Myoglobin and Hemoglobin Review

Class Overview

• Date: March 27, 2023

• Topics: Myoglobin & Hemoglobin Part I

• Assignments:

  • In-Lecture Assignment due: April 1, Tuesday

  • Chapter 7 Problem Sets I, II, III due: April 8, Tuesday

Exam II Details

• Date: March 27

• Duration: 75 minutes in class

• Materials: Bring a calculator

• Format: Short answers and multiple choice questions (MCQs)

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Introduction to Myoglobin and Hemoglobin

• Myoglobin (Mb):

  • Most studied and best understood oxygen-binding protein.

  • First 3D structure determined.

  • Classic model for critical biochemical processes.

• Hemoglobin (Hb):

  • A circulating oxygen-binding protein.

  • Crucial for oxygen transport in the bloodstream.

Oxygen Binding Mechanisms

• Challenges with Oxygen (O2):

  • Oxygen is poorly soluble in aqueous solutions.

  • Ineffective diffusion through tissues over large distances.

• Solution:

  • Utilize heme, a protein-bound organometallic compound, to bind oxygen reversibly.

Heme Prosthetic Group

• Structure:

  • Consists of a protoporphyrin ring structure with a central Fe2+ atom.

  • Iron has six coordination bonds - four with nitrogen atoms in the porphyrin ring and two perpendicular bonds.

• Function:

  • Ensures efficient binding of O2 while preventing oxidation to non-binding Fe3+.

Role of Protein Structure in Oxygen Binding

• Globins family:

  • Widespread family of proteins including myoglobin and hemoglobin.

  • Characterized by a highly conserved tertiary structure with eight α-helices.

  • Can also bind other gases, such as CO, NO, H2S, with varying affinities.

Types of Globins

• Myoglobin: Monomeric, facilitates O2 diffusion in muscle tissues.

• Hemoglobin: Tetrameric, responsible for systemic O2 transport.

• Neuroglobin: Monomeric, protects neurons from low O2.

• Cytoglobin: Monomeric, regulates nitric oxide levels for muscle relaxation.

Myoglobin Structure and Function

• Single Binding Site:

  • Comprises 153 residues and one heme molecule.

  • Distal His (His E7): Stabilizes O2 binding through hydrogen bonding.

• Binding Mechanism:

  • CO competes with O2 for binding site, demonstrating a large difference in binding affinities.

  • CO binds over 20,000 times better than O2 but is toxic.

Protein-Ligand Interactions

• Quantitative Description:

  • Equilibrium reaction: P + L ⇌ PL, where P = protein, L = ligand.

  • Association Constant (Ka): Indicates ligand affinity - higher Ka = higher affinity.

  • Dissociation Constant (Kd): Inverse of Ka - smaller Kd indicates higher affinity.

Graphical Representation of Binding Affinity

• Fraction Bound (θ or Y):

  • Allows determination of Kd based on the ligand concentration at which half of the binding sites are occupied.

  • Binding curves help visualize ligand-receptor interactions and calculate their affinity.

Hemoglobin Functionality

• Transport Mechanism:

  • Hemoglobin operates in erythrocytes (red blood cells) to carry O2:

  • Arterial blood: ~96% O2 saturation.

  • Peripheral blood: ~64% O2 saturation.

• Structural Features:

  • Tetrameric structure (α2β2) consisting of two alpha chains and two beta chains.

  • Strong subunit interactions stabilize quaternary structure.

Different Forms of Hemoglobin

• Deoxyhemoglobin vs. Oxyhemoglobin:

  • Oxygen binding alters hemoglobin's quaternary structure leading to significant functional implications.

  • Cooperativity in oxygen binding enhances efficiency of O2 uptake and release.

Summary of Key Concepts

• Binding affinity is crucial for both myoglobin and hemoglobin in their respective roles in oxygen transport and storage.

• Understanding these intricacies provides insight into broader biological processes and pathologies related to oxygen transport deficiencies or toxicant interactions.