Protein-Ligand Binding and Oxygen Transport: Hemoglobin and Myoglobin

Protein-Ligand Binding: Quantitative Description

Binding Equilibrium and Affinity

Proteins often function by binding to other molecules, called ligands. The quantitative description of this binding is essential for understanding protein function in biochemistry.

• Ligand (L): A molecule that binds specifically to a protein.

• Binding site: The region on the protein where the ligand binds.

• Equilibrium: The reversible binding of a ligand to a protein can be described by the equilibrium:

$P + L \rightleftharpoons PL$

• Association constant (Ka): $K_a = \frac{[PL]}{[P][L]}$

• Dissociation constant (Kd): $K_d = \frac{[P][L]}{[PL]}$ (inverse of $K_a$)

• Fractional saturation (θ): The fraction of binding sites occupied by ligand:

$\theta = \frac{[L]}{[L] + K_d}$

• Lower $K_d$ indicates higher affinity of the protein for the ligand.

Graphical Analysis of Binding

Binding data are often plotted as θ versus [L] to visualize binding strength and saturation.

• Hyperbolic curve for simple binding (non-cooperative).

• At $[L] = K_d$, θ = 0.5 (half of the binding sites are occupied).

Examples of Binding Strength

• High affinity: Low $K_d$ (tight binding, e.g., biotin-avidin interaction).

• Low affinity: High $K_d$ (weak binding, e.g., enzyme-substrate interactions).

Oxygen Binding by Metalloproteins

Globins: Oxygen-Binding Proteins

Globins are a family of proteins that bind oxygen using a heme prosthetic group. The two main types are myoglobin and hemoglobin.

• Myoglobin: Monomeric protein found in muscle, stores oxygen.

• Hemoglobin: Tetrameric protein in red blood cells, transports oxygen.

Structure of Porphyrin and Heme

• Heme: A prosthetic group containing an iron (Fe2+) ion coordinated in a porphyrin ring.

• Iron binds oxygen reversibly.

• Heme geometry is planar, with Fe2+ at the center.

Myoglobin: Structure and Function

• Single polypeptide chain with one heme group.

• Binds O2 with high affinity; not suitable for O2 transport due to lack of cooperativity.

Binding of Carbon Monoxide (CO)

• CO binds to heme iron with much higher affinity than O2 (toxic effect).

• CO binding can block O2 transport and lead to poisoning.

O2 Binding to Free Heme vs. Protein-Bound Heme

• Free heme binds O2 irreversibly, leading to oxidation of Fe2+ to Fe3+ (non-functional).

• Protein environment (globin) stabilizes Fe2+ and allows reversible O2 binding.

Hemoglobin: Structure and Cooperative Binding

Hemoglobin Structure

• Tetramer: 2 α and 2 β subunits, each with a heme group.

• Exhibits cooperativity in O2 binding: binding of O2 to one subunit increases affinity in others.

Cooperativity and the Hill Equation

• Cooperative binding produces a sigmoidal (S-shaped) θ vs. [O2] curve.

• Hill equation:

$\theta = \frac{[L]^n}{[L]^n + K_d}$

• Hill coefficient (n): Indicates degree of cooperativity (n > 1: positive cooperativity).

T and R States of Hemoglobin

• T (Tense) state: Low O2 affinity, stabilized by salt bridges.

• R (Relaxed) state: High O2 affinity, salt bridges broken upon O2 binding.

• O2 binding triggers conformational change from T to R state.

pH Effect on O2 Binding (Bohr Effect)

• Lower pH (higher [H+]) reduces O2 affinity (stabilizes T state).

• Metabolism produces CO2 and H+, promoting O2 release in tissues.

• Key residues (e.g., histidine, aspartate) form salt bridges at low pH, stabilizing T state.

CO2 and 2,3-Bisphosphoglycerate (2,3-BPG) Regulation

• CO2 binds to hemoglobin as carbamate, stabilizing T state and promoting O2 release.

• 2,3-BPG binds to central cavity of hemoglobin, stabilizing T state and reducing O2 affinity.

Summary Table: Factors Affecting Hemoglobin O2 Affinity

Factor	Effect on O2 Affinity	Mechanism
pH (Bohr Effect)	Decreases	Stabilizes T state via salt bridges
CO2	Decreases	Carbamate formation, stabilizes T state
2,3-BPG	Decreases	Binds central cavity, stabilizes T state
CO	Increases (abnormally)	Binds heme with high affinity, blocks O2 binding

Clinical Relevance: Sickle Cell Anemia

Hemoglobin Mutations and Disease

• Sickle cell anemia: Caused by a single amino acid substitution (Glu → Val) in β-globin.

• Mutant hemoglobin (HbS) polymerizes under low O2, distorting red blood cells.

• Leads to impaired O2 delivery and various clinical symptoms.

Key Concepts Covered

• Quantitative analysis of protein-ligand binding

• Structure and function of myoglobin and hemoglobin

• Cooperativity and allosteric regulation

• Physiological regulation of O2 binding (pH, CO2, 2,3-BPG)

• Clinical implications of hemoglobin mutations