Myoglobin and hemoglobin: structural and functional comparison
Myoglobin is a complex muscle tissue protein with a tertiary higher structure, consisting of a single polypeptide chain and containing a single heme group (one active site). Hemoglobin has a quaternary structure: four protomers (two α-globin and two β-globin chains) and four heme groups, i.e., four active sites.
Myoglobin has a high affinity for oxygen and binds it tightly, releasing it only during severe tissue hypoxia. Hemoglobin has a lower affinity and performs a transport function: it carries O₂ from the lungs to the tissues and CO₂ from the tissues to the lungs. Hemoglobin's affinity for oxygen is regulated, whereas myoglobin's is not.
Hemoglobin is an oligomeric domain protein: each of its subunits is called a protomer, and an allosteric cavity is located at the center of the molecule. Myoglobin is a non-oligomeric protein.
Heme structure and the role of histidines
The secondary structure of the globin chain consists of eight α-helices (A–H). The heme group is located in a hydrophobic pocket between the F and E helices, stabilized by two key histidines.
Heme itself is a porphyrin ring made of four pyrrole rings, whose nitrogen atoms form coordination bonds with the central iron atom Fe²⁺. The side chains of the ring are vinyl, methyl, and propionyl groups.
- His F8 (proximal) forms a coordination bond with the heme iron, anchoring it within the protein.
- His E7 (distal) ensures the correct orientation of the O₂ molecule in the pocket and increases the heme's affinity for oxygen relative to carbon monoxide (CO), reducing the risk of poisoning.
The hydrophobic environment of the heme prevents water entry, which prevents the oxidation of Fe²⁺ to Fe³⁺ (methemoglobin), which is incapable of binding oxygen. Upon O₂ binding, His F8 "pulls" the iron back into the porphyrin plane—this movement triggers cooperative changes.
Cooperative oxygen binding
The regulation of hemoglobin's affinity for O₂ begins with cooperative conformational changes of the protomers. Upon binding of the first oxygen molecule, the corresponding protomer changes its conformation and facilitates O₂ binding by the neighboring protomer. Each subsequent molecule binds more easily: the fourth molecule binds approximately 300 times more easily than the first.
Oxygen release proceeds similarly: the more O₂ is already dissociated, the easier the next molecule dissociates. This produces a characteristic sigmoid dissociation curve and ensures efficient oxygen delivery to tissues.
The Bohr effect and 2,3-BPG regulation
The Bohr effect: the more protonated hemoglobin is, the more readily it releases oxygen. In actively metabolizing tissues, CO₂ is produced, which reacts with water to form carbonic acid, dissociating into H⁺ and HCO₃⁻. The accumulation of protons and acidification of the medium stabilize the deoxytur of hemoglobin and facilitate O₂ release. CO₂ additionally binds to the N-terminal groups of the protomers, forming carbaminohemoglobin.
2,3-Bisphosphoglycerate (2,3-BPG)—an intermediate metabolite produced in erythrocytes via the Rapoport-Luebering shunt—binds in the central allosteric cavity of hemoglobin. Its synthesis increases during hypoxia. By binding to the central cavity, 2,3-BPG decreases hemoglobin's affinity for oxygen and facilitates its release to tissues.
Fetal hemoglobin (HbF) binds 2,3-BPG weakly and therefore has an increased affinity for O₂, which is necessary for oxygen transport across the placenta from mother to fetus.
Denaturation and chaperones
Denaturation is the disruption of the quaternary, tertiary, and sometimes secondary structure of a protein while preserving its primary structure. It can be reversible (with the possibility of returning to the native conformation) or irreversible.
- High temperature disrupts hydrogen bonds.
- Acids and alkalis disrupt hydrogen and ionic bonds.
- Alcohols, phenols, and urea predominantly disrupt hydrogen bonds.
Chaperones function within the cell to prevent denaturation and promote proper folding. They are divided into constitutive chaperones (constantly present) and inducible ones, such as heat shock proteins (HSPs), whose synthesis increases in response to stress.
Sickle cell anemia
Sickle cell anemia (HbS) is a hereditary hemoglobinopathy caused by a point mutation in the β-globin gene: at the sixth position of the polypeptide chain, glutamic acid (negatively charged, hydrophilic) is replaced by valine (hydrophobic, uncharged).
The appearance of a hydrophobic residue on the molecular surface creates complementary sites along which deoxygenated HbS molecules aggregate into long fibers. This deforms the erythrocytes, giving them a sickle shape, and impairs membrane elasticity and deformability.
- Sickled erythrocytes pass poorly through capillaries, causing microthrombosis and tissue ischemia.
- Their lifespan is reduced, leading to hemolytic anemia and chronic hypoxia.
- Form designations: HbA — normal adult hemoglobin, HbF — fetal, HbS — sickle cell.
The high frequency of the mutation in African countries is explained by the selective advantage of heterozygous carriers in malaria-endemic regions: the malaria parasite reproduces less effectively in sickled erythrocytes, which are destroyed faster, preventing the parasite from completing its developmental cycle.
Key points
- Myoglobin is a monomer with one heme and high affinity for O₂; hemoglobin is a tetramer (2α + 2β) with four hemes and a transport function.
- Heme is anchored in the pocket between the F and E helices; His F8 binds Fe²⁺, while His E7 orients O₂ and reduces affinity for CO.
- Cooperative O₂ binding provides a sigmoid dissociation curve and efficient oxygen delivery to tissues.
- Bohr effect: H⁺ and CO₂ decrease hemoglobin's affinity for O₂, facilitating its release in metabolizing tissues.
- 2,3-BPG binds in the central cavity and reduces O₂ affinity; its level increases during hypoxia.
- Fetal hemoglobin HbF binds 2,3-BPG weakly and has a higher affinity for O₂.
- HbS is a Glu6→Val substitution in the β-chain, leading to hemoglobin aggregation and sickle-shaped erythrocyte deformation.
- HbS heterozygosity confers resistance to malaria, which explains the high frequency of the mutation in Africa.