Hemoglobin Structure and Function

Hemoglobin Structure and Function

• Hemoglobin is a hemoprotein found exclusively in red blood cells (RBCs).
• Adult males typically have 14-16 g/dL hemoglobin.
• Adult females typically have 12-16 g/dL hemoglobin.
• Hemoglobin is composed of four polypeptide globin units and four heme groups.
• Each heme group contains an iron ion (Fe++).
• Hemoglobin binds oxygen in the lungs and releases it in the tissues.
• Hemoglobin also transports carbon dioxide from the tissues to the lungs.
• Hemoglobin acts as an acid-base buffer, its buffering power is six times greater than plasma proteins.
• The erythrocyte membrane keeps hemoglobin inside the cell.

Objectives

• Understand hemoglobin's structure and function as a hemoprotein.
• Identify types and relative concentrations of normal hemoglobin, referencing HbA1c's clinical applications.
• Recognize key genetic and biochemical aspects of hemoglobinopathies, focusing on thalassemias.

Components of Blood

• Blood consists of approximately 55% plasma (containing water, ions, proteins, nutrients, and wastes).
• Approximately 1% of blood is platelets and white blood cells.
• Approximately 44% of blood is red blood cells.

Cause of Microcytic-Hypochromic Anemia

• Iron deficiency, chronic inflammation, or malignant conditions (ACD) can all contribute to microcytic-hypochromic anemia.
• Protoporphyrin deficiencies contribute and can lead to sideroblastic anemia,
• Thalassemia (alpha and beta) can also result in microcytic-hypochromic anemia.

Red Blood Corpuscles (Erythrocytes/RBCs)

• RBCs are circular, biconcave, non-nucleated discs.
• They contain hemoglobin, a red respiratory pigment.
• RBCs lack mitochondria and ribosomes.
• The biconcave shape increases surface area, enhancing cell flexibility.
• Reduced tension on the membrane happens when RBC volume increases in the venous blood (CO2 carriage).

Hemoglobin Function

• Hemoglobin transports oxygen from the lungs to tissues.
• Hemoglobin transports carbon dioxide from tissues to the lungs.
• Hemoglobin acts as a buffer, maintaining acid-base balance.
• The erythrocyte membrane keeps hemoglobin inside the cell.
• Free hemoglobin in the plasma poses a risk as it can filter through kidneys and increase blood viscosity.

Hemoglobin: Heme + Globin

• Hemoglobin (Hb) is composed of a heme moiety (with an iron ion at its center) and four globin subunits (2 alpha and 2 beta).
• Hemoglobin is a tetramer composed of 4 polypeptide chains.
• The heme groups bind oxygen in the lungs and release it in the tissues.

Heme

• Heme is a ferrous (Fe++) protoporphyrin IX.
• Protoporphyrin contains a porphin ring, which is formed by four pyrrole rings.
• Heme is positioned within a hydrophobic pocket inside the globin chains.

Interaction of Iron with Heme & Globin

• Iron is coordinated to the four nitrogen atoms in the porphyrin ring.
• A proximal histidine residue (His F8) coordinates the fifth coordination position of the iron.
• The sixth coordination position is available for oxygen binding.

Importance of Globin

• Globin makes heme soluble, preventing its diffusion into the plasma.
• Globin also prevents the oxidation of iron within heme, retaining oxygen-carrying capacity.

Globin Structure

• Adult hemoglobin contains two alpha and two beta chains.
• Each chain consists of amino acids; the alpha chain has 141 amino acids and the beta chain has 146.
• Chains are folded into alpha-helices.
• Polar amino acids are on the surface, while nonpolar amino acids are on the inside of the globular protein.
• Four polypeptide chains form two identical dimers.
• The two dimers are held together by ionic and hydrophobic interactions, while weaker bonds link the two dimers together.

Forms of Hemoglobin

• Deoxyhemoglobin (T-form): low oxygen affinity (low oxygen binding).
• Oxyhemoglobin (R-form): high oxygen affinity (high oxygen binding).
• The binding of oxygen changes Hb conformation and results in more freedom of movement throughout the dimers.

Oxygenation vs. Oxidation of Hemoglobin

• When hemoglobin carries oxygen, the iron is in the ferrous (Fe++) state.
• When hemoglobin loses oxygen (oxidized) the iron becomes ferric (Fe+++).
• This change results in methemoglobin, a compound that is unable to carry oxygen.

Chemical Reactions of Hemoglobin

• Hb combines with carbon dioxide (CO2) to form carbaminohemoglobin.
• Hb reacts with carbon monoxide (CO) to form carboxyhemoglobin.
• Iron within Hb can be oxidized by strong oxidizing agents, creating methemoglobin (unable to carry oxygen).

Types of Adult Hemoglobin

• HbA1: major hemoglobin in adults (2 alpha-2 beta)
• HbA2: minor hemoglobin; found in adults and is increased in some forms of thalassemia.
• HbF (fetal hemoglobin): higher oxygen affinity than HbA; found in infants, eventually replaced by HbA.

Hemoglobin A1c (HbA1c)

• HbA1c is a glycated form of hemoglobin—glucose molecules bind to Hb.
• HbA1c levels reflect average blood glucose concentrations over the previous 2-3 months.
• High HbA1c levels indicate poor blood glucose control in people with diabetes mellitus (DM).

Hemoglobin-2 Hemoglobin-2 Hemoglobinopathies

• Abnormal or structurally defective versions of hemoglobin.
• Hemoglobin disorders result from insufficient synthesis of normal hemoglobin or from structural defects.

Hemoglobinopathies

• Abnormalities can affect the quality of hemoglobin synthesis or the quantity.
• Examples include HbS, HbC, and HbM, and various forms of thalassemias.

Sickle Cell Disease (HbS)

• Caused by a point mutation in the β-globin gene.
• The substitution of a nonpolar valine for a polar glutamate residue causes a "sticky patch".
• Deoxygenated HbS polymerizes, forming insoluble fibers, leading to rigid, misshapen red blood cells.
• Leads to vaso-occlusion, hemolytic anemia, and various complications.
• The sickled cells are destroyed prematurely by the spleen.
• Heterozygotes have both normal and abnormal hemoglobin, often resulting in sickle cell trait.

Sickle Cell Anemia Inheritance

• Autosomal recessive inheritance pattern
• Individuals with two copies of the mutated beta-globin gene develop sickle cell disease.
• These individuals inherit a mutant ß allele (ßS) from each parent.
• Those with one copy typically termed as having Sickle Cell trait and do not have symptoms in the same way.

Clinical Manifestations of Sickle Cell Anemia

• Mild to severe symptoms are varying.
• Frequent episodes of painful crises (sickle cell crises)
• Anemia
• Shortness of breath
• Bone deformities
• Jaundice are often linked to sickle cell anemia.
• Common organs affected by complications include the bones, lungs, abdomen, and joints.
• Complications of sickle cell anemia can include stroke, acute chest syndrome, and multiple organ failure.

Hemolysis

• Finding anemia with accelerated reticulocytosis (an increase in immature red blood cells), and evidence of red blood cell destruction via serum and urine supports hemolytic anemia.
• Hemolysis has characteristic biochemical findings such as decreased haptoglobin and elevated LDH, indirect bilirubin levels, abnormal peripheral blood smears (reticulocytes, schistocytes, spherocytes, polychromasia), and abnormal urinary findings (hemoglobinuria, and urobilinogen).

Hemoglobin Electrophoresis

• A technique used to separate and identify different types of hemoglobin in a sample.
• Used to diagnose hemoglobinopathies, like sickle cell disease and thalassemia.
• Electrophoresis provides a visual pattern of different hemoglobin types, allowing for diagnosis.

Hemoglobin C Disease (HbC)

• Glutamate in the beta-globin is replaced with lysine.
• HbC is less soluble than HbA and has a tendency towards forming hexagonal crystals.
• Dehydration of the RBCs due to crystal formation results in reduced oxygen-carrying capacity. This results in reduced lifespan of RBCs.

This quiz explores the structure and function of hemoglobin as a crucial hemoprotein found in red blood cells. Participants will learn about its composition, oxygen transport capabilities, and its role in acid-base buffering. The quiz also highlights the clinical significance of hemoglobin types and related disorders like thalassemias.