Question

Why are \(\mathrm{CN}^{-}\) and \(\mathrm{CO}\) toxic to humans?

Short answer

Both cyanide ions (CN-) and carbon monoxide (CO) are toxic to humans because they interfere with the body's ability to utilize oxygen. CN- inhibits the final stage of the respiratory chain by binding to the iron in the heme a3 group within cytochrome c oxidase, preventing the enzyme from functioning properly, and ultimately hindering ATP production. CO is toxic to humans mainly because it binds to hemoglobin more effectively than oxygen, reducing oxygen delivery to tissues and leading to hypoxia, which can cause organ and tissue damage. Both mechanisms result in oxygen deprivation and can be lethal if not treated promptly.

Step-by-step solution

1. Overview of the Toxic Effects

Both CN- ions and CO exhibit their toxic effects by interfering with cellular respiration, a crucial process for energy production in living organisms. Cellular respiration depends on the proper functioning of the respiratory chain, a series of complexes located in the inner mitochondrial membrane which perform the transfer of electrons from the high-energy molecules (like glucose) to oxygen molecules. As a result, the energy produced is harnessed to generate adenosine triphosphate (ATP), the cell's energy currency.

2. Cyanide Ion's Mechanism of Toxicity

Cyanide ions (CN-) exert their toxic effect by inhibiting the final stage of the respiratory chain, specifically the complex IV, also known as cytochrome c oxidase. This enzyme is responsible for transferring electrons from cytochrome c to oxygen molecules, forming water. CN- has a high affinity for the iron present in the heme a3 group within cytochrome c oxidase, thus preventing the enzyme from functioning properly. As a result, the electron transport chain is halted, which leads to a negative cascade effect that inhibits ATP production and therefore, cellular respiration. The lack of ATP in cells leads to cell damage and eventually cell death, which is the main cause of cyanide toxicity.

3. Carbon Monoxide's Mechanism of Toxicity

Carbon monoxide (CO) is toxic to humans mainly because it binds to hemoglobin, the protein responsible for transporting oxygen in the blood, much better than oxygen does. Hemoglobin contains iron in its heme group, which has a higher affinity for carbon monoxide than for oxygen. When CO binds to hemoglobin, it forms carboxyhemoglobin, which renders the hemoglobin unable to transport oxygen. Oxygen delivery to tissues is reduced, leading to hypoxia, or oxygen deprivation. This can cause damage to organs and tissues, particularly in the heart and brain, and ultimately lead to death.

4. Summary of Toxic Effects on Humans

In summary, both CN- and CO are toxic to humans because they interfere with the body's ability to utilize oxygen. Cyanide ions disrupt cellular respiration by inhibiting the functioning of complex IV in the respiratory chain, thus hindering ATP production. On the other hand, carbon monoxide prevents the proper oxygen transportation within the bloodstream by binding to hemoglobin more effectively than oxygen. Both mechanisms lead to oxygen deprivation and potential tissue and organ damage, which can be lethal if not promptly treated.