CHE340-Ch05-Protein-Interactions-V1

CHE 340: Biochemistry I

  • Chapter 5

  • Last Edited: 1/28/2025 10:23 PM

Protein Interactions

  • Dynamic interactions with other molecules including:

    • Catalytic Actions: Proteins act as enzymes to change chemical configurations of bound molecules.

    • Binding: Compositions of bound molecules remain unchanged.

Section 5.1: Reversible Binding of Proteins to Ligands

  • Ligands: Molecules that bind reversibly to proteins. Can include other proteins.

  • Importance:

    • Allows organisms to react to varying environmental and metabolic conditions swiftly.

Heme and Oxygen Binding

  • Oxygen Properties:

    • Poorly soluble and ineffective at diffusing through tissues.

    • Transition metals, like iron, readily bind oxygen but free ions can generate toxic byproducts.

  • Heme:

    • A prosthetic group containing iron, crucial for oxygen binding.

    • Structure: Complex organic ring (protoporphyrin) with a Fe2+ atom bound.

Coordination of Iron in Heme

  • Six coordination bonds:

    • Four to nitrogen atoms in porphyrin ring.

    • Two perpendicular bonds.

Clicker Question 1: Heme Prosthetic Group

  • Correct Answer:

    • A. Heme consists of protoporphyrin and an iron (II) ion.

Binding Sites and Specificity

  • A ligand binds at a protein's binding site that matches the ligand's size, shape, charge.

  • Protein selectively binds specific ligands, maintaining order in living systems.

Binding of Oxygen to Heme

  • Coordination bonds:

    1. One bond is from a conserved His residue to prevent oxidation and regulate binding affinity.

    2. Second bond is for binding molecular oxygen (O2).

Globins as Oxygen-Binding Proteins

  • Types:

    • Myoglobin: Monomeric; facilitates oxygen diffusion in muscles.

    • Hemoglobin: Tetrameric; responsible for blood oxygen transport.

    • Neuroglobin: Monomeric; protects the brain from low oxygen.

    • Cytoglobin: Monomeric; regulates nitric oxide levels.

Myoglobin Structure

  • Composition: 153 amino acids and one heme.

  • Naming residues specific to their positions e.g., His93, His F8.

Quantifying Protein-Ligand Interactions

  • Equilibrium Expression: Describes reversible binding between protein (P) and ligand (L).

  • Association Constant (Ka): Measures ligand affinity; higher Ka indicates higher affinity.

    • Equation: Ka = [PL]/([P][L]) where [PL] is the concentration of the bound complex.

Clicker Question 3: Affinity Comparison

  • Protein A (Ka = 10^5) and Protein B (Ka = 10^8):

    • Correct Answer: B. Protein B has higher affinity for ligand L.

Binding Equilibrium Analysis

  • When [L] remains constant and is in excess:

    • Ka = [PL]/([P][L])

    • Rearranged: Y = Ka[L]/(Ka[L] + 1)

  • Defines binding site occupancy.

Graphical Representation of Binding

  • Use of graphs to show ligand binding and determine [L] leading to half-occupied binding sites.

Dissociation Constant (Kd)

  • Kd = reciprocal of Ka.

  • Provides information on ligand release rates.

    • Interpretation of affinity: lower Kd = higher affinity.

2,3-Bisphosphoglycerate (BPG) in Oxygen Binding

  • BPG is an allosteric modulator that binds and reduces hemoglobin's affinity for oxygen, particularly in low oxygen environments (like at high altitudes).

  • Hemoglobin Structure Influence: Stabilizes the T state between β subunits.

The Bohr Effect

  • Increased CO2 leads to lower blood pH, facilitating oxygen release in tissues through ionic interactions.

Sickle Cell Anemia

  • A mutation (E6V) results in a hydrophobic patch, fostering polymerization of deoxygenated hemoglobin leading to cell distortion.

Key Takeaways: Hemoglobin Functionality

  • Oxygen release is influenced by BPG and pH levels (Bohr effect).

  • Hemoglobin’s structure allows for cooperative binding, influenced by various environmental factors.