Molecular structure and types of hemoglobin
The hemoglobin molecule has a tetrameric structure, meaning it consists of four polypeptide subunits. It always contains two $\alpha$-chains, whereas the second pair of chains varies depending on the stage of development of the organism:
- Embryonic hemoglobin ($2\alpha, 2\epsilon$) — synthesized at early stages of intrauterine development.
- Fetal hemoglobin, Hb F ($2\alpha, 2\gamma$) — replaces embryonic hemoglobin, reaching its peak by the 6th month of fetal development.
- Adult hemoglobin, Hb A ($2\alpha, 2\beta$) — the main hemoglobin after birth, accounting for about 98%.
It is the second pair of chains that determines the specific functional and gas-binding characteristics of a given hemoglobin variant.
Heme structure and hydrophobic pocket
Heme is a non-protein prosthetic group. Its chemical basis is a tetrapyrrole protoporphyrin ring. An iron atom ($Fe^{2+}$) bound to four nitrogen atoms of the pyrrole rings is located at the center of the ring. Substituents are attached to the sides of the ring: 4 methyl groups, 2 vinyl groups, and 2 propionic acid residues.
The heme is embedded in a special hydrophobic pocket between $\alpha$-helices F and E. Two histidine residues play a key role:
- Proximal histidine (His F8): forms a coordination bond with $Fe^{2+}$ and firmly anchors the heme within the protein.
- Distal histidine (His E7): is located on the oxygen-binding side. It ensures the correct orientation for the $O_2$ molecule and hinders the binding of carbon monoxide (CO).
Protective function: The hydrophobic environment of the pocket prevents water from entering. If water reached the iron, it would be oxidized to $Fe^{3+}$ (forming methemoglobin), making oxygen transport impossible.
Cooperative effect
Hemoglobin binds oxygen not simply by summing the efforts of the subunits, but cooperatively. In deoxyhemoglobin, the iron atom protrudes slightly from the plane of the porphyrin ring toward His F8. When oxygen binds to $Fe^{2+}$ (in the region of His E7), the iron atom is pulled directly into the plane of the heme.
As it shifts, the iron pulls the attached histidine F8 and the entire protein helix along with it. This triggers a chain reaction of conformational changes throughout the molecule. As a result of this shift, the fourth $O_2$ molecule binds to the tetramer 300 times more easily than the first. In tissues, the process works in reverse: the dissociation of one oxygen molecule facilitates the release of the remaining ones.
Bohr effect (influence of CO2 and H+)
In actively working tissues (e.g., muscles), high amounts of carbon dioxide and water are produced during catabolism. $CO_2$ diffuses into erythrocytes, where the enzyme carbonic anhydrase converts it into carbonic acid. This acid dissociates into bicarbonate ($HCO_3^-$) and protons ($H^+$).
Protons bind to histidine residues (His146) in the hemoglobin chains. This allosteric interaction decreases hemoglobin's affinity for oxygen. The bond between $Fe^{2+}$ and $O_2$ breaks, and oxygen is released into the tissues. The more intense the metabolism, the more $H^+$ is produced and the more oxygen the protein releases. In the lungs, the situation is reversed: a high $O_2$ partial pressure forces the protein to bind oxygen, which displaces the protons. Protons combine with bicarbonate to form $CO_2$, which we exhale.
Allosteric regulation by 2,3-BPG
2,3-Bisphosphoglycerate (2,3-BPG) is a ligand with a strong negative charge (-5) formed in erythrocytes from an intermediate of glycolysis.
In deoxyhemoglobin, additional ionic bonds form between protomers, causing the central cavity to expand. It is precisely here that 2,3-BPG integrates by interacting with positively charged amino acids of the $\beta$-chains (valine, lysine, and histidine). Its binding alters the protein conformation and decreases its affinity for oxygen, prompting it to actively release $O_2$ to cells. In the lungs, oxygen binding narrows the central cavity, and 2,3-BPG is simply displaced from the molecule.
Briefly: answers to topical questions
What are the normal blood hemoglobin concentration ranges for men and women?
The provided sources indicate the following normal/reference values for blood hemoglobin concentration:
- In men — 130–160 g/L, 132–164 g/L, or 155.0 ± 25.0 g/L.
- In women — 120–150 g/L, 120–140 g/L, 115–145 g/L, or 140 ± 25.0 g/L.
A decreased hemoglobin concentration is the primary laboratory sign of anemia of any etiology.
What is glycated hemoglobin (HbA1c) and what is its clinical significance?
Glycated hemoglobin (HbA1c) is an indicator reflecting the average glucose/glycemia level over the lifespan of an erythrocyte, which is approximately 120 days; sources also mention the estimation of average glycemia over 3 months.
Clinical significance of HbA1c:
- Diagnosis — used to diagnose diabetes mellitus and prediabetes provided the method is certified and standardized against reference values; an HbA1c level ≥ 6.5% is specified for diabetes diagnosis.
- Monitoring — in patients with type 2 diabetes mellitus, it is measured once every 3 months to assess the achievement of glycemic control targets.
- Risk stratification — used to stratify the risk of developing complications of diabetes mellitus.
What types of abnormal hemoglobins are formed in carbon monoxide and nitrate poisoning?
Upon exposure to carbon monoxide and nitrates, sources indicate the following forms of hemoglobin:
- Carboxyhemoglobin (COHb, HbCO) — an abnormal form of Hb resulting from interaction with carbon monoxide (CO); formed during carbon monoxide poisoning. Formula in the source: HbCO (Fe²⁺ + CO). Impairs $O_2$ transport.
- Methemoglobin (MetHb) — a form of hemoglobin containing $Fe^{3+}$, formed under the action of oxidizing agents. Methemoglobinemia arises due to excess nitrates; nitrates and nitrites can cause marked cyanosis due to impaired oxygen transport.
How to distinguish histidines without confusion: Proximal (F8) — Firmly Fixes (P-F, F-F) the heme. Distal (E7) — Keeps Distance (D-D) and prevents carbon monoxide binding.
Class and exam questions
Why does free heme transport oxygen less effectively than when it is part of hemoglobin?
Free heme has an affinity for carbon monoxide (CO) that is 25,000 times higher than for $O_2$. In hemoglobin, the distal histidine (His E7) creates steric hindrance, so the affinity for CO exceeds the affinity for $O_2$ by only 200 times, protecting us from endogenous carbon monoxide suffocation.
How does hemoglobin functionally differ from myoglobin?
Myoglobin is a monomer with a very high affinity for $O_2$; its function is to store oxygen in muscles. Hemoglobin is an oligomer whose structure allows it to rapidly become saturated with $O_2$ in the lungs and release it in tissues at a pressure of 20–40 mm Hg.
What happens if water enters the active center of hemoglobin?
Water will cause the oxidation of the iron atom from the $Fe^{2+}$ to the $Fe^{3+}$ state. Methemoglobin is formed, which is completely devoid of the ability to bind and transport oxygen.
What else to study on this topic
- Detailed chemical synthesis of 2,3-bisphosphoglycerate
- Spatial folding of globin $\alpha$- and $\beta$-chains
- Mechanism of the carbonic anhydrase reaction in erythrocytes
- Differences in oxygen binding between Hb A and Hb F
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