<P> A heme group consists of an iron ( Fe ) ion ( charged atom ) held in a heterocyclic ring , known as a porphyrin . This porphyrin ring consists of four pyrrole molecules cyclically linked together ( by methine bridges ) with the iron ion bound in the center . The iron ion , which is the site of oxygen binding , coordinates with the four nitrogen atoms in the center of the ring , which all lie in one plane . The iron is bound strongly ( covalently ) to the globular protein via the N atoms of the imidazole ring of F8 histidine residue ( also known as the proximal histidine ) below the porphyrin ring . A sixth position can reversibly bind oxygen by a coordinate covalent bond , completing the octahedral group of six ligands . Oxygen binds in an `` end - on bent '' geometry where one oxygen atom binds to 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 . </P>
<P> A heme group consists of an iron ( Fe ) ion ( charged atom ) held in a heterocyclic ring , known as a porphyrin . This porphyrin ring consists of four pyrrole molecules cyclically linked together ( by methine bridges ) with the iron ion bound in the center . The iron ion , which is the site of oxygen binding , coordinates with the four nitrogen atoms in the center of the ring , which all lie in one plane . The iron is bound strongly ( covalently ) to the globular protein via the N atoms of the imidazole ring of F8 histidine residue ( also known as the proximal histidine ) below the porphyrin ring . A sixth position can reversibly bind oxygen by a coordinate covalent bond , completing the octahedral group of six ligands . Oxygen binds in an `` end - on bent '' geometry where one oxygen atom binds to 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 . </P>
where is oxygen attached on a hemoglobin molecule
where is oxygen attached on a hemoglobin molecule
The iron ion
The iron ion