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

Ans. is 'b' i.e., His E-7 The heme has high affinity for carbon monoxide (about 25000 more than for oxygen)But when heme molecule is associated with Histidine E7 [( as occurs in myoglobin), its affinity for CO is drastically reduced, because the presence of His E7 creates a hindered environment for CO by disturbing the orientation of atoms in the heme moleculeSee what Harper writes- "Carbon monoxide (CO) binds to isolated heme about 25000times more strongly than does oxygen. The atmosphere contains traces of CO, and normal catabolism of heme itself forms small quantities of CO. Why then does not CO (rather than 02) occupy the sixth coordination position of the heme iron of myoglobin? The answer lies in the hindered environment of heme in myoglobin. The preferred orientation for CO to heme iron is with all three atoms (Fe, C, and O) perpendicular to heme ring.* While this orientation is possible for isolated heme, in myoglobin the distal histidine (His-E7) sterically hinders binding of CO at this angle. *Angles for bonding of oxygen and carbon monoxide to the heme iron of myoglobin. The distal E7 histidine hinders bonding of CO at the preferred (180-degree) angle to the plane of the heme ring. This forces CO to bind in a less favoured configuration and reduces the strength of the heme - CO bond over two orders of magnitude"