Heme consists of tetrapyrole ring structure (protophorphorin IX) with a Fe2+ ion bound in the center, coordinated to the
nitrogens of the pyrole rings.
The first structure that appears on your screen shows
dioxygen bound to the iron in the heme. Notice the Fe2+ is
in the heme plane.
The proximal His (F8) and
distal His (E7) from myoglobin or hemoglobin are
also shown. F8 provides the 5th coordinating ligand to the iron ion.
looking along heme plane
The geometry around the iron ion
O2-heme drawing facing heme plane
Click on the second structure which shows
deoxyheme. The iron ion is out of
the heme plane, having square planar geometry.
notice the square
pyramidal geometry around the iron ion.