The coordination number of the iron ion Fe in the heme group of the myoglobin protein is 6. The geometry of the iron ion in the heme group is octahedral. In this arrangement, the iron ion is coordinated to four nitrogen atoms from the porphyrin ring in a planar configuration, one nitrogen atom from the imidazole side chain of the histidine residue, and one oxygen atom from the bound oxygen molecule O2 .