Hemoglobin is widely known as the primary transporter of oxygen, carrying the gas from the lungs to every cell in the body. This complex protein also plays a role in the removal of metabolic waste. Hemoglobin transports carbon dioxide (\(\text{CO}_2\)), a byproduct of cellular energy production, back to the lungs for exhalation. This dual function is accomplished by forming a specific compound with carbon dioxide, known as carbaminohemoglobin.
Defining Carbaminohemoglobin and its Structure
Carbaminohemoglobin is the compound formed when carbon dioxide chemically bonds with the hemoglobin molecule inside red blood cells. Unlike oxygen, which binds to the iron atom within the heme group, carbon dioxide does not attach to this iron center. Instead, \(\text{CO}_2\) attaches covalently to the terminal amino (\(\text{NH}_2\)) groups found on the polypeptide chains of the globin protein. Carbon dioxide binds primarily to the \(\text{N}\)-terminal residues of these chains, forming a reversible carbamate group. This structural difference ensures that \(\text{CO}_2\) transport does not interfere with the sites responsible for oxygen transport.
The Mechanism of Formation and Release
The formation of carbaminohemoglobin is favored in the body’s tissues, which are actively consuming oxygen and producing high concentrations of carbon dioxide. This inverse relationship between oxygen and carbon dioxide binding is known as the Haldane effect. When hemoglobin releases oxygen to the tissues, its shape changes, increasing its capacity to bind carbon dioxide and hydrogen ions. The resulting carbaminohemoglobin is carried in the venous blood back toward the lungs. Upon reaching the lungs, high oxygen concentration triggers the reversal: oxygen binds to the heme groups, lowering hemoglobin’s affinity for \(\text{CO}_2\) and prompting its release for exhalation.
Physiological Importance in Carbon Dioxide Transport
Carbon dioxide is transported in the bloodstream using three primary mechanisms. The majority of \(\text{CO}_2\) (approximately 70%) is transported as bicarbonate ions, a process facilitated by the enzyme carbonic anhydrase inside the red blood cells. A small percentage (5% to 10%) is dissolved directly in the blood plasma. Carbaminohemoglobin accounts for the remaining 20% to 30% of the total carbon dioxide transported from the tissues to the lungs. Furthermore, the formation of carbaminohemoglobin helps buffer the blood, as the binding of \(\text{CO}_2\) releases a hydrogen ion that stabilizes the blood’s pH.
Not to Be Confused: Carbaminohemoglobin vs. Carboxyhemoglobin
Carbaminohemoglobin and carboxyhemoglobin are often confused due to their similar names, but they are chemically distinct compounds. Carbaminohemoglobin is the natural compound formed when carbon dioxide (\(\text{CO}_2\)) binds to the globin protein chains. Carboxyhemoglobin, by contrast, is a toxic compound formed when carbon monoxide (\(\text{CO}\)) binds to the hemoglobin molecule. Carbon monoxide is extremely hazardous because it binds to the same heme iron site as oxygen, but with an affinity approximately 240 times greater. This strong, stable binding blocks oxygen attachment, significantly reducing the blood’s oxygen-carrying capacity and leading to carbon monoxide poisoning.