GMS 6604 Cellular Energy I: Glycolysis & Gluconeogenesis 2024 PDF

Cellular Energy I: Glycolysis & Gluconeogenesis Andreas Seyfang, PhD Professor Department of Molecular Medicine Office: MDC 4010A  974-2332; [email protected] GMS 6604: Human Structure and Function Monday, September...

Cellular Energy I: Glycolysis & Gluconeogenesis Andreas Seyfang, PhD Professor Department of Molecular Medicine Office: MDC 4010A  974-2332; [email protected] GMS 6604: Human Structure and Function Monday, September 23, 2024 1  1. Glycolysis Describe the function of glycolysis. Explain the strategy of the pathway. Describe the key steps in glycolysis. Explain the mechanism by which the activity of the rate-limiting step is regulated. Illustrate how glycolysis interrelates with other metabolic pathways. 2  Function of Glycolysis The function of glycolysis is to convert glucose to three-carbon compounds with the formation of ATP Glycolysis occurs in all the cells of the body and the enzymes are located mainly in the cytosol. 3  Metabolic Sequence Glycolysis is a sequence of ten reactions in which glucose is converted to pyruvate. There is an initial requirement for ATP but glycolysis results in a net production of 2 ATP. There is one oxidative step in which NAD is reduced to NADH. 4  Glycolysis – Glucose to Pyruvate Summary of glycolytic pathway reactions rate-limiting step 5  rate-limiting step 6 7  Regulation of Glycolysis Glycolytic flux is controlled by need for ATP and/or for intermediates formed by the pathway (e.g., for fatty acid synthesis) Control occurs at sites of irreversible reactions Phosphofructokinase- major control point; first enzyme “unique” to glycolysis Hexokinase (or glucokinase) Pyruvate kinase Phosphofructokinase responds to changes in: Energy state of the cell (high ATP levels inhibit) H+ concentration (high lactate levels inhibit) Availability of alternate fuels such as fatty acids, ketone bodies (high citrate levels inhibit) Insulin/glucagon ratio in blood (high fructose 2,6-bisphosphate 8 levels activate)  Effect of F-2,6-BP on Phosphofructokinase Activity F-2,6-BP is a strong activator of glycolysis 9 (increases phosphofructokinase’s substrate affinity)  Regulation of Pyruvate Kinase Phosphorylated PK is less active 10  Hormone Production by the Pancreas Each hormone produced by a different subset of cells in the Islets of Langerhans. Elevated blood glucose trips an ATP-dependent switch in beta cells leading to insulin release. 11  Glucokinase (GK) catalyzes the rate-controlling step in glucose-stimulated insulin secretion of pancreatic beta cells rate-controlling step 12  Blood Glucose Control of Insulin Secretion and anti-Diabetic Drug Target Pharmacogenomics PharmGKB link: https://www.pharmgkb.org/pathway/PA153627758 Anti-diabetic Drugs: X Sulfonylurea Repaglinide (Prandin®) 13  Insulin Secretion Glucose homeostasis relies critically on detection of variations in blood glucose concentrations by pancreatic beta cells and their timely release of an appropriate amount of insulin. Glucose sensing by pancreatic beta cells requires glucose uptake and metabolism through the glycolytic pathway. Activation of the TCA cycle and oxidative phosphorylation generates ATP and the increased ATP/ADP ratio induces plasma membrane depolarization by closing an ATP-dependent K+ channel. This leads to the opening of voltage-dependent Ca2+ channels, and the entry of calcium triggers insulin granule exocytosis. The rate-controlling step in glucose-stimulated insulin secretion is the phosphorylation of glucose by glucokinase. The role of glucokinase is underlined by the finding that an autosomal dominant form of early onset type 2 diabetes is caused by a mutation in the glucokinase gene. Glucose uptake by beta cells is catalyzed by a low affinity, high-capacity glucose transporter called GLUT2. 14  Summary of Glycolysis Glycolysis is the conversion of glucose to pyruvate with ATP production. Glycolysis is particularly critical for brain and erythrocyte metabolism and exercising muscle. Hexokinase, phosphofructokinase-1 and pyruvate kinase catalyze key irreversible steps in glycolysis. Phosphofructokinase-1 catalyzes the rate-limiting step and is regulated by the levels of fructose 2,6-bisphosphate as a glycolysis activator. Pyruvate kinase is a site of secondary regulation. It undergoes phosphorylation-dephosphorylation. Under anaerobic conditions NAD+ is regenerated by 15 conversion of pyruvate to lactate.  2. Gluconeogenesis Explain the function of gluconeogenesis. Identify the precursors and requirements for gluconeogenesis. Identify the tissue distribution and intracellular location of the gluconeogenic pathway. Compare the unique reactions of gluconeogenesis relative to those of glycolysis. Identify the enzyme that catalyzes the rate-limiting step in gluconeogenesis and how it is regulated. 16  ER lumen Glycolysis Gluconeogenesis Location: rate-limiting rate-limiting Location: step step All cells; cytosol cytosol Liver Erythrocytes, Kidney Brain and CNS [mitochondria-cytosol- Muscle ER lumen] [cytosol only] Net: + 2 ATP, + 2 NADH Net: - 4 ATP, - 2 GTP, cytosol - 2 NADH mitochondria 17  The Gluconeogenic Pathway ER lumen rate-limiting step rate-limiting step F2,6BP (+) F2,6BP (-) Glucagon (+) insulin inhibition Insulin (-) Glucagon (+) mitochondria 18  Glucose 6-phosphatase is located on the luminal side of the ER 19  Gluconeogenesis is NOT appropriately down-regulated in diabetes mellitus The lack of insulin (type 1 diabetes) or insufficient insulin to overcome insulin resistance (type 2 diabetes) causes the gluconeogenesis pathway to be uninhibited even when blood glucose concentration is high. The liver pumps out glucose, contributing to the already high blood glucose. Metformin, a drug commonly used to treat type 2 diabetes, is effective largely because it stimulates liver AMP-activated protein kinase (AMPK), which results in the insulin-independent inhibition of gluconeogenesis. 20  Regulation of Gluconeogenesis in the Liver   21  Important! Liver contains glucose 6-phosphatase. Muscle does not have this enzyme. WHY? The liver releases glucose to the blood to be taken up by brain and active muscle. The liver regulates blood glucose levels (gluconeogenesis). The muscle retains glucose 6-phosphate to be use for energy. Phosphorylated glucose is not transported out of muscle cells (no gluconeogenesis). 22  What causes insulin levels to rise in the body prior to food uptake? 23  Mechanisms by which acetylcholine stimulates insulin secretion in the preabsorptive phase in food intake. 24  Mechanisms by which acetylcholine stimulates insulin secretion in the absorptive phase in food intake. 25  Cori cycle: Lactate recycled from erythrocytes (or muscle cells) to liver Alanine cycle: Alanine recycled from muscle cells to liver 26  Summary of Gluconeogenesis Gluconeogenesis is needed to maintain blood glucose levels under fasting conditions. Precursors of gluconeogenesis are lactate, glycerol, and several amino acids, but never acetyl-CoA. Gluconeogenesis requires ATP, GTP, and NADH. The liver is the primary gluconeogenic tissue; the pathway requires participation of enzymes located in the mitochondria, cytosol, and endoplasmic reticulum. The rate-limiting reaction in gluconeogenesis is catalyzed by fructose 1,6-bisphoshatase which is inhibited by fructose 2,6- bisphosphate (F2,6BP) and stimulated by glucagon. Note, reciprocally, F2,6BP stimulates phosphofructokinase in glycolysis. 27  Increased glucagon levels lower the levels of fructose 2,6- bisphosphate, removing the inhibition of fructose 1,6-bisphosphatase and promoting gluconeogenesis. The Cori cycle involves conversion of lactate to glucose in the liver via gluconeogenesis and the metabolism of glucose to lactate in the muscle or erythrocytes. Glucokinase (GK) catalyzes the rate-controlling step in glucose- stimulated insulin secretion. Glucose phosphorylation by glucokinase leads to a rise in the ATP/ADP ratio causing the membrane to depolarize. Opening of voltage-dependent Ca2+ channels (VDCC) results in a rise in intracellular [Ca2+], which stimulates insulin secretion. 28  3. Glycogen Metabolism Describe the function of glycogen relative to its mechanism of synthesis and breakdown. Identify the sites of regulation of glycogen synthesis and degradation Describe the mechanisms by which glycogen metabolism is regulated in liver and muscle Explain why a deficiency in glucose 6-phosphatase results in a glycogen storage disease 29  Glycogen Function In liver – The synthesis and breakdown of glycogen is regulated to maintain blood glucose levels. In muscle - The synthesis and breakdown of glycogen is regulated to meet the energy requirements of the muscle cell. 30  Daily Variations in Hepatic Glycogen Levels Electron micrographs showing glycogen granules (dark stained material in the liver of a well-fed rat (left) and a rat starved for 24 hours (right) 31  Branched Structure of Glycogen Note: each glycosidic bond removes two OH groups from sugar that results in two water molecules less to bind, i.e. polysaccharides are less osmotically active than monosaccharide. 32  Action of Branching Enzyme 33  Glycogenin Provides a Primer for Glycogen Synthesis 2-D Cross-sectional view of a glycogen molecule: A core protein of glycogenin is surrounded by branches of glucose units. The entire globular complex may contain approximately 30.000 glucose units. 34  Degradation of glycogen in liver contributes to blood glucose but not degradation of glycogen in muscle (a Gluconeogenesis enzyme) Liver Muscle (not capable of carrying out gluconeogenesis) 35  Glycogen Degradation 36  Regulation of Glycogen Synthesis When blood glucose levels are high, insulin stimulates glycogen synthesis that helps to reduce blood glucose levels. This is accomplished through a complex highly regulated signal transduction pathway. Remember: Glycogen metabolism in liver regulates blood glucose levels. 37  Epinephrine and Glucagon Stimulate Glycogen Breakdown Muscle is responsive to epinephrine. Liver is responsive to glucagon and somewhat responsive to epinephrine. Both signal a cascade of molecular events leading to glycogen breakdown. 38  Summary of Glycogen Metabolism Glycogen synthesis and breakdown helps to maintain blood glucose levels. The highly branched structure of glycogen provides a large number of sites for addition or removal of glucose to facilitate a more rapid response to bodily needs. Its proteinaceous core (glycogenin) is a self-glucosylating enzyme that provides the poly-glucose primer for glycogen synthesis. Glycogen synthase and glycogen phosphorylase catalyzes the rate-limiting steps of glycogen synthesis and degradation, respectively. A sugar nucleotide, UDP-glucose, provides the glucose residues for the synthesis of glycogen. 39