Proteins in Action: Oxygen Transport – Myoglobin & Haemoglobin

Structure of Myoglobin

• Primary: $\sim150$ amino acids
• Secondary: eight $\alpha$-helices (A–H) with connecting loops
• Tertiary: compact globin fold; hydrophobic pocket for haem
• Quaternary: monomeric (single polypeptide)

Structure of Haemoglobin

• Tetramer: $2\alpha+2\beta$ globin subunits (non-covalent association)
• Each subunit ⟹ one haem ⟹ binds one $\mathrm{O_2}$ (max $4$ per tetramer)
• Globin fold almost identical to myoglobin per subunit

Haem (prosthetic group)

• Planar porphyrin (four pyrrole rings)
• $\text{Fe}^{2+}$ has six coordinate bonds:
  • $4$ to haem N atoms
  • $1$ to His F8 (proximal)
  • $1$ reversible site for $\mathrm{O_2}$ (distorted by His E7)
• Electronic orbitals → red colour; $\mathrm{O_2}$ binding shifts Fe from out-of-plane to in-plane

Oxygen Binding Characteristics

• Myoglobin: hyperbolic curve, high affinity
  • Saturated at low $p\mathrm{O<em>2}$ • Releases $\mathrm{O</em>2}$ only when cellular $p\mathrm{O_2}$ is very low
• Haemoglobin: sigmoidal curve (co-operative)
  • Low affinity in tissues (T-state), high in lungs (R-state)
  • Enables loading at $\sim100\,\text{Torr}$ & unloading at $\sim20\,\text{Torr}$

Cooperativity & Allostery

• Requires multiple interacting subunits (present in Hb, absent in Mb)
• Two interconvertible states:
  • Tense (T) – low $\mathrm{O<em>2}$ affinity • Relaxed (R) – high $\mathrm{O</em>2}$ affinity
• Generates sigmoidal binding curve → steep response within physiological $p\mathrm{O_2}$ range
• Allostery: modulators bind sites other than haem; can occur in mono- or oligomeric proteins

Quantifying Oxygen Binding

• Spectroscopy + Beer–Lambert Law: absorbance $\propto$ concentration
• Colour shift (dull → bright red) monitors $\mathrm{O_2}$ saturation in vitro & in vivo

Key Numbers / Facts

• $[\text{Hb}]_{blood} \approx 5\,\text{mmol L}^{-1}$
• Myoglobin stores $0.5!–!0.7\,\text{mmol L}^{-1}$ $\mathrm{O_2}$ in muscle; exhausted in $\sim7\,\text{s}$ without supply

Quick Self-Check

• Role of Mb? Tissue $\mathrm{O_2}$ storage
• Role of Hb? $\mathrm{O_2}$ transport lung ⇄ tissue
• Components? Globin + haem
• Dominant secondary structure? $\alpha$-helix
• Coordination sites on $\text{Fe}^{2+}$? $6$ (see above)
• Function of His E7? Distorts 6th site → lowers affinity → facilitates release
• Mb saturation: high at low $p\mathrm{O<em>2}$; releases only at very low $p\mathrm{O</em>2}$
• Max $\mathrm{O_2}$ per Hb tetramer? $4$
• Structural change on binding? Fe moves into haem plane; T → R transition in Hb