Alpha-Lactalbumin: compact state with fluctuating tertiary structure?

The denaturation of some globular proteins by Gu-HCl shows a non-simultaneous change of different optical properties: the CD spectrum in near UV (aromatic) region changes at smaller concentrations of Gu-HCl than the CD spectrum in far UV (peptide) region [l--4]. This suggests the existence of stable, partly denatured (intermediate) forms of these proteins with a more or less symmetrical environment of aromatic groups but with a more or less native-like secondary structure. For both bovine [2] and human [4] a-lactalbumins as well as for some other proteins [S] a similar state exists also at acid pH. While the partly denatured (P) form at moderate Gu-HCl concentrations usually can be obtained only in mixture with native (N) and/or unfolded (U) forms, the acid (A) forms of these proteins can be studied in pure state. It was shown that for bovine (u-LA U + A transition takes <l ms while A + N transition needs as much as 0.1-I s which suggests that the intermediate state similar to the A-form can be also a kinetic intermediate in the course of protein folding [6]. nounced than that of the native protein. This paper presents the results of investigation of acid and some similar forms of bovine and human a-lactalbumins which show that these forms are compact, have secondary structure similar to the native one but their tertiary structure can slowly fluctuate. More detailed experimental data and their discussion will be published later.