Lack of N--F transition in the N-terminal fragment (domain I + II) of bovine serum albumin.

A fragment of bovine serum albumin corresponding to positions 1-385 of the protein sequence has been prepared by peptic digestion. Acid-induced conformational changes in the fragment as well as in the intact albumin have been studied by hydrodynamic and spectral methods. Special emphasis has been given to the pH zone, which is known to cause the N-F transition in bovine serum albumin. In agreement with earlier reports we have shown that bovine serum albumin undergoes N-F transition in the neighbourhood of pH 4.0. Our results on the fragment showed that it was unable to exhibit the N-F transition under identical experimental conditions. Since bovine serum albumin and the peptic fragment differ in that the latter lacks the C-terminal portion (i.e. residues 386-582) of its parent molecule, we conclude that the separation of C-terminal region (from rest of bovine serum albumin molecule) and/or its unfolding is responsible for the N-F transition in bovine serum albumin.