Question

As hemoglobin is formed in the body, how does the heme coenzyme associate with this protein?

A. Hydrogen Bonds

B. They are covalently attached to the protein

C. They are a modified "R" group of a non-standard amino acid

D. Hydrophobic force

E. Coordination of the iron atom with the distal histidine residue

Answer 1

Ans. Correct options- A, C, D

Option A. Correct: The O- and H-atoms of –COOH group in ring C and D of the heme group may form H-bonds with adjacent residues.

Option. B. Incorrect. The heme group is NOT linked to protein with any covalent bond.

Option. C. Incorrect. Heme group is a substituted tetrapyrrole, but NOT modified R group of any amino acid residue.

Option. D. Correct. Hydrophobic force: The bulk of heme is hydrophobic with which it exerts hydrophobic interactions with adjacent hydrophobic regions in the groove of protein. Thus interaction also is crucial for positioning the heme group in protein’s hydrophobic pocket.

Option E. Correct. Coordination of the iron atom with distal histidine residues.

Heme B (heme present in Hb and Myb) prosthetic group consists of four pyrrole groups (tetrapyrrole) connected by acetylene group between adjacent rings. A Fe²⁺ is held at the center of tetrapyrrole with five coordination bonds- four coordination bonds (one between the Fe²⁺ ion and N-atom of each rings) with the ring, and one below the plane between Fe²⁺ ion and an imidazole N-atom of histidine residue of the subunit.

Note: All the three types of interactions (H-bond, coordination bond, and hydrophobic interactions) are crucial for correctly positioning the heme prosthetic group in hemoglobin monomer. Any single interaction would NOT be sufficient enough to the stable interaction between heme and protein.