver after a meal, when blood glucose levels are high. 1. Synthesis of glucose-1-phosphate. Glucose-6-phosphate is reversibly converted to glucose-1-phosphate by phosphoglucomutase, an enzyme that contains a phosphoryl group attached to a reactive serine residue: 2. Synthesis of UDP-glucose. Derivatizing the sugar with a good leaving group provides the driving force for most sugar transfer reactions. For this reason, sugar-nucleotide synthesis is a common reaction preceding sugar transfer and polymerization processes. Uridine diphosphate glucose (UDP-glucose) is more reactive than glucose and is held more securely in the active site of the enzymes catalyzing transfer reactions. Because UDP-glucose contains two phosphoryl bonds, it is a highly energized molecule. Formation of UDP-glucose is catalyzed by UDP-glucose pyrophosphorylase: 3. Synthesis of glycogen from UDP-glucose.  The formation of glycogen from UDP-glucose requires two enzymes: a. Glycogen synthase, which catalyzes the transfer of the glucosyl group of UDP-glucose to the non-reducing ends of glycogen, and b. Amylo-α(l,4 to 1,6)-glucosyl transferase (Branching enzyme), which creates the α(l ,6) linkages for branches in the molecule.  Glycogen synthesis is believed to be initiated by the transfer of glucose from UDP-glucose to a specific tyrosine residue in a "primer" protein called glycogenin. 3. Synthesis of glycogen from UDP-glucose. Glycogenolysis Glycogen degradation requires the following two reactions: 1. Removal of glucose from the nonreducing ends of glycogen. Using inorganic phosphate (Pi), glycogen phosphorylase cleaves the α (1,4) linkages on the outer branches of glycogen to yield glucose-1-pbosphate. Glycogen phosphorylase stops when it comes within four glucose residues of a branch point. 2. Hydrolysis of the α (l,6) glycosidic bonds at branch points of glycogen. Amylo-α(l,6)- glucosidase, also called Debranching enzyme, begins the removal of α(l,6) branch points by transferring the outer three of the four glucose residues attached to the branch point to a nearby nonreducing end. It then removes the single glucose residue attached at each branch point. The product of this latter reaction is free glucose. Cori's disease: It is glycogen-storage disease. Patients with Cori's disease, caused by a deficiency of debranching enzyme, have enlarged livers (hepatomegaly) and low blood sugar concentrations (early fasting hypoglycemia). Glycogenolysis Glycogenolysis Glycogenolysis Regulation of Glycogen Metabolism  Both glycogen synthesis and degradation are controlled through a complex mechanism involving insulin, glucagon, and epinephrine.  The binding of glucagon to liver cells stimulates glycogenolysis and inhibits glycogenesis. After glucagon binds to its receptor, adenylate cyclase is stimulated to convert ATP to the intracellular signal molecule 3'-5' cyclic AMP (cAMP). Then cAMP initiates a reaction cascade that amplifies the original signal to initiate release of thousands of glucose molecules.  When insulin binds to its receptor, the receptor becomes an active tyrosine kinase enzyme that causes a phosphorylation cascade: the enzymes of glycogenolysis are inhibited and the enzymes of glycogenesis are activated. Insulin also increases the rate of glucose uptake into several types of target cells, but not liver or brain cells.  Emotional or physical stress releases epinephrine from the adrenal medulla. Epinephrine promotes glycogenolysis and inhibits glycogenesis. In emergency situations, when epinephrine is released in relatively large quantities, massive production of glucose provides the energy required to manage the situation (flight-or fight response). Regulation of Glycogen Metabolism  Glycogen synthase and glycogen phosphorylase have both active and inactive conformations that are interconverted by covalent modification.  The active form of glycogen synthase, known as the I (independent) form, is converted to the inactive or D (dependent) form by phosphorylation.  In contrast, the inactive form of glycogen phosphorylase (phosphorylase b) is converted to the active form (phosphorylase a) by the phosphorylation of a specific serine residue. The phosphorylating enzyme is called phosphorylase kinase.  Phosphorylation of both glycogen synthase and phosphorylase kinase is catalyzed by a protein kinase, which is activated by cAMP.  Glycogen synthesis occurs when glycogen synthase and glycogen phosphorylase have been dephosphorylated. This conversion is catalyzed by phosphoprotein phosphatase. Phosphoprotein phosphatase also inactivates phosphorylase kinase. Hexose MonoPhosphate Shunt  The Pentose Phosphate Pathway It is an alternative metabolic pathway for glucose oxidation in which no ATP is generated. Its principal products are NADPH, a reducing agent required in several anabolic processes, and ribose-5- phosphate, a structural component of nucleotides and nucleic acids. The pentose phosphate pathway occurs in the cytoplasm in two phases: oxidative and nonoxidative. In the oxidative phase, the conversion of glucose-6-phosphate to ribulose-5-phosphate is accompanied by the production of two molecules of NADPH. The nonoxidative phase involves the isomerization and condensation of a number of different sugar molecules. Three intetmediates in this process that are useful in other pathways are ribose-5-phosphate, fructose-6-phosphate, and glyceraldehyde-3-phosphate.  The oxidative phase of the pentose phosphate pathway It consists of three reactions: In the 1st reaction, glucose-6-phosphate dehydrogenase (G-6-PD) catalyzes the oxidation of glucose-6-phosphate. This is the key reaction. 6-Phosphogluconolactone and NADPH are products in this reaction. In the 2nd rexn., 6-Phosphogluconolactone is hydrolyzed by gluconolactonase to produce 6-phosphogluconate. In the 3rd rexn., another molecule of NADPH is produced during the oxidative decarboxylation of 6-phosphogluconate by 6-phosphogluconate dehydrogenase, a reaction that yields ribulose-5-phosphate.  The nonoxidative phase of the pentose phosphate pathway It begins with the conversion of ribulose-5-phosphate to ribose-5-phosphate by ribulose-5-phosphate isomerase or to xylulose-5-phosphate by ribulose- 5-phosphate epimerase. During the remaining reactions of the pathway, transketolase and transaldolase catalyze the interconversions of trioses, pentoses, and hexoses. Transketolase is a TPP-requiring enzyme that transfers two-carbon units from a ketose to an aldose. (TPP, thiamine pyrophosphate). Two reactions are catalyzed by transketolase. In the first transketolase- catalyzed reaction, the enzyme transfers a two-carbon unit from xylulose-5- phosphate to ribose-5-phosphate, yielding glyceraldehyde-3-phosphate and sedoheptulose-7-phosphate.  The nonoxidative phase of the pentose phosphate pathway In the reaction catalyzed by transaldolase, a three-carbon unit is transferred from sedoheptulose-7-phosphate to glyceraldehyde-3-phosphate. The products formed are erythrose-4-phosphate and fructose-6-phosphate. (Erythrose-4-phosphate is used by some organisms to synthesize aromatic amino acids). In the second transketolase-catalyzed reaction, a two-carbon unit from another xylulose-5-phospbate molecule is transferred to erythrose-4- phosphate to form a second molecule of glyceraldehyde-3-phosphate and fructose-6-phospbate. The result of the nonoxidative phase of the pathway is the synthesis of ribose-5-phosphate and the glycolytic intermediates glyceraldehyde-3- phosphate and fructose-6-phosphate.  Glycolysis & The HMP Shunt When pentose sugars are not required for biosynthetic reactions, the metabolites in the nonoxidative portion of the pathway are converted into glycolytic intermediates that can then be further degraded to generate energy or converted into precursor molecules for biosynthetic processes. For this reason the pentose phosphate pathway is also referred to as the hexose monophosphate shunt (HMP shunt). In plants, the pentose phosphate pathway is involved in the synthesis of glucose during the dark reactions of photosynthesis.  NADPH A substantial amount of the NADPH required for reductive processes is supplied by pentose phosphate pathway: This pathway is most active in cells in which relatively large amounts of lipids are synthesized (lipogenesis and steroid synthesis), for example, adipose tissue, adrenal cortex, mammary glands, and the liver. In the eye, NADPH is important for retinal reductase enzyme activity to transform retinal into retinol. Also, NADPH is an important source of energy to the cornea, retina and eye lens. NADPH is required for dihydrofolate reductase enzyme activity to convert folic acid to its biologically active form (THF).  NADPH & RBCs NADPH is also a powerful antioxidant. (Antioxidants are substances that prevent the oxidation of other molecules). Consequently, the oxidative phase of the pentose phosphate pathway is active in cells that are at high risk for oxidative damage, such as red blood cells. Glutathione peroxidase utilizes reduced glutathione (GSH) to reduce the peroxides generated by cellular aerobic metabolism. GSH is regenerated from its oxidized form, GSSG, by glutathione reductase. NADPH, the reducing agent in this reaction, is supplied by the pentose phosphate pathway.  NADPH & RBCs Under normal conditions, a few percent of hemoglobin molecules become oxidized. The oxidized product of hemoglobin, called methemoglobin, with its heme- Fe3+ group is no longer able to bind oxygen. GSH functions as coenzyme for methemoglobin reductase which keeps iron of hemoglobin in the ferrous state, thus, preserving its capacity to carry oxygen. In absence of GSH, RBCs become fragile because the lipid peroxidation caused by H2O2 damages the cell‘s plasma membrane. When such cells pass through narrow blood vessels, they may rupture. If the oxidative stress is severe, hemolytic anemia results. A similar condition known as favism results when glucose-6-phosphate dehydrogenase-deficient individuals develop a severe hemolytic anemia after eating fava beans or intake of antimalarial drugs. In patients with favism, ribose-5-P is synthesized by the reversal of HMP shunt. G-6-P (Isomerase) F-6-P (Reversal of HMP Shunt) R-5-P  Regulation of The HMP Shunt The pentose phosphate pathway is regulated to meet the cell‘s requirements for NADPH and ribose-5-phosphate. The oxidative phase is very active in cells such as red blood cells or hepatocytes in which demand for NADPH is high. In contrast, the oxidative phase is virtually absent in cells (e.g., muscle cells) that synthesize little or no lipid. G-6-PD catalyzes a key regulatory step in the pentose phosphate pathway. Its activity is inhibited by NADPH and stimulated by GSSG (the oxidized form of glutathione) and glucose-6-phosphate. In addition, insulin and diets high in carbohydrate stimulate HMP shunt via increasing the synthesis of both G-6-PD and 6-phosphogluconate dehydrogenase.