Aerobic Cellular Respiration and Oxygen Transport

Overview

• Aerobic cellular respiration uses oxygen to produce ATP, the energy source for cells.
• Oxygen is delivered to cells by two proteins: myoglobin and hemoglobin.

Myoglobin

• Consists of a single polypeptide chain.
• Found in muscle cells.
• Stores oxygen and releases it when oxygen concentration is low.

Hemoglobin

• Composed of four polypeptide chains: two alpha (α1, α2) and two beta (β1, β2) subunits.
• Has a quaternary structure, allowing cooperative oxygen binding.
• Delivers oxygen from lungs to tissues and returns CO2 to the lungs for expulsion.

Oxygen Binding Mechanism

• Both myoglobin and hemoglobin contain a prosthetic group called the heme group.

Structure of Heme Group

• Organic Component (Protoporphyrin): Contains carbon, nitrogen, hydrogen, and oxygen atoms.
• Inorganic Component (Iron Atom): Central Fe atom responsible for oxygen binding.

Iron Atom in Heme Group

• Bound to four nitrogen atoms; typically in a ferrous (Fe2+) state.
• Forms a fifth bond with an amino acid (histidine) from the polypeptide chain, called proximal histidine.
• Can form a sixth bond with an oxygen molecule.

Binding Process

• In deoxyhemoglobin/myoglobin, the Fe atom is slightly below the protoporphyrin plane.
• Upon oxygen binding, oxygen pulls electron density from Fe, reducing its size, allowing it to fit into the protoporphyrin plane.

Stability and Resonance Structures

• The oxygen-iron complex can be described using resonance structures:
  • Neutral diatomic oxygen with Fe2+.
  • Superoxide ion (O2⁻) with a ferric ion (Fe3+).
• Distal Histidine: Forms a hydrogen bond with the negatively charged oxygen for stabilization.

Conclusion

• Both myoglobin and hemoglobin transport oxygen via the heme group, with the iron atom directly binding to oxygen.
• The stabilization of the bound oxygen by distal histidine is crucial for oxygen release to tissues.