Red Cell Metabolism and Function PDF

Red Blood Cell Metabolism RECAP OF LAST LECTURE Erythropoiesis Haemoglobin formation Types of haemoglobins Case study on Microcytic hypochromic anaemia RBC Metabolism Learning Objectives 1. Support on group work assignment 2. Describe the function of RBCs 3. Recognise main metabolic pathwa...

Red Blood Cell Metabolism RECAP OF LAST LECTURE Erythropoiesis Haemoglobin formation Types of haemoglobins Case study on Microcytic hypochromic anaemia RBC Metabolism Learning Objectives 1. Support on group work assignment 2. Describe the function of RBCs 3. Recognise main metabolic pathways within the RBC 4. Analyse the role of GLUT-1 (DHA and Glucose) 5. Discuss the relationship between characteristic features and structure of the RBC membrane. 6. Identify the causes of major RBC metabolic disorders  RBC are small cells (6-8μm) Carr O from y CO r t 2 spo lung 2 , to the Tran gs to the es. s lun al tissu pher peri RBC Function D l of ispo osa H+ sa sp ion l Di f CO 2 s o RBC Metabolism-Introduction RBCs are not true cells? (Yes/No) Composed of Membrane surrounding a solution of haemoglobin (95%). Bag filled with Hb. RBCs contain no nucleus nor nucleic acids and thus, can not reproduce. RBCs contain no cell organelles (as mitochondria, Golgi, ER or lysosomes) and thus possess no synthetic activities. RBC Metabolism-Introduction No protein biosynthesis, No lipid synthesis & No carbohydrate synthesis. RBCs must be able to squeeze through some tight spots in micro-circulation. For that RBCs must be easily & reversibly deformable RBC Metabolism RBC metabolic processes require energy for Maintenance of membrane phospholipid distribution Maintenance of skeletal protein deformability Maintenance of functional Hb with ferrous Iron (Fe 2+) Protecting cell proteins from oxidative state Glycolysis initiation and maintenance Glutathione synthesis Biochemical composition of RBC Red cells contain about 35 % solids. Haemoglobin, the chief protein of the red cells. Other proteins are present in combination with lipids and oligosaccharide chains, forming the stroma and cell membrane. K, Mg and Zn concentrations in RBCs are higher than in the plasma RBC Metabolism RBCs contain no mitochondria, so there is no respiratory chain, no citric acid cycle, and no oxidation of fatty acids or ketone bodies. The RBC is highly dependent upon glucose as its energy source. Energy in the form of ATP is obtained ONLY from the glycolytic breakdown of glucose with the production of lactate (anaerobic glycolysis). RBC Metabolism-ATP ATP produced in the RBC is used for Keeping the biconcave shape of RBCs In the regulation of transport of ions & water in and out of RBCs. To drive the Na+–K+-ATPase and the anion exchange protein RBC Metabolism-Glucose transport Glucose is transported through RBC membrane by facilitated diffusion through glucose transporters (GLUT-1). Glucose transporters (GLUT-1) are independent of insulin i.e., insulin does not promote glucose transport to RBCs It functions by generating a gated pore in the membrane to permit passage of glucose;  Main role of GLUT1 is to transport glucose following its chemical gradient. Intracellular glucose is immediately turned into glucose-6-phosphate (G6P) G6P can either be used for glycolysis or the PPP. Normal glycolysis produces 2 molecules of ATP out of 1 molecule of glucose. Increasing intracellular [ATP] inhibit glucose uptake through GLUT1. provides a negative feedback regulation. GLUT1 also transports dehydroascorbic acid (DHA, an oxidized form of vitamin C). DHA used to balance intracellular RedOx reactions  RBC Metabolism: Glycolysis Glucose is metabolized in RBCs through anaerobic glycolysis Anaerobic glycolysis does not require mitochondria oxygen One molecule of glucose yields 2 ATP molecules by one anaerobic glycolytic pathway. In addition, 2 molecules of lactate are produced. Lactate is use in gluconeogenesis. Gluconeogenesis Lactate is transported to blood & in the liver it is converted to glucose. Anaerobic glycolysis Important in red blood cells Energy production: -it is the only pathway that supplies RBCs with ATP. Reduction of methaemoglobin: -glycolysis provides NADH for reduction of metHb by NADH- cytob5 reductase In red cells 2,3 DPG binds to Hb, decreasing its affinity for O2, and helps its availability to tissues. RBC Metabolism  Genetic defects in enzymes of glycolysis Genetic defects of one of the enzymes of glycolysis in RBCs results in a reduced rate of glycolysis in RBCs & by this way deprive RBCs of the only means for producing energy. Reduced energy causes compromised RBC flexibility Compromised RBC flexibility causes haemolytic anaemia 95% of cases of genetic defects in glycolytic enzymes is caused by PK deficiency. 4% is caused by phosphoglucose isomerase deficiency Production of 2,3 DPG: Luebering Rapoport shunt In RBCs, some of glycolytic pathways are modified so that 2, 3 DPG is formed (by DPG mutase). 2, 3 DPG decreases affinity of Hb for O2 So, it helps oxyhaemoglobin to unload oxygen. Storing blood results in reduced 2,3-DPG leads to high oxygen affinity Hb. This leads to oxygen trap. 6-24 hours are needed to restore the depleted 2,3 DPG. Maximum storage time for RBCs is 21-42 days RBC Metabolism: Pentose phosphate pathway Pentose phosphate pathway (HMP-SHUNT) RBCs contain an active pentose phosphate pathway (PPP) for glucose that supplies NADPH PPP is the only source for NADPH in RBCs NADPH is important in keeping glutathione as reduced glutathione. Reduced glutathione plays a vital role in the survival of the RBCs. (prevents oxidation of membrane)  Pentose α&β phosphate pathway  RBC Metabolism: PPP G6PD Deficiency: Glucose 6-phosphate dehydrogenase is the first enzyme of pentose phosphate pathway & its deficiency leads to reduced production of NADPH ending in acute haemolytic anaemia RBC Metabolism The RBCs contain carbonic anhydrase CO2 combines with water only after it enters the red cells where Hb, the most important buffer for carbonic acid, is present. CO2 + H2O → HCO3- + H+ The red cell also contain rhodanese enzyme responsible for the detoxification of cyanides RBC Membrane Membrane of the Human Red Blood Cell is about 50% protein, 40% fat & up to 10% carbohydrate. RBCs membrane comprises: a lipid bilayer -determine the membrane fluidity proteins -responsible for flexibility Proteins are either peripheral or integral penetrating the lipid bilayer & carbohydrates that occur only on the external surface. RBC Membrane RBC Membrane The major lipid classes in membrane are phospholipids and cholesterol; Major Phospholipids are Phosphatidylcholine (PC), Phosphatidylethanolamine (PE), and Phosphatidylserine (PS) along with Sphingomyelin (Sph). – The choline-containing phospholipids, predominate in the outer leaflet RBC Membrane The major phospholipids are Phosphatidyl Choline and Sphingomyelin – The amino-containing phospholipids predominate in the inner leaflet. Phosphatidyl Ethanolamine and Phosphatidyl Serine Glycosphingolipids (GSLs) 5–10% – (neutral GSLs, gangliosides, and ABO blood group substances) RBC Metabolism The major membrane Proteins The membrane skeleton is four structural proteins that include α & β spectrin, ankyrin, protein 4.1 & actin. Spectrin is major protein of the cytoskeleton & its two chains (α & β) are aligned in an antiparallel manner RBC Metabolism α & β chains are loosely interconnected forming a dimer, one dimer interact with another, forming a head to head tetramer. Ankyrin binds spectrin & in turn binds tightly to band 3 securing attachment of spectrin to the membrane  RBC Membrane structure Band 3 is anion exchange protein It permits exchanges of Cl- for HCO3+ Actin binds to the tail of spectrin & to protein 4.1 which in turn binds to integral proteins, glycophorins A, B & C. Glycophorins A,B,C are transmembrane glycoproteins; Defects of proteins may explain some of the abnormalities of shape of RBCs membrane as hereditary spherocytosis & elliptocytosis Fate of RBC When RBCs reach the end of their lifespan, the globin is degraded to amino acids Amino acids are reutilized in the body, Fe is released from haeme and recycled The tetrapyrrole component of haeme is converted to bilirubin, which is mainly excreted into the bowel via the bile Protection of RBCs from Oxidative Stress & Damage Reactive oxygen species (ROS) Oxidants produced during metabolism, in blood cells and most other cells of the body include: 1. Superoxide (O2), 2. hydrogen peroxide (H2O2), 3. peroxyl radicals (ROO·), and 4. hydroxyl radicals (OH·) Free radicals are atoms or groups of atoms that have Protection of RBCs from Oxidative Stress & Damage OH· is a particularly reactive molecule and can react with proteins, nucleic acids, lipids, and other molecules to alter their structure and produce tissue damage. Recreate the glycolysis diversion pathways (Shunt) HMP Met Hb pathway Rapoport-Lubering Pathway