Hemoglobin is a complex protein within red blood cells, primarily managing the transport of oxygen throughout the body. Carbon monoxide (CO), a colorless and odorless gas produced by the incomplete combustion of carbon-containing fuels, interferes with this transport system. CO’s toxicity lies in its ability to hijack the oxygen-carrying sites on hemoglobin, preventing oxygen delivery to tissues and organs. Understanding this molecular interaction reveals why even small concentrations of the gas can rapidly become lethal.
Hemoglobin’s Normal Function in Oxygen Transport
The hemoglobin molecule is a globular protein built from four polypeptide chains, known as a tetramer. Each chain contains a specialized pocket called a heme group, centered around a single iron atom. This iron atom is the site where oxygen molecules bind during respiration.
Oxygen binds to hemoglobin in the lungs, forming a temporary, reversible bond that allows the molecule to pick up four oxygen molecules. This reversibility is necessary because hemoglobin must release oxygen when it reaches tissues that are actively consuming it.
The Molecular Mechanism of Carbon Monoxide Binding
Carbon monoxide (CO) is a competitive inhibitor, vying directly with oxygen for the same binding sites on the hemoglobin molecule. When inhaled, CO enters the bloodstream and binds to the iron atom within the heme group, displacing any oxygen present. The resulting compound is called carboxyhemoglobin (COHb).
CO binding is chemically very stable; the molecule does not readily detach, effectively taking that heme site out of commission for oxygen transport. This binding also triggers a significant conformational change in the hemoglobin molecule’s remaining subunits.
This change forces the unoccupied heme sites to hold onto their oxygen molecules much more tightly. This phenomenon, known as a left-shift in the oxygen-hemoglobin dissociation curve, makes it difficult for the oxygen still carried by the blood to be released into the body’s tissues.
Affinity Comparison and Systemic Consequences
CO’s superior chemical attraction to hemoglobin compared to oxygen makes it dangerous. CO has an affinity for the heme iron that is approximately 200 to 250 times greater than that of oxygen. Even a small concentration of carbon monoxide in the air can quickly lead to a high percentage of carboxyhemoglobin in the blood.
Since CO molecules rapidly displace oxygen, the blood’s capacity to transport oxygen is reduced, leading to functional anemia. This tissue oxygen deprivation, or hypoxia, affects the brain and heart first due to their high metabolic demand. Initial symptoms like headache, dizziness, and nausea result from the brain being starved of oxygen.
Continued exposure leads to severe consequences, including confusion, loss of consciousness, and permanent organ damage. Carboxyhemoglobin can also interfere with cellular respiration independent of hemoglobin, causing direct toxicity at the cellular level.
Treatment Strategies for Carbon Monoxide Poisoning
Treatment for carbon monoxide poisoning involves competitively reversing the strong bond between CO and hemoglobin. The primary strategy is administering 100% pure oxygen through a non-rebreather mask. This high concentration creates a steep gradient, flooding the blood with oxygen molecules to outcompete and displace the bound carbon monoxide.
Breathing 100% oxygen at normal atmospheric pressure reduces the half-life of carboxyhemoglobin from several hours to approximately 60 to 90 minutes. For severe cases, hyperbaric oxygen therapy (HBOT) is employed. This treatment places the patient in a chamber where 100% oxygen is delivered under pressures two to three times greater than normal.
The increased pressure and oxygen concentration accelerate the displacement of CO, reducing the half-life of carboxyhemoglobin to as little as 15 to 30 minutes. HBOT also increases dissolved oxygen in the blood plasma, which bypasses the compromised hemoglobin and provides temporary delivery to oxygen-starved tissues, particularly the brain.