In hemoglobin, affinity for carbon monoxide is diminished by presence of ?

Heme is a derivative of porphyrin (protoporphyrin) porphyrin is composed by fusion of four pyrrole ring linked by methenyl (=CH) bridges, i.e. tetrapyrrole ring. Since an atom of iron is present, heme is a ferroprotoporphyrin. Four methyl, two vinyl and two propionate side chain groups are attached to the porphyrin ring. The iron is held in the center of porphyrin ring in ferrous form (Fe+2). Iron has six coordinated bonds: (i) four bonds are formed between the iron and nitrogen atoms of the porphyrin ring system; (ii) fifth bond is formed between nitrogen atoms of histidine residue of globin chain, known as proximal histidine (His F8); (iii) sixth bond is formed with oxygen. The oxygenated form of hemoglobin is stabilized by the hydrogen bond between oxygen and side chain of another histidine residue of globin chain, known as distal histidine (His E7).