Describe the sequence of events that lead to conformational changes upon binding of O2 to hemoglobin. Include in your discussion a detailed description of the protein structure. Discuss the models that have been proposed to describe this cooperative binding event. Solution A hemoglobin can bind 4 oxygen molecule. Hemoglobin contains 4 heme group bound to 4 protien chains. The 4 protien chains contains 2 alpha (containing 141 amino acid) and 2 beta chain( containing 146 amino acid) each. Due to this kind of tetrameric structure its oxygen binding property is different to that of myoglobin. Upon oxygenation of hemoglobin the conformation of the heme groups and also the protien chains changes. The deoxy form of hemoglobin is called T state and the oxy form of hemoglobin is called as R state.The affinity to dioxygen is more in case of R state. When dioxygen interacts with heme group, position of the protien chain attached to it changes this in turn changes the other protien positions. This leads ultimately change the protien structure. Due to this change increase in the affinity to bind dioxygen occurs from T to R state. It is known that high spin Fe2+ has a larger radius than that of the low spin Fe2+ by 17 pm. When oxygen is not bonded to hemoglobin Fe is in high spin state. When oxygen binds to hemoglobin Fe is in low spin state. Prior to oxygen binding iron atom lie above the porphyrin plane. After oxygen binding the iron atom sits into the plane. The inhibition of the movement of the iron atom is attributed to the steric interaction with the histidine of the protien chain. The above discussed facts can be seen in dioxygen binding to hemoglobin. When first oxygen binds to heme group it discourages this binding. The same thing happens to the second O2 also. Addition of the third O2 converts the T state to the R state. After that the fourth O2 binds easily. In tissuses where O2 is releasesd from hemoglobin this cooperative effect is very important. The last O2 molecule when released is pushed off from the T state due to considerable strain. .

1. Describe the sequence of events that lead to conformational changes upon binding of O2 to
hemoglobin. Include in your discussion a detailed description of the protein structure. Discuss
the models that have been proposed to describe this cooperative binding event.
Solution
A hemoglobin can bind 4 oxygen molecule. Hemoglobin contains 4 heme group bound to 4
protien chains. The 4 protien chains contains 2 alpha (containing 141 amino acid) and 2 beta
chain( containing 146 amino acid) each. Due to this kind of tetrameric structure its oxygen
binding property is different to that of myoglobin. Upon oxygenation of hemoglobin the
conformation of the heme groups and also the protien chains changes. The deoxy form of
hemoglobin is called T state and the oxy form of hemoglobin is called as R state.The affinity to
dioxygen is more in case of R state. When dioxygen interacts with heme group, position of the
protien chain attached to it changes this in turn changes the other protien positions. This leads
ultimately change the protien structure. Due to this change increase in the affinity to bind
dioxygen occurs from T to R state.
It is known that high spin Fe2+ has a larger radius than that of the low spin Fe2+ by 17 pm.
When oxygen is not bonded to hemoglobin Fe is in high spin state. When oxygen binds to
hemoglobin Fe is in low spin state. Prior to oxygen binding iron atom lie above the porphyrin
plane. After oxygen binding the iron atom sits into the plane.
The inhibition of the movement of the iron atom is attributed to the steric interaction with the
histidine of the protien chain.
The above discussed facts can be seen in dioxygen binding to hemoglobin. When first oxygen
binds to heme group it discourages this binding. The same thing happens to the second O2 also.
Addition of the third O2 converts the T state to the R state. After that the fourth O2 binds easily.
In tissuses where O2 is releasesd from hemoglobin this cooperative effect is very important. The
last O2 molecule when released is pushed off from the T state due to considerable strain.