Final answer:
A) Oxygen binding
The distal histidine (DH) decreases the CO affinity of hemoglobin and myoglobin to avoid CO poisoning by forcing CO to bind in a bent mode, lessening its binding affinity compared to O₂.
Step-by-step explanation:
The function of the distal histidine in hemoglobin and myoglobin is primarily to decrease the affinity of these proteins for carbon monoxide (CO), thus preventing self-poisoning. This is achieved because the imidazole group of the distal histidine forces CO to bind in a bent mode rather than in a linear fashion, which is less favorable for CO than for oxygen (O₂). If the distal histidine were not present, CO would bind with much higher affinity, potentially leading to fatal carbon-monoxide poisoning. The shortened abbreviation for distal histidine is typically 'DH'.