<P> Hemoglobin has a quaternary structure characteristic of many multi-subunit globular proteins . Most of the amino acids in hemoglobin form alpha helices, and these helices are connected by short non-helical segments . Hydrogen bonds stabilize the helical sections inside this protein, causing attractions within the molecule, which then causes each polypeptide chain to fold into a specific shape . Hemoglobin's quaternary structure comes from its four subunits in roughly a tetrahedral arrangement . </P> <P> In most vertebrates, the hemoglobin molecule is an assembly of four globular protein subunits . Each subunit is composed of a protein chain tightly associated with a non-protein prosthetic heme group . Each protein chain arranges into a set of alpha - helix structural segments connected together in a globin fold arrangement . Such a name is given because this arrangement is the same folding motif used in other heme / globin proteins such as myoglobin . This folding pattern contains a pocket that strongly binds the heme group . </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> <P> Even though carbon dioxide is carried by hemoglobin, it does not compete with oxygen for the iron - binding positions but is bound to the protein chains of the structure . </P>

Where is oxygen attached on a hemoglobin molecule