Bovine serum albumin oligomers in the E- and B-forms at low protein concentration and ionic strength

The manuscript describes the study of the oligomerization process of bovine serum albumin (BSA) in two different structural monomeric forms: the extended-form (E) at pH 2.0 and the basic-form (B) at pH 9.0. The study was conducted at low protein concentration (1 mg/ml) and relatively short incubation time (maximum 56 days) in order to investigate early oligomerization events rather than the formation of mature fibrils. The comparison between the two isoforms show that oligomers form much faster (∼6 days) in the E-form than in the B-form where formation of oligomers requires ∼4 weeks. The oligomers appear to be limited to a maximum of tetramers with size <30 nm. Hydrophobic interactions from exposed neutral amino acid residues in the elongated E-form are the likely cause for the quick formation of aggregates at acidic pH. We used an array of biophysical techniques for the study and determined that oligomerization occurs without further large changes in the secondary structure of the monomers. Under the conditions adopted in this study, aggregation does not seem to exceed the formation of tetramers, even though a very small amount of much larger aggregates seem to form.