Biochemistry: A Short Course 9

Hemoglobin and Myoglobin: Structure, Function, and Clinical Relevance

9.1 Hemoglobin Displays Cooperative Behavior

Hemoglobin is an allosteric protein that exhibits cooperative binding of molecular oxygen. This property is essential for its physiological role in oxygen transport.

• Allosteric protein: A protein whose function at one site is affected by the binding of a regulatory molecule at a separate site.

• Cooperative binding: The binding of one oxygen molecule increases the affinity for subsequent oxygen molecules, resulting in a sigmoidal (S-shaped) oxygen dissociation curve.

• Physiological significance: Cooperative binding allows hemoglobin to efficiently pick up oxygen in the lungs (high O2 concentration) and release it in tissues (low O2 concentration) where it is needed for metabolism.

9.2 Myoglobin and Hemoglobin Bind Oxygen in Heme Groups

Myoglobin is a largely α-helical protein that binds the prosthetic group heme. Heme is crucial for oxygen binding in both myoglobin and hemoglobin.

• Heme structure: Composed of protoporphyrin (four linked pyrrole rings) and a central iron ion in the ferrous state ($Fe^{2+}$).

• Iron coordination: The iron ion is coordinated to the side chain of a histidine residue (proximal histidine) in myoglobin.

• Oxygen binding: One oxygen atom binds to an open coordination site on the iron ion. Partial electron transfer causes the iron to move into the plane of the porphyrin ring upon oxygen binding.

• Myoglobin vs. Hemoglobin: Myoglobin is monomeric (single polypeptide), while hemoglobin is tetrameric (four polypeptides).

9.3 Hemoglobin Binds Oxygen Cooperatively

Hemoglobin consists of four polypeptide chains: two α chains and two β chains. Each chain is similar in sequence and structure to myoglobin.

• Quaternary structure: Hemoglobin's four subunits allow for cooperative binding of oxygen.

• T (tense) and R (relaxed) states: Oxygen binding induces a transition from the T state (low affinity) to the R state (high affinity). The two αβ dimers rotate by ~15° during this transition.

• Structural transmission: Changes at the iron sites are transmitted to the αβ interface, influencing the T-to-R equilibrium and thus oxygen affinity.

9.4 An Allosteric Regulator Determines the Oxygen Affinity of Hemoglobin

The oxygen affinity of hemoglobin is modulated by 2,3-bisphosphoglycerate (2,3-BPG), an allosteric effector present in red blood cells.

• 2,3-BPG binding: Binds tightly to the T state, stabilizing it and lowering hemoglobin's oxygen affinity.

• Fetal hemoglobin: Binds 2,3-BPG less tightly, resulting in higher oxygen affinity. This facilitates oxygen transfer from maternal to fetal blood.

• Physiological role: 2,3-BPG ensures efficient oxygen release in tissues.

9.5 Hydrogen Ions and Carbon Dioxide Promote the Release of Oxygen (The Bohr Effect)

Hemoglobin's oxygen-binding properties are affected by pH and carbon dioxide concentration, a phenomenon known as the Bohr effect.

• pH effect: Lower pH (higher [H+]) decreases oxygen affinity by stabilizing the T state.

• CO2 effect: CO2 is converted to carbonic acid, lowering pH, and also forms carbamates with hemoglobin, further stabilizing the T state.

• Physiological significance: In metabolically active tissues (high CO2 and H+), hemoglobin releases more oxygen.

Equation for carbonic acid formation:

$CO_2 + H_2O \leftrightarrow H_2CO_3 \leftrightarrow H^+ + HCO_3^-$

9.6 Mutations in Genes Encoding Hemoglobin Subunits Can Result in Disease

Genetic mutations affecting hemoglobin subunits can lead to clinically significant diseases.

• Sickle-cell disease: Caused by a mutation in the β chain (Glu → Val), creating a hydrophobic patch that promotes polymerization of deoxyhemoglobin, distorting red blood cells into a sickle shape.

• Thalassemias: Result from reduced production of either α or β chains, leading to hemoglobin tetramers with only one type of chain and impaired oxygen transport.

• Clinical significance: Sickle-cell disease was the first disease linked to a specific amino acid change in a protein.

Appendix: Biochemistry in Focus – Additional Globins in the Human Genome

The human genome encodes several globin proteins beyond myoglobin and hemoglobin, with distinct expression patterns and functions.

• Globin gene family: Includes genes for myoglobin, α-hemoglobin, β-hemoglobin, fetal hemoglobin (γ chain), δ chain, ε chain, and ζ chain.

• Neuroglobin: Monomeric, expressed in the brain and retina, may protect neural tissue from hypoxia.

• Cytoglobin: Monomeric, expressed widely in the body, function still under investigation.

• Oxygen binding: In neuroglobin and cytoglobin, both proximal and distal histidines coordinate the iron in the deoxy form; oxygen binding displaces the distal histidine.

Clinical Application: Carbon Monoxide Poisoning and Neuroglobin

Carbon monoxide (CO) is a colorless, odorless gas that binds hemoglobin with high affinity, inhibiting oxygen transport and cellular respiration.

• CO poisoning: More than 50,000 emergency room visits and ~500 deaths per year in the US.

• Treatment: High-pressure oxygen therapy is the standard treatment.

• Neuroglobin as therapy: Modified neuroglobin that binds CO 500-fold more tightly than hemoglobin can strip CO from hemoglobin, offering potential as a life-saving therapy.

Oxygen Dissociation Curves and Hemoglobin Function

The oxygen dissociation curve plots the fractional saturation of hemoglobin (Y) against the partial pressure of oxygen ($pO_2$). The sigmoidal shape reflects cooperative binding.

• Sigmoidal curve: Indicates allosteric (cooperative) binding; hemoglobin is highly saturated in the lungs and releases oxygen in tissues.

• Mutant hemoglobins: Some mutants may bind oxygen but fail to release it efficiently in tissues, making them physiologically ineffective.

• Ideal curve: Hemoglobin should be saturated in the lungs and poorly saturated in tissues, ensuring maximal oxygen delivery.

Hemoglobin State	Oxygen Affinity	Stabilized by
T (Tense)	Low	2,3-BPG, H+, CO2
R (Relaxed)	High	Oxygen binding

Key Terms: cooperative effect, heme, protoporphyrin, distal histidine, α subunit, β subunit

Additional info: The notes above expand on the original content by providing definitions, physiological context, and clinical relevance for each topic, as well as a summary table comparing hemoglobin states.