Myoglobin prosthetic group, structure

It is quite evident that the ferrous complexes of porphyrins, both natural and synthetic, have extremely high affinities towards NO. A series of iron (II) porphyrin nitrosyls have been synthesized and their structural data [11, 27] revealed non-axial symmetry and the bent form of the Fe-N=0 moiety [112-116]. It has been found that the structure of the Fe-N-O unit in model porphyrin complexes is different from those observed in heme proteins [117]. The heme prosthetic group is chemically very similar, hence the conformational diversity was thought to arise from the steric and electronic interaction of NO with the protein residue. In order to resolve this issue femtosecond infrared polarization spectroscopy was used [118]. The results also provided evidence for the first time that a significant fraction (35%) of NO recombines with the heme-Fe(II) within the first 5 ps after the photolysis, making myoglobin an efficient N O scavenger. 

Let us return to the cases of myoglobin and hemoglobin and recognize something explicitly these two proteins contain a nonprotein constiment. This constituent, or prosthetic group, is heme, a complex polycyclic structure, protoporphyrin IX, containing an atom of iron (as Fe (II) or Fe +) at its center. 

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. 

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