Comparative characteristics of myoglobin and hemoglobin
Myoglobin is a red muscle protein that functions as an oxygen store. Its highest structure is tertiary; the molecule consists of a single polypeptide chain and a single heme (prosthetic group), meaning it has one active site. Myoglobin has a high affinity for oxygen, so it releases O₂ only during severe tissue oxygen deficit.
Hemoglobin is a complex oligomeric domain protein with quaternary structure. It consists of four protomers: two α-globin and two β-globin chains, each bound to a heme. Thus, the hemoglobin molecule has four active sites. Its function is to transport O₂ from the lungs to tissues and CO₂ (as well as protons) from tissues to the lungs. Its oxygen affinity is lower than that of myoglobin, which is necessary for the efficient delivery of O₂ to tissues.
Structure of heme and the role of histidine residues
Heme is represented by protoporphyrin IX—four pyrrole rings linked by methene bridges, with side vinyl, methyl, and propionyl groups. The nitrogen atoms of the pyrrole rings form four coordinate bonds with the Fe²⁺ ion. The fifth coordinate bond of iron goes to the proximal histidine His F8, and the sixth goes to the O₂ molecule.
The secondary structure of globin is represented by eight α-helices, designated by the letters A–H. Heme is located in a hydrophobic pocket between helices E and F.
- His F8 (proximal) forms a covalent bond with Fe²⁺ and holds the heme in place.
- His E7 (distal) ensures the correct orientation of O₂, increases the affinity of heme for oxygen, and simultaneously decreases the affinity for carbon monoxide (CO), which would otherwise bind much more tightly.
The hydrophobic environment of the heme prevents water entry and prevents the oxidation of Fe²⁺ to Fe³⁺ (formation of methemoglobin, which is incapable of carrying oxygen).
Cooperative oxygen binding
The binding of the first O₂ molecule to one of the protomers induces a conformational transition from the T-state (low affinity) to the R-state (high affinity). This facilitates O₂ binding by subsequent protomers: the fourth oxygen molecule binds approximately 300 times easier than the first. The reverse process works similarly—O₂ release is cooperative. Cooperativity is manifested in the S-shaped (sigmoidal) oxyhemoglobin dissociation curve, whereas the myoglobin curve is hyperbolic.
Bohr effect
Bohr effect is the decrease in hemoglobin affinity for oxygen upon medium acidification and an increase in the partial pressure of CO₂. Working tissues accumulate carbon dioxide, which, reacting with water in the presence of carbonic anhydrase, forms carbonic acid that dissociates into H⁺ and HCO₃⁻.
- Protons protonate certain residues in hemoglobin protomers, stabilizing the T-form, which facilitates O₂ release to tissues.
- CO₂ binds to the terminal α-amino groups of globins to form carbaminohemoglobin.
- In the lungs, the process is reversible: at low pCO₂ and higher pH, hemoglobin binds O₂ again.
Regulation by 2,3-bisphosphoglycerate
An allosteric site is located in the central cavity of the hemoglobin tetramer, binding 2,3-bisphosphoglycerate (2,3-BPG), an intermediate metabolite of erythrocytic glycolysis (Rapoport–Luebering shunt). During hypoxia, the concentration of 2,3-BPG increases; it binds to deoxyhemoglobin, stabilizing the T-form and decreasing O₂ affinity, which facilitates oxygen release to tissues.
Fetal hemoglobin (HbF), consisting of two α- and two γ-chains, binds 2,3-BPG more weakly, due to which it has a higher O₂ affinity and effectively "steals" oxygen from maternal HbA across the placenta.
Denaturation and chaperones
Denaturation is the disruption of the quaternary, tertiary, and secondary structures of a protein while preserving the primary structure. It can be reversible (renaturation is possible) and irreversible. Denaturing factors include high temperature, extreme pH values, heavy metals, organic solvents, and detergents—they destroy hydrogen, ionic, and hydrophobic interactions, as well as disulfide bonds.
Chaperones are proteins that ensure the correct folding of polypeptides and prevent the aggregation of denatured molecules. There are constitutive chaperones (present in the cell constantly) and inducible ones (e.g., heat shock proteins, whose synthesis increases in response to stress factors).
Sickle cell anemia
Sickle cell anemia is a hereditary hemoglobinopathy caused by a point mutation in the β-globin gene: at position 6 of the polypeptide chain, glutamic acid (polar, negatively charged) is replaced by valine (hydrophobic, uncharged). Abnormal hemoglobin is designated as HbS; normal adult hemoglobin is HbA, and fetal hemoglobin is HbF.
The appearance of a hydrophobic residue on the surface of the β-chain creates a complementary "sticky" patch. Under hypoxic conditions, deoxygenated HbS molecules polymerize, forming long fibers that deform the erythrocyte, giving it a sickle shape.
- Sickled erythrocytes lose their plasticity and pass poorly through capillaries, causing microthromboses and tissue hypoxia.
- The lifespan of erythrocytes is reduced—hemolytic anemia develops.
- Heterozygous carriers have increased resistance to malaria: Plasmodium falciparum multiplies less efficiently in sickle erythrocytes, which explains the high frequency of the mutation in malaria-endemic regions of Africa.
Key points
- Myoglobin is a monomer with a hyperbolic saturation curve; hemoglobin is a tetramer (2α + 2β) with a sigmoidal curve and cooperative O₂ binding.
- His F8 covalently binds heme Fe²⁺, while His E7 orientates O₂ and decreases affinity for CO.
- The hydrophobic environment of heme prevents the oxidation of Fe²⁺ to Fe³⁺ (methemoglobin).
- Bohr effect: an increase in [H⁺] and pCO₂ decreases hemoglobin affinity for O₂, facilitating its release to tissues.
- 2,3-BPG binds in the central cavity of the tetramer, stabilizes the deoxy form, and decreases O₂ affinity; HbF binds 2,3-BPG weakly, and therefore has a high oxygen affinity.
- Chaperones (including heat shock proteins) prevent aggregation and ensure correct protein folding.
- HbS arises from the Glu6→Val substitution in the β-chain; hydrophobic valine ensures the polymerization of deoxy-HbS and the sickle-shaped deformation of erythrocytes.
- Sickle cell anemia is accompanied by hemolysis, hypoxia, and vascular occlusions, but in the heterozygous state, it protects against malaria.