Question

Explain why the heme group is mostly flat, particularly the large ring of a hemoglobin molecule.

Answer 1

Heme group is consist of active metal site Iron atom with a porphyrin ring. Porphyrin ring is a conjugated molecule of 11pi bonds (conjugated), so all carbons atoms in ring are sp2 hybridized i.e. planar & metal centre Fe(II) is in high or low spin state depending upon oxygen is attached to it or not. Fe is coordinated by planar porphyrin ring by 4 Nitrogen atoms and in 5th site when oxygen is attached to it, it is in plane due to low spin Fe and is in small size in compare to high spin Fe that is present when oxygen is not attached. So it is slighly deviate from the planarity.

So overall heme group is planar due to planar porphyrin ring and suitable size metal ion present in the gap of porphyrin ring.