The oxygen carrier haemoglobin (hemoglobin, Hb), when oxygenated forms oxyhaemoglobin (oxyhemoglobin, HbO₂),

Hb  +  O₂    Hb-Fe-O₂

However, there are four heme groups to which an oxygen molecule can be attached (see diagram below), so at any given time the oxygenated hemoglobin molecule may be carrying between one and four O₂ molecules.

This explains why iron is an essential mineral nutrient in our diet.

Oxyhemoglobin gives strong absorption in many regions of the visible light spectrum except in the visible red region, so it appears as red molecule - it is effectively a transition metal complex of iron(II), though ligand system is a bit more complicated than usual.

Iron must be in the iron(II) state as square planar bonding arrangement in the complex involving the Fe²⁺ ion.

Hb-Fe^II  +  O₂    Hb-Fe^II-O₂

Deoxyhaemoglobin (deoxyhemoglobin) is a slightly pale crimson colour with a greater absorbance in the red visible light region, BUT NOT blue as indicated for convenience in many biology textbooks and my website biology pages too!

 

The crucial part of the haemoglobin molecule and where it fits into the whole structure

In haemoglobin, the two hydrogen atoms of a porphyrin ring (shown in the left) are replaced by the Fe²⁺ ion.

The porphyrin ring as 26 pi electrons and forms a large conjugated system with the Fe²⁺ ion, a complex in which the energies required for electron excitation include the energies of visible light photons e.g. in the blue to yellow regions.

The co-ordination number of the iron complex is 6 with an octahedral arrangement of 6 covalent bonds.

There are four covalent bonds from the four nitrogen atoms of the porphyrin ring in a square planar arrangement.

Below the plane, a protein molecule is coordinated with the Fe²⁺ ion and below the plane either a water molecule or an oxygen molecule is coordinated to the Fe²⁺ ion to complete the octahedral arrangement.

The haemoglobin molecule is effectively acting as a polydentate ligand in forming the iron(II) complex.

See transition metals Appendix 2. for an Introduction to complexes & ligands

The porphyrin ring - iron(II) association amounts a typical transition metal complex and the oxygen molecule is effectively a fifth ligand.

Apart from the four heme groups capable of carrying an oxygen molecule, the rest of the molecular structure, for each heme group, includes polypeptide sequences of 141 alpha amino acids and 146 beta amino acids. Its a big molecule!

The whole molecule is an association of two alpha and two beta subunits, shown in red and blue on the above diagram.

The four heme groups are shown in green on the same diagram of haemoglobin.

The haemoglobin molecule image was adapted from https://en.wikipedia.org/wiki/Hemoglobin

 

Hemoglobin and medical conditions

(i) Unfortunately, the iron(II) ion can bind with other molecules including deadly carbon monoxide.

One of carbon monoxide's several mechanisms of toxicity is binding with circulating hemoglobin to form carboxyhemoglobin, resulting in a functional anemia

Hb-Fe^II  +  CO    Hb-Fe^II-CO

Unfortunately, carbon monoxide binds to haemoglobin 240 x more strongly than oxygen, so the more CO you breathe in, the less oxygen you have for respiration, hence its toxic nature.

So it can actually displace oxygen from carboxyhaemoglobin.

Hb-Fe^II -O₂  +  CO    Hb-Fe^II-CO  + O₂

Carbon monoxide poisoning can be reversed by treating the patient with oxygen to move the equilibrium to the left - the concentration rule of Le Chatelier's equilibrium principles.

(ii) Sickle cell anaemia

Hemoglobin (HbS) has existed in humans for thousands of years.

HbS arise from a mutation in the gene controlling the building of the beta-globin chain in the haemoglobin molecule.

It causes the formation of insoluble polymers that severe damage to the red blood cell membrane causing a considerable deficiency in transportation of oxygen around the body.

Damage to the red blood cell membrane gives rise to reduced cell survival and chronic hemolytic anemia (haemolytic anemia).