Direct evidence for the involvement of domain III in the N-F transition of bovine serum albumin.

The domain III of bovine serum albumin containing residues 377-582 of the protein sequence was isolated and its behaviour in acid solution was studied. The fragment was found to undergo structural transformations over the pH range 3.5-4.5 known to cause N-F transition in serum albumin. On the other hand, an albumin fragment that was devoid of domain III was unable to exhibit such a transition. These results were consistent with a mechanism where N-F transition involves the separation of domain III from the rest of the albumin starts at about pH 4.3 and is completed at pH 3.5.