Describe the active (binding) sites of Hb, Hc and Hr.

Expert Solution

Hemoglobin, hemocyanin and hemoerythrin all three serve as oxygen transporter in various animal systems.

While hemoglobin is main oxygen binding molecule in higher vertebrates. Hemocyanin transports oxygen in invertebrates like Arthropods and molluscs. whereas hemoerythrin transports oxygen in several marine invertebrates.

The active site is different in all three.

Hemoglobin (Hb). Hemoglobin is a tetramer. It contains 4 subunit ( 2 alpha and 2 beta in case of human adult). It is found inside red blood cells in most vertebrates.

So,active site contains 4 heme groups as prosthetic group in each monomer. A heme is iron bound with porphyrin (heterocyclic). Porphyrin contains 4 pyrrole rings where i ion remain complexed in the centre of pyrrole rings by binding with N atoms. Iron binds covalently to N of proximal histidine residue (F8) covalently hence strongly. The 6 th position can attach oxygen by forming a covalent coordinate bond. The binding of oxygen shows cooperativity. It means that binding of oxygen to one iron makes oxygen binding at other 3 binding sites easier.

Hemocyanin (Hc)- In case of Hc the molecule which binds to oxygen is Cu. Cu remains as prosthetic group in the active site and forms coordination complex with histidine residues.

The active site therefore is composed of 2 copper (I) cations which form coordination complex with 6 histidine residue's imidazole ring.

Unlike hemoglobin the hemocyanin is not found in any blood cell but remains a part of hemolymph. Oxygenation converts Cu (I) colorless form to Cu (II) blue color form. It also does not show cooperative binding mostly as happens in case of hemoglobin

Hemoerythrin. (Hr)- it is an oligomer protein. Active site contains 2 iron centre. iron centres form coordination complex with 5 histidine residues as well as carboxyl side chain of glutamic amino and aspartic amino acid.

Oxygen transport by hemoerythrin is most unusual in terms of formation of hydroperoxide intermediate while in hemoglobin oxygen forms di oxygen complex.

gladiator answered 1 year ago

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