Hemoglobin Structure & Function (HMIM224)

HMIM224 Block SEM 242 Hemoglobin-1 Structure & Functions of Hemoglobin https://next.amboss.com/us/article/Introducing%20Labs%20in%20our %20block s?q=hemoglobin#Z94b6c5cf373e3de2a4f32b9f3fb10b...

HMIM224 Block SEM 242 Hemoglobin-1 Structure & Functions of Hemoglobin https://next.amboss.com/us/article/Introducing%20Labs%20in%20our %20block s?q=hemoglobin#Z94b6c5cf373e3de2a4f32b9f3fb10b4e  Objective s 1- Understandingthe main structural & functional details of hemoglobin as one of the hemoproteins. 2- Identify type & relative concentrations of normal adult hemoglobins with reference to HBA1c with its clinical application. 3- Recognize some of the main genetic & biochemical aspects of hemoglobinopathie s with some implications on clinical features (with focusing on thalassemias ). Red Blood Corpuscles (Erythrocytes/RBC’S) Functions of RBC’s: A.Functions of Hemoglobin: 1. 02 transport from the lungs to the tissues. 2. C02 transport from the tissues to the lungs 3. Buffering function: ¾ Being a protein, hemoglobin acts as an acid-base buffer. ¾ The buffering power of hemoglobin is 6 times that of plasma proteins B.Function of the membrane of the RBC’s: The membrane of erythrocytes keeps hemoglobin inside the cell. Hazards of free hemoglobin in the plasma: ¾ Hb filters through the glomeruli of the kidneys, ¾ block the renal tubules. ¾ Hb increases the viscosity of the blood ¾ +++ the  Hemoglobin : Heme + Globin Hb is a circulating globular protein composed of a heme moiety with a central iron ion and four subunits of globin. Hemoglobin is found exclusively in RBCs. 16 ± 2 Adult males gm/dl : ±2 14 Adult females:gm/dl Hemoglobin = 4 non protein (HEME) + 4 protein (GLOBIN part The hemoglobin is a ) tetramer, formed of 4 composed of a polypeptide subunits, each chain subunit attached is to one heme molecule ( Quaternary structure of HB)  Heme Heme is a ferrous (Fe++ ) protoporphyrin IX. ◆ Protoporphyrins contain porphin ring. ◆ Porphin ring is formed of 4 pyrrole rings linked by methenyl bridges (-HC=) ◆ 4 Pyrrole rings are: I, II, III & IV. Substituent positions on the rings: 1, 2, 3, 4, 5, 6, 7 & 8. ◆ Methenyl bridges are: a, β, Y and δ Heme Ring: present in the hydrophobic core of polypeptide chain in a pocket between E and F helices of globin chain.  Interaction of Iron with Heme & globin 1. 4 coordination bonds are formed between Fe II & 4 nitrogen of the porphyrin ring, which lie in the SAME PLANE of the ring. 2. 5th coordination is for proximal histidine (His F8). 3. 6th coordination is for oxygen. It is unoccupied in absence of O2. 4. Both 5th & 6th are PERPENDICULAR to the plane of the ring. Importance of the Protein Part (globin) of hemoglobin 1. Makes heme soluble (due to the surface polar amino acids) 2. Prevents Heme diffusion into the plasma (due to its large size) 3. Prevents oxidation of iron (heme ---- hematin) (heme-O2-heme complex) (due to the nonpolar amino acids lining of heme pocket)  Globin of Tetramer 2 identical (a) &HbA1 2 identical (β) chains 1ry Structure: each a chain consists of 141 amino acids, while β chain is of 146 amino acids. 2ry Structure: each a chain is folded into 7 a-helices, while each β chain is folded into 8 a-helices. 3ry Structure: each chain is further folded to form Globular protein with polar amino acids toward the surface & nonpolar amino acids at the inner part. 4ry Structure: 4 polypeptide chains interact together forming 2 identical dimers (aβ)1 & (aβ)2 # The 2 chains within dimers are held tightly by ionic & hydrophobic interactions (prevent relative movement to each other) # However, the 2 dimers are linked with each other by Weaker noncovalent bonds (e.g., hydrogen bonds) (2 dimer move freely during oxygenation & deoxygenation) Forms of hemoglobin: A. T form: Deoxy “T" for taut or (tense) (low oxygen-affinity form) deoxyhemoglob in B. R form: Oxygenated “R," for relaxed} (high oxygen-affinity form) oxyhemoglobin Binding of O2 to Hb causes rupture of some noncovalent bonds between 2 dimers till reach full oxygenated form with more freedom of movement Oxygenation vs oxidation of hemoglobin: When Hb carries oxygen, it is then oxygenated, and its iron remains in ferrous state. If Hb is oxidized its iron is then becomes in a ferric state and it loses its oxygen carrying capacity and is named Met-Hb and heme is named Chemical Reactions of Hemoglobin: 1. Hb loosely oxygen ¾ unites with oxy hemoglobin. (02) O2 is attached to the iron ¾ remains in Fe2+ state Types of Adult Hemoglobin: A1- A2- F 1 HbA1: The Major hemoglobin in humans (2 alpha- 2 beta) 2 HbA2: First appears 12 weeks after birth (2 alpha- 2 delta) A Minor component of normal adult hemoglobin. Increased in thalassemia 3 HbF: Normally synthesized only during Fetal development (2alpha-2 gamma) in fetus.. HbF has more affinity to O2 than HbA for extraction of O2 from maternal blood. 3–6 After birth, HbF is changed to HbA ¾ completed by % the age of 4 months. Increased in thalassemia why? HbA1C (Less than 5.7% of HbA1): has glucose residues attached to β- globin chains in DM HbA1C concentration is increased  Hemoglobin A1c (HBA1c) subtype of Hb A1 Some of hemoglobin A 1 is glycated (non-enzymatic binding of sugar (hexose) to protein). Extent of glycation depends on the plasma concentration of glucose). The most abundant form of glycated hemoglobin is HBA1c which has a glucose residues attached to N- terminal valine residue of β-globin chain s in hemoglobin RBCs. Increased amounts of HBA1c are found in RBCs of patients with diabetes mellitus (DM).  Hemoglobin-2 Hemoglobinopathies  Hemoglobinopathies Hemoglobinopathies are members of a family of genetic disorders caused by: 1- Production of a abnorm hemoglobin structurally ( Qualitative al molecule HbS, HbC, HbS-C, Hb Or hemoglobinopathies): M : 2- Synthesis insuffi cie quantities of normal nt of ( Quantitative hemoglobin Thalassemia hemoglobinopathies): s Type of Abnormalities Hemoglobin Hb S Glutamic acid at position 6 of ¾ subunit is replaced by VALINE (termed ¾S-chain) Hb C Glutamic acid at position 6 of ¾ subunit is replaced by lysine Hb M Proximal or distal histidine of a or β-chain of Hb is replaced by tyrosine a-Thalassemias Defects in synthesis of a -globin chains β-Thalassemias Defects in synthesis of β -globin chains  1- Sickle Cell Disease Hb S It’s a genetic disease caused by Point mutation in the β-globin gene (polar glutamate at posi six has been replaced with a nonpolar valine). HbS contains two normal a-globin chains and mutant β-globin (βs) chains. The nonpolar amino acid generates a hydrophobic “sticky patch the surface of the β subunit of both oxy HbS and deoxy HbS. At low PO2, deoxy HbS polymerize to form long, insoluble fibers producing rigid, mis-shaped erythrocytes Sickled block the flow of blood in the narrow capillaries causes localized anoxia in the tissue causing and infarction spleen removes sickle cells at a faster rate than normal cells, leading to hemolytic anemia than 20 days, compared to 120 days of normal) Sickle cell disease homozygotes have HbS but no HbA. HbSS Sickle cell traits (Heterozygotes) have both hemoglobins in their red cells. HbAS  Inheritance of Sickle Cell Anemia It is an inherited, lifelong condition, people are born with it, and can pass the theirgene on to autosomal recessive children. It is Individuals (AR). who inherit two copies of the sickle cell gene (mutant β globin gene 11), one from each parent are considered chromosome (SS) ( RBCs contain homozygous Their individuals totally HbS. These individuals Sickle Cell Disease(Sickle. have Cell Anemia) Individuals who inherit one sickle cell (mutant β) gene from and a one geneparent from the other parent are considered heterozygous normal (AS )and have individuals condition sickle cell a the Individuals with sickle cell trait don’t have trait.manifestations), called no clinical (show condition but they carry one of mutant genes. Their RBCs contain both HbS and normal HbA.  Clinical Manifestations of Sickle Cell Anemia Sickle cell trait ((heterozygous individuals) usually show no clinical symptoms. Sickle cell disease (homozygous individuals) ¾ The symptoms vary. Some people have mild symptoms while others have it so severe that they need to be hospitalized. ¾ Present at birth, many infants doesn’t show signs until after 4 -6 months of age (when sufficient HbF is replaced by HbS) The most common signs and symptoms are linked to anemia and pain. Fatigue (tiredness), pale skin and nail beds, jaundice, and shortness ¾ Anemia: of breath.and delayed growth. Bone deformities ¾ Pain (Sickle Cell Crisis): Periodic episodes of pain of variable intensity and duration,. The most common sites affected are the bones, lungs, abdomen, and joints ¾ Complication of Sickle Cell Anemia such as Hand-Food Syndrome, Infections, Acute Chest syndrome, Stroke, Eye problem, ulcers of the leg and Multiple Organ Failure  Clinical Manifestations of Sickle Cell Anemia Hydroxyurea, an anticancer drug which was found to reduced the frequency of painful crises and acute chest syndrome. Patients taking the drug needed fewer blood transfusions. It prompts the body to make fetal hemoglobin ( Hb F), (increase the expression of gamma Y-globin gene , mutated β- globin with a γ-globin chain which is non-pathological, reduce sickling of RBCs hence decreasing the number of vaso-occlusive events and infarction.  2- Hemoglobin C disease Hb C Pathophysiology β-globin mutation (glutamate replaced by lysine) Glutamic acid can also be replaced with a lysine, creating hemoglobin C. as HbC precipitates crystal → ↑ RBC rigidity and ↓ deformability → s extravascular hemolysis. HbC is less soluble than HbA and tends to form hexagonal crystals, which lead to RBC dehydration (↑ MCHC). RBCs have reduced oxygen- binding capacity and a shorter lifespan. 1. Hemoglobin C disease: homozygous for the hemoglobin C mutation (HbCC) 2. Hemoglobin C trait: heterozygous carriers of the hemoglobin C mutation (HbAC)  Hemolysis workup While no single test can be used to confirm hemolysis, the finding of anemia in the presence of accelerated erythropoiesis (i.e., reticulocytosis) in addition to evidence of RBC destruction in serum and/or urine studies is highly suggestive of hemolytic anemia. Typical biochemical findings in hemolysis include ↓ haptoglobin ? ↑ LDH concentration, ↑ indirect bilirubin concentration, peripheral blood smear abnormalities (e.g., ↑ reticulocytes, schistocytes, spherocytes, polychromasia), urinalysis abnormalities (e.g., hemoglobinuria, and urobilinogen).  Hemoglobin Electrophoresis + - - - + -