Question

1. a. What are the important amino acids around heme in myoglobin/hemoglobin.

b. Why is Fe a +2 charge in the protein environment but able to oxidize to fe 3+ in a non-protein environment? Why is this pertinent to Oxygen binding?

Answer 1

1. a. The important aminoacid that helps in binding of oxygen to the heme in myoglobin and all the four subunits of hemoglobin is HISTIDINE. There exists two coordination sites one above and below the heme plane. One of the coordination site is occupied by the imidazole ring of the hisitidine aminoacid. This is called the proximal histidine. The other site which is free helps in binding oxygen which is further stabilised by a distal hisitidine.

2. As explained in the answer above, the heme prosthetic group is held in the protein by interactions that are hydrophobic and by a coordinate interaction of the imidazole side chain in the aminoacid histidine with the Fe atom. Opposite side of the heme plane has distal hisitidine which helps in preventing the oxidation of Fe (II) by other oxidising agents. In the absence of such interactions offered by the distal histidine in the globin, Fe(II) can oxidise to Fe(III).

The oxygen binds to Fe(II) and forms a Fe(II)-O2 complex. The oxidised form of Fe, i.e. Fe(III) cannot bind oxygen.