Lecture 3 PDF: HMP Shunt, Uronic Acid Pathway, and Glycogen Metabolism

HMP shunt, Uronic acid pathway and Glycogen metabolism Dr. Yasser Elghobashy Medical Biochemistry and Molecular Biology  Hexose monophosphate shunt (HMP shunt) Definition: - It is an alternative pathway of glucose oxidation in which ATP is neither p...

HMP shunt, Uronic acid pathway and Glycogen metabolism Dr. Yasser Elghobashy Medical Biochemistry and Molecular Biology  Hexose monophosphate shunt (HMP shunt) Definition: - It is an alternative pathway of glucose oxidation in which ATP is neither produced nor utilized. Site:- Cytoplasm of many tissues Steps:- The reactions of HMP-shunt occurs in 2 phases 1. Oxidative (irreversible) phase:- – Where 3 molecules of glucose are converted into 3 molecules of ribulose-5-P with production of NADPH+H+ and CO2. 2. Non-oxidative (reversible) phase:- – Where 3 molecules of ribulose-5-P are converted into 2 molecules of glucose-6-P and 1 molecule of glyceraldhyde-3-P.  Functions of HMP shunt A. Production of pentoses (ribose-5-p) used for synthesis of - DNA, RNA - ATP, GTP....etc - NAD, FAD....etc B. Production of NADPH+H+:- It is important for synthesis; – Fatty acids and steroid hormones. – Non-essential amino acids. – Malate from pyruvate by malic enzyme. – Reduced glutathione in erythrocytes; Glutathione reductase G-S-S-G 2 G-SH NADPH+H+ NADP (Oxidized glutathione) (Reduced glutathione)  Reduced glutathione is needed for; 1. Maintenance of the normal integrity of RBCs by removal of hydrogen peroxide (H2O2), which is a toxic compound that increases cell membrane fragility. Glutathione peroxidase 2 G-SH + H2O2 2 H2O + G-S-S-G Glutathione reductase NADP NADPH+H+  Regulation of HMP shunt Glucose-6-phosphate DH is the key enzyme of HMP shunt: - – It is stimulated by insulin and NADP+ – And inhibited by NADPH+H+ and acetyl CoA.  Uronic acid pathway Definition: - It is an alternative pathway of glucose oxidation in which glucose is converted into glucuronic acid. Site: - It occurs in the cytoplasm of many tissues. Importance of uronic acid pathway: - A. Production of glucuronic acid in the form of UDP-glucuronic acid which is used in :- 1) Synthesis of substrates as – Glycosaminoglycans. – Vitamin C (L-ascorbic acid) in animals only not in human. (Humans can't convert glucuronic acid into ascorbic acid due to absence of L- gluconolactone oxidase)  2) Conjugation reactions:- with many substances e.g bilirubin to make them more water soluble and easily excreted. 3) Detoxification reactions:- to make the toxic compounds less toxic. B. Production of pentoses  Glycogen Metabolism Glycogen is the storage from of carbohydrates in animals. It is formed of α-D glucose units linked together by α1-4 glucosidic bonds and by α 1-6 glucosidic bonds at the branch points. It is stored mainly in the liver and muscles.  Liver glycogen Muscle glycogen o Occurs in the liver. o Occurs in muscles. o Constitutes up to 6% of liver o Rarely exceeds 1% of ms mass. mass. o Its function is to maintain blood o Acts as a source of glucose - glucose during fasting. 6-P for glycolysis of muscle only. o Depletes after 12-18 hrs of o Depletes after prolonged fasting. vigorous muscle exercise. o Glucagon stimulates o No effect glycogenolysis  1. Glycongenesis Definition: - It is the synthesis of glycogen from glucose. Site: - Cytoplasm of liver and muscle cells. Sources of glucose units:- A. For liver glycogen: it includes. 1. Blood glucose. 2. Other hexoses: galactose & fructose. 3. Non-carbohydrate sources by gluconeogenesis e.g lactate & glycerol. B. For muscle glycogen: - 1. Blood glucose only.  Steps 1. Formation of UDP - glucose (UDP-G) as follow: Hexokinase in muscles Glucokinase in liver Phosphoglucomutase Glucose Glucose-6-P Glucose-1-P UDP-glucose UTP Phosphorylase PPi Uridine diphosphate glucose (UDP-G) 2. Formation of glycogen from UDP - G units needs: A. Glycogen primer which is formed of – Few molecules of glucose linked by α1-4 linkage. or – A protein called glycogenin where UDP - glucose residues are added to the hydroxyl group of its tyrosine residue. B. Glycogen synthase enzyme. – The key regulatory enzyme of glycogenesis. – It catalyzes the formation of α 1-4 glucosidic bond between C1 of the activated glucose of UDP-G and C4 of a terminal glucose residue of a glycogen primer. Glycogen synthase UDP-G + Elongated glycogen primer Glycogen primer + UDP – It elongates the primer up to 11 glucose units. C. Branching enzyme: - It transfers about 6 glucose units of the elongated chain to a neighboring chain forming α1-6 glucosidic bond forming a branch point in the molecule which is elongated by glycogen synthase.  2. Glycogenolysis Definition: - It is the process of degradation of glycogen into glucose units (in liver) or glucose-6-p (in muscles). Site: - Cytoplasm of liver and muscle cells. Steps: - Glycogenolysis needs the following 1. Phosphorylase enzyme:- – The key regulatory enzyme of glycogenolysis. – It acts on branches containing more than 4 glucose units. – It breaks down α l - 4 glycosidic bonds by phosphorylysis (i.e breakdown by addition of phosphate) giving glucose-1- P. 2. Glucan transferase enzyme:- – It acts on the branch containing 4 glucose units where it transfers 3 glucose units to other branch leaving the last one which is linked by α 1 - 6 linkage. 3. Debranching enzyme:- – It removes the last glucose units that is attached by α 1 - 6 linkage by hydrolysis (i.e breaking down by addition of water) giving glucose. – Note:- G-1-P is converted to G-6-P by phosphoglucomutase. Fate of glucose -6- phosphate:- In liver:- The liver contains glucose - 6 - phosphatase so it converts g - 6 - p into glucose that is released to the blood. In muscle:- It contains no glucose - 6 - phosphatase, so muscle glycogenolysis ends with G-6-P which can’t leave the muscle. Regulation of glycogenesis and glycogenolysis There is a coordinated regulation of glycogenesis and glycogenolysis i.e conditions stimulating glycogenesis inhibit glycogenlysis and vice versa. The key regulatory enzymes include:- 1. Glycogen synthase: - present in 2 forms. – Active dephosphorylated form (glycogen synthase a). – Inactive phosphorylated form (glycogen synthase b). 2. Phosphorylase :- present in 2 forms – Active phosphorylated form (phosphorylase a). – Inactive dephosphorylated form (phosphorylase b).  A. During fasting 1. The blood glucose tends to decrease causing release of epinephrine in muscle and liver and glucagon in liver only. 2. These hormones bind to beta receptors in the cell membrane activating adenylate cyclase. 3. Active adenylate cyclase catalyzes formation of cAMP from ATP. 4. cAMP activates cAMP dependent protein kinase. 5. The active cAMP dependent protein kinase causes: – Phosphorylation and inactivation of glycogen synthase inhibiting glycogenesis. – Phosphorylation and activation of phosphorylase kinase that phosphorylates and activates phosphorylase enzymes producing glycogenolysis.  B. After meals Blood glucose level tends to be increased; this stimulates insulin secretion which decreases blood glucose by stimulation of glycogenesis and inhibition of glycogenolysis. This occurs by:- – Stimulation of phosphodiestrase enzyme which breaks down cAMP into 5AMP → no stimulation of protein kinase – Stimulation of phosphatase enzyme which removes the phosphate group from enzymes.