Structure Of Heme

The Beta-chain of Hb begins with valine and histidine. Both these amino acids are important in the synthesis and structure of heamoglobin.

 Alignment according to helical designation makes homology evident:

Residue F8 is the proximal heme-linked histidine, and the histidine on the distal side of the heme is E7. 

The iron atom is linked by a coordinate bond to the imidazole nitrogen (N) of histidine F8.

The E7 distal histidine, on the other side of the heme plane, is not bonded to the iron atom but is very close to the ligand-binding site.

Ref: Natarajan K., Townes T.M., Kutlar A. (2010). Chapter 48. Disorders of Hemoglobin Structure: Sickle Cell Anemia and Related Abnormalities. In J.T. Prchal, K. Kaushansky, M.A. Lichtman, T.J. Kipps, U. Seligsohn (Eds), Williams Hematology, 8e

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.

Ans. is ‘d’ i.e., All of the above

• Hemoproteins are proteins which have heme as prosthetic group.
• Important hemoproteins are hemoglobin, myoglobin, cytochromes (cytochrome C, cytochrome P₄₅₀),catalase, peroxidase, tryptophan pyrrolase and nitric oxide synthase.