L1 - Haemoglobin Synthesis PDF

HAEMOGLOBIN SYNTHESIS Dr. Mudiana Muhamad (PhD.) Associate Professor Biochemistry & Molecular Medicine H & L Module Year 2 08/10/2024 1  Specific Learning Objectives At the end of this lecture, students...

HAEMOGLOBIN SYNTHESIS Dr. Mudiana Muhamad (PhD.) Associate Professor Biochemistry & Molecular Medicine H & L Module Year 2 08/10/2024 1  Specific Learning Objectives At the end of this lecture, students should be able to: Compare and contrast the structures of different types of haemoglobin (Hb). Describe the synthesis of Hb and its regulation. Describe the catabolism of Hb. Explain the causes and consequences of abnormal Hb including sickle cell anaemia and Thalassemia Explain the abnormalities resulting from heme synthesis. 2 Haemoglobin = globin + heme Heme – Porphyrin ring contain Iron (Fe2+) Globin – Protein part *Heme containing proteins (hemoprotein): Haemoglobin (transport O2 ) Myoglobin (store O2 ) Enzymes that contain heme as part of their prosthetic groups (e.g. catalase, peroxidases, cytochrome) 3 Types of Hb In adult HbA, most common with over 95% of total Hb HbA2, minor component with ~2% of total Hb HbF During foetal development HbF normally synthesized only during foetal development HbA gradually replaces HbF HbA1c – glycosylated Hb, glucose residues attached β- globin chains – diabetes mellitus 4  HbA The major hemoglobin in adults A tetramer (4 subunits) → 2 α- and 2 β- chains α- globin chain has 141 amino acids β- globin chain has 146 amino acids *Each globin chain contains a heme group 5  Composition of haemoglobins found in normal human development Globin Chains Haemoglobin Stage of development α2β2 A Adult α2δ2 A2 Adult α2γ2 F Foetus α2ε2 Gower 2 Embryo ζ2ε2 Gower 1 Embryo ζ2γ2 Portland Embryo *Embryonic stage: Portland → Gower 1 → Gower 2; Foetal development: HbF; Adults: HbA, HbA2 **Different globin chains made up different types of Hb 6  Synthesis of Haemoglobin Synthesis of globin Synthesis of heme ✓ Adequate supply and delivery of iron ✓ Adequate synthesis of protoporphyrins (heme precursor) ✓ Adequate globin synthesis 7  Synthesis of Haemoglobin - and -genes Succinyl CoA + glycine transcription mRNAs Protoporphyrin translation Fe2+ - and -globin chains Heme Hemoglobin 8  Various genes codes for different types of globin chains → types of haemoglobin epsilon gamma delta beta Throughout embryonic and foetal development, activation of globin genes progresses from zeta to alpha, and from epsilon to gamma to delta to beta zeta alpha 9  Synthesis of Haemoglobin - and -genes Succinyl CoA + glycine transcription mRNAs Protoporphyrin translation Fe2+ - and -globin chains Heme Hemoglobin 10  Biosynthesis of Heme - The bone marrow and liver - Occurs in both mitochondria and cytosol Precursors: Succinyl CoA and Glycine ALA synthase → The rate- limiting step of heme synthesis Iron is added at the last step by ferrochelatase 11 12  Catabolism of Haemoglobin RBCs life span is approximately 120 days! → Recognized as senescent cells (membrane changes) → Removed and engulfed by reticuloendothelial system Heme Fe2+ (iron) released and reutilized Globin chain Protoporphyrin IX converted to bilirubin Denatures (spleen and liver) Hydrolyzed to release amino acids 13  CO Heme oxygenase will remove iron → open-up the protopophyrin ring Heme oxygenase producing biliverdin Biliverdin converted to bilirubin by biliverdin Biliverdin reductase reductase 14 ▪ Bilirubin is transported from reticuloendothelial cells to the liver (hydrophobic thus bound to albumin) ▪ In the liver, bilirubin is conjugated → more soluble form ▪ Conjugated bilirubin is excreted via gastrointestinal tract 15 16  Synthesis of Haemoglobin Synthesis of globin Synthesis of heme Defective Porphyrias Hemoglobinopathies Altered Synthesis structure 17 Defect in globin synthesis → Abnormal haemoglobins Haemoglobinopathies Genetic mutations result in → Structurally abnormal Hb molecule → Synthesis insufficient quantities of Hb *Rarely both Affects the functions of Hb E.g. Sickle cell anaemia, thalassemia 18 HbS (Sickle cell anaemia) - Autosomal recessive disorder/ inheritance - HbS: Irregular crescent-like shape, sickle → Increases RBCs rigidity, hinders their free passage through capillaries → Shorten RBCs lifetime to approximately 20 days - Homozygous HbS: α subunits identical but β subunits differ from HbA Abnormality : Point mutation in the -globin gene Single base substitution (A to U) in the -globin gene results in a change of amino acid (no. 6) in the −globin chain of HbS. Normal  6 (Glu) -GAG- Glutamic acid Abnormal  6 (Val) -GUG- Valine 19 → Residue on the surface of the β-globin chain had changed from glutamate (charged) to valine (non-polar) → Difference in the charges (HbA vs HbS) can be distinguished by electrophoresis. Normal Carrier Sickle Cell 20 → Alteration reduces the solubility of the deoxygenated but not the oxygenated form of haemoglobin. → Under deoxygenated state, haemoglobin exposes a hydrophobic patch on the alpha chains. → The hydrophobic side chain of the valine residue at position 6 of the beta chain in haemoglobin is able to associate with the hydrophobic patch, causing HbS molecules to aggregate and form fibrous precipitates. → Sickling occurs when there is high concentration of deoxygenated form of haemoglobin S. 21 Other point mutations Hb Hammersmith, unstable: (Phe → Ser) HbC: Beta chain – Glu → Lys Explain the signs and symptoms seen in a patient with sickle cell anaemia. Find out on your own !! 22 Thalassemia - The synthesis of either α- or β-globin chain is defective. - Can be caused by a variety of gene mutations. → Entire gene deletions (usually seen with α-globin chain gene) → Substitutions → Deletions of one or many nucleotides in the DNA - Classified as : αo or βo thalassemia – no globin chain synthesized α+ or β+ thalassemia – chain synthesized but at reduced amount 23 α Thalassemia Synthesis of the α-globin chain is defective (usually because of gene deletion). Excess beta chain precipitates, become homotetramer (HbH) → high affinity for oxygen → ineffective for O2 delivery 24 β Thalassemia Synthesis of the β-globin chain is defective: nucleotide deletion Promoter mutations Splice-junction mutations Thus, excess α-globin chains form α4 which is insoluble, precipitate and causes premature death to developing cells → Ineffective erythropoiesis. There are only 2 copies of β-globin gene in each cell (1 on each chromosome 11). Therefore, individuals with β-globin gene mutation have either: β-thalassemia trait or β-thalassemia minor β-thalassemia major 25  Synthesis of Haemoglobin Synthesis of globin Synthesis of heme Defective Porphyrias Hemoglobinopathies Altered Synthesis structure 26 *Deficiency of an enzyme in heme biosynthesis → Porphyrias 27  Porphyrias: Inherited disorders of heme synthesis due to enzyme deficiency Intermediates of the heme synthetic pathway accumulate and may be toxic to nervous systems → Neuropsychiatric symptoms Porphyrinogens are converted to porphyrins by light. Porphyrins react with oxygen to form ROS which cause skin damage → patients are photosensitive. 28  THANK YOU References:  Textbook of Biochemistry with Clinical correlations, Devlin T.M. Wiley  Mark’s Basic Medical Biochemistry. 4th Edition.  Lippincott’s Illustrated Biochemistry Reviews 5th Edition, Harvey R A, Champe P C and Ferrier D R 29