Noun
any of various pigments distributed widely in living tissues
Source: WordNetAt the same time, the imidazole side-chain of the histidine residue interacting at the other pole of the iron is pulled toward the porphyrin ring. Source: Internet
Iron's shift to a higher oxidation state in Hb-O 2 decreases the atom's size, and allows it into the plane of the porphyrin ring, pulling on the coordinated histidine residue and initiating the allosteric changes seen in the globulins. Source: Internet
Heme is a good example: the iron atom is at the centre of a porphyrin macrocycle, being bound to four nitrogen atoms of the tetrapyrrole macrocycle. Source: Internet
It is thus too large to fit inside the porphyrin ring, which bends instead into a dome with the Fe 2+ ion about 55 picometers above it. Source: Internet
Since many of these proteins contain globins and the heme moiety (iron in a flat porphyrin support), they are often called hemoglobins, even if their overall tertiary structure is very different from that of vertebrate hemoglobin. Source: Internet
This results in the Fe 2+ ion switching to a low-spin configuration, resulting in a 20% decrease in ionic radius so that now it can fit into the porphyrin ring, which becomes planar. Source: Internet