Question

What are the R and T forms of hemoglobin how does the heme structure differ in each? What forces stablize the T state?

Answer 1

R and T form:

hemoglobin exists in two distinct states: the T-state and the R-state. The T-state of hemoglobin is the more "Tense" of the two; this is the deoxy form of hemoglobin (meaning that it lacks an oxygen species) and is also known as "deoxyhemoglobin. The R-state of hemoglobin is more "Relaxed" and is the fully oxygenated form; it is also known as "oxyhemoglobin."

The change between the T and R structures is the result of a rotation of 15 degrees between the two alpha-beta dimers. So this isthe difference in their structure . This rotation changes the bonds between the side chains of the alpha-beta dimers in the F helix and therefore causes the heme molecule to change positions. In the T structure, the iron ion is pulled out of the plane of the porphyrin ring and becomes less accessible for oxygen to bind to it, thus reducing its affinity to oxygen. In the R structure the iron atom is in the plane of the porphyrin ring and is accessible to bind oxygen, thus increasing its oxygen affinity. The transformation from the T to R structure occurs when oxygen binds to the T structure under the high oxygen pressure environment in the lungs, which causes the rotation of the two dimers and shifts the remain iron atoms so that they become more accessible to oxygen. Likewise, the transformation from the R to T structure occurs when oxygen is released under the low oxygen pressure environment of the tissues which causes the dimers to rotate back and shifts the iron atoms so that they become less accessible to oxygen. Thus, the cooperativity of the hemoglobin molecule can be explained by its unique structure which allows it to shift between the T and R structures in the presence or absence of oxygen