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Module 5: Hemoglobin Structure and Function

Overview

  • Module 5 discusses hemoglobin structure and function including:

    • Allosteric control of oxygen transport by hemoglobin.

    • Mechanisms of transport of polar molecules across cell membranes by membrane proteins.

    • Actin-myosin motor mechanisms that mediate muscle contraction in response to calcium release.

Lecture Topics and Textbook Readings

  • Module 5 Topics:

    1. Hemoglobin: Allostery and Evolution

    • Textbook Reading: Chapter 6.2

    1. Membrane Transport Proteins

    • Textbook Reading: Chapter 6.3

    1. Passive Transport

    • Textbook Reading: Chapter 6.3

    1. Active Transport

    • Textbook Reading: Chapter 6.3

    1. Muscle Contraction

    • Textbook Reading: Chapter 6.4

Hemoglobin: Allostery and Evolution

Key Concepts

  • Allosteric Effector Molecules:

    • Bind to hemoglobin and cause conformational changes that affect oxygen (O2) binding.

    • Shift the equilibrium between:

    • R state (relaxed state) - Higher affinity for O2.

    • T state (tense state) - Lower affinity for O2.

  • Functional Types of Allosteric Effector Molecules:

    • Positive Allosteric Effector:

    • O2: Shifts equilibrium from T state to R state, enhancing O2 binding to the heme group (homotropic effector).

    • Negative Allosteric Regulators:

    • CO2, H+, and 2,3-bisphosphoglycerate (BPG): Shift equilibrium from R state to T state, decreasing O2 binding to the heme group (heterotropic effectors).

  • Sickle Cell Anemia:

    • Caused by a mutation (E6V) in the β subunit that leads to the association of βS subunits forming large polymeric fibers, potentially destroying erythrocytes.

Allosteric Control of O2 Binding: The Bohr Effect

  • CO2 as a Byproduct:

    • CO2 generated from aerobic respiration binds and needs removal from tissues.

    • Most CO2 is converted by the enzyme carbonic anhydrase into:

    • Carbonic Acid (H2CO3) and Bicarbonate (HCO3−) + H+,

      • Results in proton (H+) generation that decreases blood pH from 7.6 (lungs) to 7.2 (tissues).

  • Effects on Hemoglobin:

    • Both α and β subunits of hemoglobin can be protonated, impacting T state and R state equilibrium.

  • Heterotropic Allosteric Effect of CO2:

    • CO2 binds reversibly to hemoglobin and forms additional ion pairs stabilizing the T state.

Allosteric Control of O2 Binding: BPG Binding

  • 2,3-Bisphosphoglycerate (BPG):

    • Binds to hemoglobin, stabilizing it in the T state by interacting with various residues in the center of the tetramer.

    • A single molecule of BPG inhibits O2 binding across all four hemoglobin subunits.

O2 Transport Dynamics

  • High O2 Concentrations:

    • Lungs favor the R state; BPG molecules are usually not bound.

  • In Tissues:

    • Reduced O2 levels and the effects of the Bohr effect promote stability of the T state, enhanced by BPG.

Fetal Hemoglobin (Hbγ)

  • Composition: Fetal hemoglobin has a structure of α2γ2.

    • Displays lower affinity for BPG due to an H143S substitution in the γ subunit.

    • Leads to a greater shift towards the R state compared to maternal hemoglobin.

High Altitude Adaptation

  • Physiological Response at High Altitudes:

    • Above 4000 meters, the body increases synthesis of BPG to enhance oxygen unloading in the tissues.

    • While elevated BPG levels reduce O2 binding, efficient O2 delivery compensates for this effect.

Evolution of the Globin Gene Family

  • Gene Divergence:

    • Hemoglobin and myoglobin genes diverged approximately 500 million years ago.

    • Hbα and Hbβ genes diverged roughly 100 million years later.

  • Bioinformatic Analysis of Sequences:

    • Sequence alignment features eight α helices and key residues required for O2 binding (His E7 and His F8).

Globin Gene Mutations and Sickle Cell Anemia

Anemia Causing Mutations

  • Alterations in Amino Acid Sequences:

    • Identified mutations in human Hbα and Hbβ subunits, specifying positions in the helices.

  • Deoxyhemoglobin with Val6 Mutation:

    • The Val6 mutation (deoxyHbS) can result in extended polymer formation due to hydrophobic interactions among βS residues.

    • Involves strong interactions between βS variants across different tetramers.

Characteristics of Sickle Cell Anemia

  • Sickle Cell Morphology:

    • Characterized by deformed red blood cells with a sickle appearance due to accumulation of deoxyHbS fibers.

    • This condition is an autosomal recessive genetic disease; individuals require two defective gene copies for the disease.

  • Malaria Resistance in Heterozygous Individuals:

    • Heterozygous individuals with the βS mutation show greater resistance to malaria compared to those with normal hemoglobin.

Base Editing Technique for Gene Modifications

  • Base Editing Definition:

    • A method developed for specific targeting of DNA regions utilizing “replacement” sequences via a modified CRISPR/Cas9 system.