L3_Structure_of_hemoglobin_and_synthesis_of_heme_Biochemistry.pdf

New Mansoura University MBBCh program Locomotor System & Integument Module BMS 202 Structure of hemoglob...

New Mansoura University MBBCh program Locomotor System & Integument Module BMS 202 Structure of hemoglobin & synthesis of heme Dr. Ghada Helal Assistant Professor of Medical Biochemistry  Learning outcomes At the end of this lecture, students should be able to:. 1-Describe the structure of heme. 2-Locate the main site of heme biosynthesis. 3-Outline heme synthetic pathway & its key regulatory step. 4-Summarize porphyria's as enzymatic defect in heme synthesis.  Contents I. RBCs II. Hemoglobin III. Heme synthesis pathway and disorders  Shape: biconcave discs Diameter: 7 microns Life span: 120 days in peripheral circulation Synthesized: in Bone marrow I. RBCs RBCs Organelles: no nucleus, no mitochondria Function of mature RBCs: contain hemoglobin  A Respiratory pigment synthesized inside immature RBCs Conjugated protein (heme + Globin) Transports O2 & CO2 II. Hemoglobin Normal concentration of Hb in the blood: Adult males 13.5 – 16.5 gm/dl Adult females 12.0 – 14.8 gm/dl Types of hemoglobin: Structure of adult hemoglobin (Hb A):  Types of Hemoglobin Normal: Adult HB type A (HbA) (97%). *** Adult Hb type A2 (HbA2) (2%) Fetal Hb (HbF) (1%) Glycosylated Hb (HbA1c) Abnormal: Carboxy Hb Met Hb Sulf Hb  Structure of adult Hemoglobin Adult Hemoglobin (Hb A) is formed of tetramer consisting of: 4 Heme groups + 4 Globin chains I. Globin chains (4 polypeptide subunits): 2 α & 2 β chains held together by noncovalent interactions -α chain: contain 141 amino acids - β chain: contain 146 amino acids II. Heme groups: 4 heme groups (1group at the center of each globin subunit)  Structure of heme Heme is an iron porphyrins I. Porphyrin: u It is a cyclic compound formed of: § four pyrrole rings § linked by methenyl bridges (-HC=) II. Iron: u Fe form complexes with nitrogen atoms of the pyrrole rings of the porphyrin  Importance of heme u Heme is a constituent of many hemoproteins’ compounds such as: 1. Hemoglobin 2. Myoglobin 3. Cytochromes (C & P450) 4. Catalase 5. Peroxidase  1- Sites of heme synthesis 2- Pathway of heme synthesis III. Heme 3- 1st step of heme synthesis synthesis 4-Last step of heme synthesis 5-Lead poisoning & heme synthesis 6-Regulation of heme synthesis 7- Disorders of Heme synthesis (Porphyrias)  1. Sites of heme synthesis Organs: the majority in: - ü Bone marrow: for synthesis of Hemoglobin ü Liver: for formation of cytochrome P450 Cellular sites: ü Mitochondria: 1st reaction and last three reactions of heme synthesis ü Cytosol: intermediate steps of Heme synthesis  2. Pathway of heme synthesis I) In mitochondria 1. Synthesis of ALA in mitochondria II) In cytosol 2.Synthesis of Porphobilinogen (PBG) 3.Synthesis of Uroporphyrinogen III 4. Synthesis of Coproporphyrinogen III III) In mitochondria 5.Synthesis of Protoporphyrinogen IX 6.Synthesis of Protoporphyrin IX 7.Synthesis of Heme  3. 1st step of heme synthesis (Synthesis of ALA) u 1st step of heme synthesis occurs in the mitochondria u Delta (5) Aminolevulinic acid (5 ALA) is synthesized from Succinyl CoA and Glycine amino acid ( starting materials ) uGlycine,: a non-essential amino acid uSuccinyl CoA :,from Krebs cycle ALA Synthase catalyze the reaction. u This is the Key regulatory (rate controlling) step in heme synthesis  4. Last step of heme synthesis (Generation of heme) u Last step of heme synthesis occurs also in the mitochondria u Fe (in the ferrous form = Fe+2) is attached to Protoporphyrin IX u Ferrochelatase (Heme Synthase) catalyze this reaction.  5-Lead poisoning & heme synthesis u Lead poisoning: inhibits two enzymes in heme synthesis pathway: 1. Ferrochelatase à accumulation of Protoporphyrin IX in blood à urine 2. ALA dehydrataseà accumulation of ALA in bloodà urine  6. Regulation of heme synthesis 1.Heme Via regulation of the Key regulatory step: 2.Hemin(oxidized ALA synthase Inhibited by heme Fe3+) 3.Glucose Liver 4.Steroids ALAS 1 1. Fe 2. Hypoxia ALA Synthase Stimulated by 3. Erythropoietin 4. insecticides (ALAS) 5. Carcinogens Erythroblast Fe availability ALAS 2  7. Disorders of Heme synthesis (Porphyrias) u Definition: Increased the blood porphyrins (???) above the normal level and its excretion in urine. u Causes: Due to deficiency of one of the enzymes of the heme synthesis pathway. Ø Manifestations of Porphyria ü Porphyria: (Porphyrins excretion in urine)à darkness of urine ü Photosensitivity, skin rash ü Peripheral and central neuropathy. ü Hypertension and abdominal colic (acute abdomen).  Classification of Porphyrias Inherited Acquired Chronic Lead RBCs Hepatic poisoning alcoholic liver disease Erythropoietic Acute intermittent interferes with porphyria porphyria ferrochelatase Interferes with (Photosensitivity & skin (Abdominal pain & ALA synthase rash) Neuropsychiatric) ALA dehydratase Ø Diagnosis of Porphyrias Measurements of: 1. ↑ Porphobilinogen (PBG) in urine during the acute attack 2. ↑ Metabolites (intermediates) of heme synthesis in plasma, urine, feces and erythrocytes 3. Defective enzyme (according to the type of porphyria).  True or false 1. Heme synthesis occurs entirely in the mitochondria. 2. ALA synthase catalyzes the 1st step in heme synthesis. 3. AlA is synthesized from succinyl CoA and alanine. 4. Hemin and glucose increase the activity of ALA1 synthesis  MCQ Besides erythroid precursor cells, which of the following is the site for the synthesis of heme? a) Kidney b) Spleen c) Liver d) Heart  MCQ Which of the following is the rate-limiting enzyme (key enzyme) for the synthesis of heme ? a) Uroporphyrinogen synthase III b) Protoporphyrinogen oxidase c) Ferrochelatase d) delta-aminolevulinic acid synthase 1  MCQ The heme, hematin, and Cytochrome P450 represses the synthesis of the following enzyme thereby reducing heme synthesis. a) delta-aminolevulinic acid synthase 1 b) Uroporphyrinogen synthase III c) Protoporphyrinogen oxidase d) Ferrochelatase  Case discussion A 3-year-old boy is brought to the emergency room with abdominal pain, mental status changes, and fatigue. On history, the patient lives in an old house and has been sucking on the paint chips that have crumbled in the windowsills, making the doctor suspicious for lead poisoning. Lead typically interferes with which of the following enzymes? a) Cytochrome oxidase b) Protoporphyrinogen oxidase c) UMP synthase d) ALA dehydratase e) Porphobilinogen deaminase  Take home messages ü The major sites of heme biosynthesis are the erythrocyte-producing cells of the bone marrow, which are active in hemoglobin synthesis and the liver ( active in synthesis of cytochrome P450). ü Porphyrin synthesis start with glycine and succinyl CoA. ü The committed step in heme synthesis is δ -aminolevulinic acid (ALA) synthesis. ü This reaction is catalyzed by ALA synthase-1 in liver (inhibited by hemin), and ALA synthase-2 in erythroid tissues (regulated by iron). ü Porphyrias are caused by inherited or acquired defects in heme synthesis, resulting in the accumulation and increased excretion of porphyrins or porphyrin precursors.  References q Lippincott’s Illustrated Reviews: Biochemistry, 8th edition q Chatterjee's Textbook of Medical Biochemistry, 8th edition. q Vasudevan's Textbook of Biochemistry For Medical Students, 6th Edition.