Nature of haemoglobin and myoglobin
Both are having the same structure excepting the fact that myoglobin is a monomer and haemoglobin is a tetramer. Both are iron-porphyrin complex.
These are biocoordination complexes formed between porphyrin and iron in its +2 oxidation state (Fe2+). The iron-porphyrin complex is called the heme group, which is a part of haemoglobin. Each haemoglobin molecule consists of four subunits, each unit is being a folded chain.
The working part of haemoglobin is a hemegroup containing an Fe2+ cation coordinated to four nitrogen atoms of porphyrin group and one nitrogen atom of histidine group. The sixth octahedral site is available to bind oxygen molecule.