In most humans, the haemoglobin molecule is an assembly of four globular protein subunits. Each subunit is composed of a protein chain tightly associated with a non-protein heme group. Each protein chain arranges into a set of alpha-helix structural segments connected together in a globin fold arrangement, so called because this arrangement is the same folding motif used in other heme/globin proteins such as myoglobin.[10][11] This folding pattern contains a pocket which strongly binds the heme group.
A heme group consists of an iron (Fe) ion (charged atom) held in a heterocyclic ring, known as a porphyrin. The iron ion, which is the site of oxygen binding, coordinates with the four nitrogens in the center of the ring, which all lie in one plane. The iron is also bound strongly to the globular protein via the imidazole ring of the F8 histidine residue below the porphyrin ring. A sixth position can reversibly bind oxygen, completing the octahedral group of six ligands. Oxygen binds in an "end-on bent" geometry where one oxygen atom binds Fe and the other protrudes at an angle. When oxygen is not bound, a very weakly bonded water molecule fills the site, forming a distorted octahedron.
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2016-10-16 11:05:44 UTC
Hemoglobin is a tetramer. It has 2 beta chains and a pair of alpha chains. to each chain is expounded a protoporphyrin ring containing an iron atom in ferrous form. Protoporphyrins plus iron is suggested as heme. Hemoglobin interior the cellular is greater effectual in choosing up and giving off oxygen simply by cellular environment.
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