Comprehensive Study Notes on Haemoglobin and Red Cell Biological Pathways

Introduction to Haemoglobin (Hb)

Definition and Function: Haemoglobin is a specialized protein responsible for the delivery of $O_2$ to tissues and the transport of $CO_2$ to the lungs for excretion.
Molecular Density: There are approximately 640 million haemoglobin molecules per individual red blood cell.
Site of Synthesis: * Haemoglobin is synthesized mainly within the mitochondria and ribosomes. * The synthesis occurs during two specific cell stages: nucleated stages (65%) and the reticulocyte stage (35%).
Essential Regulatory Elements: * Key Enzyme: $\delta\text{-aminolaevulinic acid (ALA) synthase}$ is the primary rate-limiting enzyme for haemoglobin synthesis. * Co-enzyme: Vitamin $B_6$ (Pyridoxal phosphate) acts as a necessary cofactor.

Types of Haemoglobin

Normal Adult RBC Composition: * HbA: Comprises 95% to 97% of adult haemoglobin, consisting of $\alpha_2\beta_2$ chains. * HbA2: Comprises 2% to 3% of adult haemoglobin, consisting of $\alpha_2\delta_2$ chains. * HbF (Fetal Haemoglobin): Comprises 1% to 2% of adult haemoglobin, consisting of $\alpha_2\gamma_2$ chains.
Structure of HbA: Consists of a globin tetramer (two pairs of globin peptide pairs: 2 alpha and 2 beta) and four heme groups (comprising protoporphyrin rings plus $Fe^{2+}$ ).
Fetal to Adult Switch: * HbF and HbA2 utilize $\gamma$ and $\epsilon$ chains instead of $\beta$ chains. * The major transition/switch from fetal to adult haemoglobin typically occurs between 3 to 6 months after birth.

The Four Levels of Haemoglobin Structure

Primary Structure: Refers to the specific number and sequence of amino acids in each polypeptide chain. * Alpha chains consist of 141 amino acids. * Beta chains consist of 146 amino acids.
Secondary Structure: Involves the twisting of the amino acid chain around an axis into a helical conformation (Alpha-helix).
Tertiary Structure: Involves the bending of the twisted chain into a three-dimensional "pretzel" shape.
Quaternary Structure: Describes the four-dimensional arrangement of the chains alongside their respective heme groups, forming a globular protein with a molecular weight of approximately 64,460.

Heme Synthesis and Iron States

Iron Configuration: * The heme group features an iron ( $Fe$ ) atom as its central atom. * Iron must be reduced to the ferrous ( $Fe^{2+}$ ) state to successfully bind oxygen.
Methaemoglobin: * Occurs when iron is oxidized to the ferric state (Fe^{3+). This form cannot bind oxygen. * Can be caused by hereditary factors, drugs, or toxic substances (e.g., infections, specific medications). * Clinical Presentation: Patients typically exhibit cyanosis (bluish skin discoloration).
Definitions of Oxygen Deficiency: * Hypoxia: Insufficient oxygen to meet metabolic demands. * Anaemia: Hypoxia specifically caused by an erythrocyte or haemoglobin defect.

The Eight-Step Heme Biosynthesis Pathway

Heme production relies on iron delivery (via transferrin), protoporphyrin synthesis, and globin synthesis. The synthesis process utilizes eight enzymes; four function in the mitochondria and four in the cytosol.

Mitochondria: $\text{ALA synthase}$ links glycine and succinyl coenzyme A ( $\text{CoA}$ ) to form $\text{ALA}$ . This is the major rate-limiting step influenced by erythropoietin and requiring Vitamin $B_6$.
Cytosol: $\text{ALA dehydratase}$ takes two molecules of ALA to produce $\text{porphobilinogen (PBG)}$ .
Cytosol: $\text{Porphobilinogen deaminase}$ takes four molecules of PBG to produce $\text{hydroxymethylbilane (HMB)}$ .
Cytosol: $\text{Uroporphyrinogen III cosynthase}$ converts HMB into $\text{uroporphyrinogen III}$ .
Cytosol: $\text{Uroporphyrinogen decarboxylase}$ converts uroporphyrinogen III into $\text{coproporphyrinogen III}$ .
Mitochondria: $\text{Coproporphyrinogen oxidase}$ transforms coproporphyrinogen III into $\text{protoporphyrinogen IX}$ .
Mitochondria: $\text{Protoporphyrinogen oxidase}$ converts protoporphyrinogen IX to $\text{protoporphyrin IX}$ .
Mitochondria: $\text{Ferrochelatase}$ adds iron ( $Fe$ ) to protoporphyrin IX to produce the final heme molecule.

Globin Synthesis and Genetic Control

Location: Occurs in RBC-specific cytoplasmic ribosomes.
Inheritance: Globin production is directed by structural genes.
Gene Distribution in Diploids: * Four alpha ( $\alpha$ ), two beta ( $\beta$ ), four gamma ( $\gamma$ ), two delta ( $\delta$ ), two epsilon ( $\epsilon$ ), and two zeta ( $\zeta$ ) genes. * Chromosomal Locations: According to provided materials, $\alpha$ and $\zeta$ are on chromosome 11, while the remaining genes are on chromosome 16.
Developmental Phases: * Embryonic: Utilizes $\epsilon$ and $\zeta$ plus $\alpha$ and $\gamma$ . * Fetal: Primarily HbF ( $\alpha_2\gamma_2$ ). * Adult: Alpha chains are always present; beta chain production rises gradually after birth to reach adult levels at 3-6 months.

Haemoglobin Function and the Dissociation Curve

Cooperativity: When the first $O_2$ molecule binds, spaces between chains widen to attract more molecules (Relaxed State). In tissues, spaces relax to unload oxygen (Tense State).
2,3-Diphosphoglycerate (2,3-DPG): * Controls Hb affinity for oxygen. * Relaxed State: Beta chains are pulled together, 2,3-DPG is expelled; high oxygen affinity. * Tense State: Beta chains widen, 2,3-DPG binds; lower oxygen affinity to facilitate unloading.
Oxygen Dissociation Curve: * Follows a sigmoid (S-shaped) form. * $P_{50}$ : The partial pressure of $O_2$ at which Hb is 50% saturated. Normal blood $P_{50} = 26.6\,mmHg$ . * Arterial Blood: Approximately 95% saturation at $95\,mmHg$ $O_2$ tension. * Venous Blood: Approximately 70% saturation at $40\,mmHg$ $O_2$ tension.
Curve Shifts: * Shift to the Right (Decreased Affinity): Caused by increased $pCO_2$ , increased $H^+$ (low pH), increased 2,3-DPG, increased temperature, and sickle cell haemoglobin. * Shift to the Left (Increased Affinity): Caused by decreased $pCO_2$ , decreased $H^+$ , decreased 2,3-DPG, decreased temperature, and HbF (which cannot bind 2,3-DPG effectively).

Clinical Testing for Haemoglobin

Quantification: Haemoglobin cyanide method using spectrophotometry recorded at $540\,nm$ absorbance. Normal range is approximately $11-14\,g/dl$ .
Electrophoresis: Separates globins based on electrical charges in buffered solutions (Cellulose Acetate at pH 8.4). Essential for identifying abnormal haemoglobins like HbS (Sickle), D, C, and E.
Additional Tests: * Visual inspection of smears (light microscope). * Hb solubility (screening for specific types). * Spectra analysis (detects variants like carboxy-haemoglobin and sulfhaemoglobin).

Red Cell Metabolism Pathways

Because mature RBCs lack mitochondria, they cannot use the aerobic Krebs cycle and must produce energy (ATP) via anaerobic pathways.

Embden-Meyerhof Pathway: * Non-oxidative/anaerobic; generates 90% of the cell's ATP. * Yields 2 ATP per 1 glucose molecule. * Maintains cell volume, shape, and electrolyte balance via the Na pump ( $\text{ATPase}$ ).
Leubering-Rapoport Shunt: * Produced 2,3-DPG to regulate oxygen affinity.
Hexose Monophosphate (Pentose Phosphate) Shunt: * Oxidative pathway using the enzyme $G6PD$ . * Produces $NADPH$ , which protects the cell from oxidative stress. * Defects lead to denatured Hb precipitates called Heinz Bodies, visible with supravital stains (Methylene Blue, Crystal Blue).
Methaemoglobin Reductase Pathway: * Uses $\text{NADH}$ (from Embden-Meyerhof) to reduce functional methaemoglobin back to the active ferrous ( $Fe^{2+}$ ) state.

Red Cell Membrane Structure and Function

Dimensions: Normal RBC is 6-8\,mu m; must deform to fit through 3\,mu m splenic capillaries.
Layers: * Outer hydrophilic (glycolipids, glycoproteins). * Central hydrophobic (proteins, cholesterol, phospholipids). * Inner hydrophilic (proteins).
Composition: 50% Protein, 20% Phospholipid, 20% Cholesterol, 10% Carbohydrates.
Membrane Lipids: Phospholipids have hydrophilic heads and hydrophobic tails. Increased blood cholesterol can increase surface area, leading to target cells or acanthocytes.
Membrane Proteins: * Peripheral (Cytoskeleton): Spectrin ( $\alpha$ and $\beta$ ), Ankyrin, Protein 4.1, and Actin. Maintains shape and deformability. * Integral: Band 3 (anion transport), Glycophorins A, B, and C, and transferrin receptors.
Permeability: Freely permeable to water and anions (Cl^{-} and HCO_3^{-}). Impermeable to cations (Na^{+} and K^{+}), which are maintained by energy-dependent pumps.
Pathology: Defects in proteins lead to Hereditary Spherocytosis or Elliptocytosis. Defects in lipids lead to target cells or acanthocytes.