Myoglobin and Hemoglobin

The Iron

Central iron (heme) binds oxygen loosely

The Protoporphyrin IX Ring

Organic ring structure around the iron

4 nitrogen atoms

The Heme

Oxygen binding prosthetic group embedded within hydrophobic pocket

The Proximal Histidine

His F8 (helix, residue)

Binds to iron of heme

The Distal Histidine

His E7 (helix, residue)

Stabilizes bound O2 through hydrogen bonding, forces bent oxygen geometry

The Hydrophobic Pocket

Houses heme group shielding iron fand O2 from water

Prevents oxidation

The Globin

Protect iron from O2 and CO

Provides a precise environment for reversible oxygen binding

Additional Hemoglobin Globin Roles

Cooperative binding

4 heme groups increase affinity as they bind to O2

Hemoglobin vs Myoglobin

Hemoglobin → Oxygen transport (4 hemes)
Myoglobin → Oxygen storage (1 heme)

Saturation

The fraction of total oxygen-binding sites (in Hb or Mb) that have oxygen bound

Hyperbolic Oxygen Binding Curve

Myoglobin

Simple binding with no cooperativity, affinity stays constant

Sigmoidal Oxygen Binding Curve

Hemoglobin

Positive cooperativity, binding to one subunit increases affinity of other 3

Cooperativity

The interaction between binding sites such that binding of one ligand affects the affinity at other sites (positive or negative)

Allostery

Regulation of a protein’s activity by binding of an effector molecule at a site other than the active site, inducing conformational change

Allosteric Effectors

Homo → ligand (O2)
Hetero → not ligand (H+, CO)

Ligand

A molecule that binds to a specific site on a protein

Conjugated Protein

Non-protein component
heme

Prosthetic Group

Tightly bound non-protein component

heme

Oligomeric Protein

Multiple polypeptide subunits

hemoglobin

Oxygen Affinity

The strength of binding between oxygen and a protein’s heme group

R state

relaxed state

high O₂ affinity, stabilized by O₂ binding

T state

tense state

low O₂ affinity, stabilized by deoxygenated conditions and effectors (H⁺, CO₂, BPG)

Methemoglobin and Metmyoglobin

Iron is oxidized

Conservative aa Substitution

Similar properties (e.g., Leu → Ile)

minimal structural change

Non-conservative aa Substitution

Very different properties (e.g., Glu → Val)

can disrupt structure/function

aB dimer

each aB unit are closely associated, and tend to move together.

In solution, Hemoglobin will decompose into two aB units

Physiological role of myoglobin

Myoglobin stores oxygen in muscle tissue and releases it when oxygen levels are low, supporting muscle metabolism during intense activity

Physiological role of hemoglobin

Hemoglobin transports oxygen from the lungs to tissues and facilitates CO₂ and proton transport back to the lungs

Allosteric effectors on hemoglobin

O2 → Stabilizes R state, increases affinity
H+ → Stabilizes T state, decreases affinity
CO2 → Stabilizes T state
Cl- → Stabilizes T state, binds deoxyhemoglobin

Bohr effect

A decrease in pH (↑H⁺) or increase in CO₂ decreases Hb’s oxygen affinity, promoting O₂ release to tissues

-Protonation of specific residues stabilizes the T state; oxygen binding shifts Hb toward the R state, releasing protons (H⁺)

Oxygen binding effect in hemoglobin

In the R state, some acidic residues are exposed to less polar environments, lowering their pKa and promoting proton release during oxygenation

Hemoglobin equilibrium

Hyperventilation: ↓CO₂ → ↑pH → increased O₂ affinity → reduced O₂ delivery.

Hypoventilation: ↑CO₂ → ↓pH → decreased O₂ affinity → enhanced O₂ delivery.