Functions of erythrocyte cytoplasmic proteins
In addition to hemoglobin itself, which transports $O_2$ from the pulmonary alveoli to peripheral tissues and $CO_2$ in the opposite direction, the enzyme carbonic anhydrase plays a crucial role in the cytoplasm.
Carbonic anhydrase is responsible for catalyzing the reversible hydration reaction of carbon dioxide ($CO_2 \leftrightarrow HCO_3^-$), which allows the transport of significant amounts of carbonic acid in the form of soluble bicarbonate ions.
The mechanism of this process strictly depends on the localization of the erythrocyte:
- In tissue capillaries, where a high concentration of $CO_2$ is recorded, the enzyme directs the reaction towards the synthesis of $HCO_3^-$ ions. These ions leave the erythrocyte cytoplasm and enter the blood plasma.
- In pulmonary capillaries, where the level of $CO_2$ is minimal, $HCO_3^-$ ions return inside the erythrocyte. Carbonic anhydrase transforms them back into gaseous $CO_2$, which easily diffuses into the alveoli for elimination from the body.
Chemical structure of the molecule
Hemoglobin belongs to the class of chromoproteins — complex proteins consisting of a protein part (globin) and a non-protein part (heme).
Protein part (Globin) Globin has a tetrameric shape, meaning it consists of four subunits. In the classic version, these are two $\alpha$-chains and two $\beta$-chains. The spatial configuration of this structure is reliably stabilized due to intermolecular interaction of amino acid residues of different subunits.
Non-protein part (Heme) Exactly one heme is attached to each of the four protein subunits. It is a complex polycyclic structure (porphyrin) with a ferrous iron atom ($Fe^{2+}$) located right in its center. The iron atom has six coordination bonds distributed as follows:
- Four bonds firmly fix the iron itself inside the porphyrin ring.
- The fifth bond serves as a kind of "bridge", connecting the heme to the polypeptide chain of globin.
- The sixth bond is functionally active. It is this bond that reversibly binds oxygen or carbon dioxide molecules.
Heme metabolism and pigment formation
The characteristic color of blood directly depends on the presence of $Fe^{2+}$ ions in the porphyrin ring. At the level of an individual erythrocyte, this pigment gives a yellowish tint, but in the total blood mass, it forms a rich red color.
When the lifespan of an erythrocyte comes to an end, it breaks down. This process is localized predominantly in the spleen and triggers a cascade of component utilization:
- Iron ($Fe$) is carefully cleaved off and sent for reutilization so the body can use it to synthesize new molecules.
- The heme residue (already stripped of iron) is transformed into specific bile pigments: biliverdin and bilirubin.
Subsequently, the pigments are excreted with bile into the intestinal lumen, where their fate splits into two elimination pathways:
- Intestinal pathway (via feces): pigments are modified into stercobilin and leave the body.
- Renal pathway (via urine): part of the substances is reabsorbed back into the bloodstream, where it is converted into urobilin and then filtered by the kidneys.
Age-related heterogeneity
During human ontogenesis, various types of hemoglobin are synthesized, differing in their amino acid composition. This difference is of critical physiological significance as it directly affects the protein's affinity for oxygen.
The following main types are distinguished:
- Embryonic hemoglobin: the earliest forms functioning at the initial stages of development.
- Hb F (fetal): predominates in fetal blood. Its key feature is increased oxygen affinity compared to adult hemoglobin. Thanks to this difference, $O_2$ easily diffuses from maternal hemoglobin (Hb A) to fetal hemoglobin, ensuring adequate tissue respiration of the developing fetus.
- Hb A and Hb A₂ (adult): adult hemoglobin.
In a healthy adult, the composition of hemoglobin fractions is strictly balanced: Hb A is the absolute dominant (accounting for about 96%). Minor fractions — Hb A₂ and residual Hb F — account for approximately 2% each.
Briefly: answers to topical questions
What are the physiological compounds of hemoglobin with oxygen and carbon dioxide called?
The physiological compounds of hemoglobin are oxyhemoglobin and carbaminohemoglobin.
- Oxyhemoglobin is a compound with oxygen ($HbO_2$).
- Carbaminohemoglobin is a compound with carbon dioxide ($HbCO_2$). About 20% of carbon dioxide is transported from the body in this form.
What pathological compounds of hemoglobin exist and under what conditions do they form?
Pathological compounds of hemoglobin include carboxyhemoglobin and methemoglobin.
- Carboxyhemoglobin ($HbCO$) is formed by the reaction of hemoglobin with carbon monoxide (CO).
- Methemoglobin (MetHb) is formed when heme iron is oxidized to ferric ($Fe^{3+}$) under the action of strong oxidizing agents (nitrates, inorganic nitrites, sulfonamides, local anesthetics).
What is the normal concentration of hemoglobin in the blood of men and women?
The normal hemoglobin content depends on sex:
- In men, the norm is 130–160 g/L.
- In women, the norm is 120–150 g/L.
To avoid confusing the end products of heme breakdown: Stercobilin is excreted in Stool (feces), while Urobilin is excreted in Urine (urine).
Class and exam questions
Which chemical bond in hemoglobin is responsible for gas exchange?
The reversible binding of oxygen or carbon dioxide is mediated by the sixth coordination bond of the divalent iron atom ($Fe^{2+}$).
Why must fetal hemoglobin differ from maternal hemoglobin?
Fetal hemoglobin (Hb F) has a different amino acid composition, which provides it with a higher oxygen affinity. This is physiologically necessary for the successful diffusion of $O_2$ from maternal blood into fetal blood.
In what form is the bulk of carbon dioxide transported?
In the form of bicarbonate ions ($HCO_3^-$), which are formed in erythrocytes under the action of the enzyme carbonic anhydrase and then diffuse into the plasma.
What else to study on this topic
- Mechanisms of stabilization of the globin tetrameric structure
- Specifics of $CO_2$ interaction with amino acid residues
- The process of iron cleavage by splenic macrophages
- Dynamics of embryonic hemoglobin synthesis
- Transport pathways of bicarbonate ions across the membrane
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