B4 Myoglobin and hemoglobin

Myoglobin was the first protein to have its three-dimensional structure solved by X-ray crystallography. It is a globular protein made up of a single polypeptide chain of 153 amino acid residues that is folded into eight a-helices. The heme prosthetic group is located within a hydrophobic cleft of the folded polypeptide chain. 

Hemoglobin has quaternary structure as it is made up of four polypeptide chains two a-chains and two (3-chains (a2 32), each with a heme prosthetic group. Despite little similarity in their primary sequences, the individual polypeptides of hemoglobin have a three-dimensional structure almost identical to the polypeptide chain of myoglobin. 

Oxyhemoglobin has a different quaternary structure from deoxyhemoglobin. As 02 binds to the Fe2+ it distorts the heme group and moves the proximal histidine. This in turn moves helix F and alters the interactions between the four subunits. 

C02 and 2,3-bisphosphoglycerate are allosteric effectors, promoting the release of 02 from hemoglobin. H+ and C02 bind to different parts of the polypeptide chains, while 2,3-bisphosphoglycerate binds in the central cavity between the four subunits. 

Hemoglobin F (HbF) which consists of two a-chains and two y-chains (a2y2) is present in the fetus. HbF binds 2,3-bisphosphoglycerate less strongly than adult hemoglobin (HbA) and thus has a higher affinity for 02 which promotes the transfer of Oz from the maternal to the fetal circulation. 

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