Question

9. Carbon monoxide (CO) binds Hb in a similar fashion to O₂, but with 200-fold higher affinity. When presented with a mixture of O₂ and CO, individual heme molecules in a single tetramer may bind to either molecule. Does the presence of small amounts of CO increase, decrease, or not change the affinity of Hb for O₂? Explain (a) your choice and (b) how CO binding Hb starves tissues of O₂. In your answer, please consider R and T states of Hb.

Answer 1

The presence of CO affects the affinity of Hb for O2.

1) Because CO is a stronger Π-acceptor than O2 .

2)CO is a stronger ligand than O2.

So due to these two properties in the presence of CO , Hb will first bind to CO molecules then there will be less sites left for dioxygen binding because dioxygen is a weak ligand as well as weak Π-acceptor when compared to CO.

The oxy form of Hb is known as relaxed(R) state and the deoxy form of Hb is known as tesnsed (T) state.

This is because in oxy form the binding of dioxygen reduces the steric hindrance and relaxes the Hb molecule

Wheresas in deoxy form the Hb molecule is under tension. So tensed Hb finds it hard to bind with first O2 , but after the first O2 bonding the further O2 bonding process eases eventually due to release in strain.

(a) So the presence of CO will reduce the affinity of Hb for O2 due to previously explained properties of CO.

(b) Deoxy (T)form of Hb has higher affinity for CO due to previously explained 2 properties of CO molecule. CO binding with Hb starves the muscles because less sites are available on Hb to carry oxygen to the muscles and it may cause death eventually due to Asphyxia.