What is true about hemoglobin regarding carbon monoxide?

Hemoglobin binds carbon monoxide (CO) much more tightly than it binds oxygen (O2). This means that the affinity of hemoglobin for carbon monoxide is significantly higher, as indicated by a lower dissociation constant (Kd) for CO compared to O2. The Kd is a measure of the concentration at which a complex (in this case, hemoglobin and gas) is half-saturated. A lower Kd value signifies a stronger binding affinity.

Therefore, since carbon monoxide has a Kd that is lower than that of oxygen, hemoglobin will preferentially bind to CO when it is present, leading to potential toxicity as it can inhibit oxygen transport in the bloodstream. This highlights the danger of carbon monoxide exposure, as even relatively low levels can effectively outcompete oxygen, impairing the ability of hemoglobin to carry oxygen to tissues.

The other options suggest incorrect relationships between the binding affinities of hemoglobin to CO and O2, particularly the idea that CO binds less tightly or has a higher Kd than oxygen. Understanding this binding behavior is crucial in biochemistry, particularly in contexts involving respiration and poisoning by gases such as carbon monoxide.