L. Ragona et al., Equilibrium unfolding CD studies of bovine beta-lactoglobulin and its 14-52 fragment at acidic pH, BIOPOLYMERS, 49(6), 1999, pp. 441-450
Bovine beta-lactoglobulin represents an interesting example of context-depe
ndent secondary structure induction. In fact, secondary structure predictio
ns indicated that this beta-barrel protein has a surprisingly high alpha-he
lical preference, which was retained for short fragments. Cooperative trans
itions from the native beta-sheet to alpha-helical structures were addition
ally induced by organic solvents, in particular trifluoroethanol. As a resu
lt of this high alpha-helical preference, it has been proposed that non-nat
ive alpha-helical intermediates could be formed in the unfolding pathway of
this protein. In order to provide a better under-standing of the processes
that underlie conformational plasticity in this protein, CD measurements i
n the presence of increasing amounts of urea and in the presence of organic
solvents were performed. Urea unfolding studies, performed at pH 2.1 and 3
7 degrees C, revealed an apparent two-state transition, and afforded no evi
dence of non native alpha-helical inlet-mediates. The protein treated with
up to 6M urea, refolded to the native structure, while treatment with highe
r molar concentration urea. lend to partial misfolding. A 29-mer peptide co
vering the region of strands strands a and b of the intact protein, charact
erized by the presence of 4/3 heptad repeats, was synthesized and studied b
y CD in the presence of different solvents. On the basis of the obtained re
sults, a mechanism was proposed to explain the structural transition from t
he beta to alpha structure, provoked by organic solvents in the intact prot
ein. (C) 1999 John Wiley & Sons, Inc.