In hemoglobin, the innate affinity of heme for carbon monoxide is diminished by the presence of :

The heme has high affinity for carbon monoxide (about 25000 more than for 02). But when heme molecule is associated with Histidine E 7 (as occurs in myoglobin), its affinity for CO is drastically reduced because His E 7 creates a hindered environment for CO by disturbing the orientation of atoms in heme molecules.