Hemoglobin: NMR evidence for transition between T state and i conformation.

Fan et al (2013) measured changes in chemical shifts (if any) of most amino acids in carbonmonoxyhemoglobin with and without the polyanionic effector, inositol hexaphosphate. See their Figure 2. As expected, amino acids at or near the accepted polyanion binding site between beta chains experienced shift changes greater than average. However there were many amino acids elsewhere that also experienced above average changes of chemical shift. About 6 of these were internal and in contact with heme or proximal or distal histidines of alpha and beta chains. This indicates a change in the immediate electronic environment. Perhaps the effector caused CO to dissociate from Hb, or almost so. Of the other changes, most can be explained by a transition between the t and i conformations shown in the Nativeproteins Gallery . This transition involves change of neighbouring subunit from alpha chain to beta chain across the extensive Chain A - Chain B interface and the smaller Chain A - Chain C interface. There is no change across the Chain A - Chain D interface. The NMR reflects this. Alpha 38, 39, 41, 86, 93, 94, 95, 96, 97, and 137 experienced above average NMR changes and make up most of the A - B interface. Similarly, beta 41, 96, 98, 99, 101, 142, and 146 experienced above average changes and make up most of the A - B interface. Alpha 100 and beta 34 and 43 also experienced above average NMR changes and are at the A -C interface. There were no above average NMR changes at the A - D interface. As a consequence I have renamed the t2 conformation the i conformation, reflecting the fact that it is possibly an intermediate state between r conformation and t conformation and not just a hypothetical structure. Possibly the i conformation is the true R state, pending further investigation.