How many of iron's bonds does heme coordinate?

Heme, which is an iron-containing compound crucial for various biological functions, particularly in oxygen transport, coordinates iron through its porphyrin structure. In the structure of heme, the iron atom is at the center of a planar porphyrin ring composed of four pyrrole nitrogen atoms.

Each nitrogen in the porphyrin ring forms a coordinate bond with the iron atom, thus accounting for four of iron’s bonded interactions. In addition, iron can form two additional bonds in a typical heme environment, one with a histidine residue (specifically, the imidazole nitrogen of the side chain from histidine that is part of hemoglobin or myoglobin) and the other with an oxygen molecule when it is in the oxy form.

Therefore, while iron coordinates with four bonds directly from the porphyrin structure, its total coordination in the biological context can reach six, taking into account the bonds formed with histidine and oxygen. However, in the context of the heme structure alone, the four bonds formed with the nitrogen atoms are the primary focus.

Hence, the answer accurately reflects the number of bonds directly coordinated by iron in the heme structure itself.