Hemoglobin Structure & Function

Objectives of the Lecture structuralfunctional 1- Understanding the main structural & functional details of hemoglobin as one of the hemoproteins. types HBA1c 2- Identify types & relative concentrations of normal adult hemoglobin with reference to HBA1c with its clinical application. methemoglobinopathies thalassemias 3- Recognize some of the main genetic & biochemical aspects of methemoglobinopathies with some implications on clinical features (with focusing on thalassemias).

Hemoglobin is a globular hemoprotein Hemeproteins hemeHemeproteins are a group of specialized proteins that contain heme as a tightly bound prosthetic group. Heme protoporphyrin IX ferrous iron (Fe2+).Heme is a complex of protoporphyrin IX & ferrous iron (Fe2+). ironThe iron is held in the center of the heme molecule by bonds to the four nitrogens of the porphyrin ring. heme Fe2+The heme Fe2+ can form two additional bonds, one on each side of the planar porphyrin ring. histidine amino acid oxygen One of these positions is coordinated to the side chain of a histidine amino acid of the globin molecule, whereas the other position is available to bind oxygen

Globin of hemoglobin is a globular protein with a quaternary structure

Structure of heme Hemeprotoporphyrin IX ferrous iron (Fe2+). Heme is a complex of protoporphyrin IX and ferrous iron (Fe2+). iron The iron is held in the center of the heme molecule by bonds of the four nitrogens of the protoporphrin ring. Heme F2+ can form two additional bonds, one on each side of the porphyrin ring. histidine amino acid oxygen One of these positions is coordinated to the side chain of histidine amino acid of the globin molecule, whereas the other position is available to bind oxygen.

Structure & function of hemoglobin HemoglobinRBCs Hemoglobin is found exclusively in RBCs. function Its main function is to transport oxygen from lungs to the tissues & carbon dioxide & hydrogen protons from tissues to lungs. Hemoglobin A Hemoglobin A is the major hemoglobin in adults, is composed of four polypeptide chains, 2 alpha (  ) & 2 beta (  ) chains, held together by noncovalent interactions Each day, 6-7 grams of hemoglobin is synthesized to replace lost through normal turn over of RBCs. Each subunit Each subunit has stretches of  -helical structure & a heme binding pocket.

Structure & function of hemoglobin Structure & function of hemoglobin (cont.)

Quaternary structure of hemoglobin The hemoglobin tetramer can be envisioned as being composed of two identical dimers, (αβ)1 and (αβ)2, in which the numbers refer to dimers one and two. tightlyhydrophobic interactionsThe two polypeptide chains within each dimer are tightly held together, primarily by hydrophobic interactions polar bonds.In contrast, the two dimers are able to move with respect to each other, being held together primarily by polar bonds. weaker deoxyhemoglobin oxyhemoglobinThe weaker interactions between these mobile dimers result in the two dimers occupying different relative positions in deoxyhemoglobin as compared with oxyhemoglobin Structure & function of hemoglobin Structure & function of hemoglobin (cont.)

oxygenation & deoxygenation of hemoglobin oxygenation & deoxygenation of hemoglobin (oxyhemoglobin & deoxyhemoglobin) Oxyhemoglobin Relaxed structure Deoxyhemoglobin Taut structure

Types of adult hemoglobin 3–6 % HBA: HBA: the major hemoglobin in humans HBA 2 HBA 2 : first appears 12 weeks after birth- a minor component of normal adult HB HBF: HBF: normally synthesized only during fetal development HBA 1C : HBA 1C : has glucose residues attached to  -globin chains – increased amounts in DM

Hemoglobin A1c (HBA1c) HbA1c could be used as a monitor for the control of the blood glucose level during the last 2 months for diabetic patients hemoglobin A Some of hemoglobin A is glycosylated Extent of glycosylation depends on the plasma concentration of a particular hexose (as glucose). HBA1c The most abundant form of glycosylated hemoglobin is HBA1c which has a glucose residues attached to  -globin chains in hemoglobin RBCs. HBA1c Increased amounts of HBA1c are found in RBCs of patients with diabetes mellitus (DM).

Hemoglobinopathies Hemoglobinopathies Hemoglobinopathies are members of a family of genetic disorders caused by: structurally abnormal 1- Production of a structurally abnormal hemoglobin molecule Qualitative hemoglobinopathies (Qualitative hemoglobinopathies) insufficient quantities Or: 2- Synthesis of insufficient quantities of normal hemoglobin Quantitative hemoglobinopathies (Quantitative hemoglobinopathies) both (rare) Or: 3- both (rare).

Thalassemias Thalassemias Thalassemias Thalassemias are hereditary hemolytic diseases in which an imbalance occurs in the synthesis of globin chains. single gene disorders They are most common single gene disorders in humans. Normally Normally, synthesis of  - and  - globin chains are coordinated, so that each  -globin chain has a  -globin chain partner. This leads to the formation of  2  2 (HbA). In thalassemias In thalassemias, the synthesis of either the  - or  -globin chain is defective.

Thalassemia Thalassemia can be caused by a variety of mutations, including: Entire gene deletionswhole gene is absent 1- Entire gene deletions (whole gene is absent) Substitutions or deletions of one or more nucleotides in the DNA. Or: 2- Substitutions or deletions of one or more nucleotides in the DNA. Each thalassemia can be classified as either Each thalassemia can be classified as either: no globin chains 1- A disorder in which no globin chains are produced (  o - or  o -thalassemia) a reduced rate Or: 2- Some  -chains are synthesized, but at a reduced rate. (  + - or  + - thalassemia). Thalassemias Thalassemias (cont.)

1-  -thalassemias:  -globindecreasedabsent  Synthesis of  -globin chains are decreased or absent, whereas  -globin synthesis is normal. premature death of RBCsending in chronic hemolytic anemia   -globin chains cannot form stable tetramers & therefore precipitate causing premature death of RBCs ending in chronic hemolytic anemia  Also,  2  2 (HbF) &  2  2 (HbA 2 ) are accumulated. Thalassemias Thalassemias (cont.)

There are only two copies of the  -globin gene in each cell (one on each chromosome 11).  -globin gene defects So, individuals with  -globin gene defects have either: 1-  -thalassemia minor (  -thalassemia trait): one if they have only one defective  -globin gene. 2-  - thalassemia major (Colley anemia): 2-  - thalassemia major (Colley anemia): both if both genes are defective. Thalassemias Thalassemias (cont.)

both Mutation in both  -globin genes  -thalassemiamajor one Mutation in one of  -globin genes  -thalassemiaminor Thalassemias Thalassemias (cont.)  -thalassemia

Some clinical aspects of  -thamassemias: 1- As  -globin gene is not expressed until late fetal gestation, the physical  -thalassemias appear only after birth manifestations of  -thalassemias appear only after birth.  -thalassemias minor 2- Individuals with  -thalassemias minor, make some  -chains, and no usually require no specific treatment.  - thalassemias major healthy at birth 3- Infants born with  - thalassemias major seem healthy at birth, but severely anemic during the first or second years of life become severely anemic during the first or second years of life. They require regular transfusions of blood. Thalassemias Thalassemias (cont.)

2-  -thalassemia:  -globindecreasedabsent Synthesis of  -globin chains is decreased or absent. Each individual's genome contains four copies of the  -globin (two on each chromosome 16), there are several levels of  -globin chain deficiencies Thalassemias Thalassemias (cont.)

Types: If one of the four genes is defective silent carrier of  - thalassemia the individual is termed a silent carrier of  - thalassemia as no physical manifestations of the disease occur. If two  -globin genes are defective,  -thalassemia trait the individual is designated as having  -thalassemia trait. If three  -globin genes are defective; hemoglobin H (HbH the individual has hemoglobin H (HbH) disease which is a mildly to moderately hemolytic anemia. Synthesis of unaffected  - and then  - globin chains continues, resulting in the accumulation of  tetramer in the newborn (  4, Hb Bart's) or  -tetramers (  4, HbH). The subunits do not show heme-heme interactions. So, they have very high oxygen affinities. Thus, they are essentially useless as oxygen carriers to tissues If four  -globin genes are defective, hydrops fetalis hydrops fetalis & fetal death (death at birth), occurs as  -globin chains are required for the synthesis of HbF Thalassemias Thalassemias (cont.)

Types of  -thalassemias