Haemoglobin Structure

• Each of the four globin subunits contains a non-protein component known as a prosthetic group, specifically a haem group.

• The haem group consists of a flat porphyrin ring with a central Iron(II) ion ($Fe^{2+}$) at its core.

• This $Fe^{2+}$ ion is the actual site of oxygen binding; it can bind to one molecule of oxygen ($O_2$) reversibly.

• Because there are four haem groups per haemoglobin molecule, one complete haemoglobin can carry a maximum of four oxygen molecules (or eight oxygen atoms).

• Haemoglobin exhibits cooperative binding, a phenomenon where the binding of the first oxygen molecule triggers a conformational change in the protein's shape.

• This structural shift makes it significantly easier for the second and third oxygen molecules to bind to the remaining haem groups.

• Conversely, the release of one oxygen molecule makes it easier for the subsequent ones to be released, allowing for efficient loading in the lungs and unloading in the tissues.

• This mechanism is the reason why the oxygen dissociation curve is sigmoidal (S-shaped) rather than linear.

Comparison of Structural Components

• Haemoglobin vs. Simple Proteins: Unlike simple monomeric proteins, haemoglobin's quaternary structure allows for allosteric regulation, meaning events at one site affect the others.

• Check the Oxidation State: Always specify that the iron in haem is Iron(II) or $Fe^{2+}$. A common distractor in exams is $Fe^{3+}$, which cannot bind oxygen effectively.

• Stoichiometry Precision: Distinguish between oxygen molecules ($O_2$) and oxygen atoms. One haemoglobin binds 4 molecules, which equals 8 atoms.

• Solubility Logic: If asked why R-groups are arranged a certain way, link the outward-facing hydrophilic groups directly to the requirement for solubility in the blood.

• Structure-Function Link: When discussing quaternary structure, always mention that it is the interaction between subunits that enables cooperative binding.