Haem

Try to identify in the model the substituents of the porphyrin ring system.

Fe²⁺ in haem can form 6 coordination bonds:

of them, located in the haem plane, are set with the nitrogens in the pyrrolic rings of protoporphyrin IX.

The fifth coordination bond, perpendicular to the haem plane, is set between the iron ion and an oxygen molecule (forming oxyhaem, a component of oxyhaemoglobin). This happens in lungs, where oxygen is abundant, and later this is released in tissues that need it for their metabolism (haemoglobin hence becoming deoxyhaemoglobin).

The coordination bond of iron, also perpendicular to the haem plane, is set with the nitrogen of the sidechain in a histidine, called proximal histidine and located in helix F.

On the same face of haem where oxygen is bound, a sidechain approaches from another histidine residue, called distal histidine and which is part of helix E; this histidine does not bind Fe²⁺, since it is farther apart, but sterically hinders the binding of CO in place of oxygen.
another good point of view.
Affinity of isolated haem for CO is 25 000 times that of oxygen, while for haem in haemoglobin it is just 200 times higher.

Show the volume of the haem group (dotted spheres).