Question

List the changes that occur to the iron ion, the distal and proximal histidines, and to the α- helices that the histidines are in when oxygen binds to the iron in the heme prostethic group

Answer 1

Heme is a porphyrin molecule that is coordinated with Fe(II) ion. The iron in the heme is bonded to the four nitrogen atoms of the porphyrin ring and also bonded to a nitrogen atom from the histidine residue. When it is not bound to oxygen, the heme group is non planar. The Fe becomes slightly smaller when it is binded to oxygen and this shrinking in size allow it to move into the plane of the porphyrin ring and thus the porphyrin changes to a planar configuration. The shrinking in the size of Fe atom and the planar allignment with the porphyrin ring pull up the proximal histidine bound to the Fe atom and thus cause a structural change in the alpha helix attached to the histidine and this develops a greater affinity towards oxygen. The distal histidine also binds to the oxygen and thus stops the release of reactive oxygen.