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Kinetics of folding and unfolding of goat α‐lactalbumin
Author(s) -
Chedad Allel,
Van Dael Herman
Publication year - 2004
Publication title -
proteins: structure, function, and bioinformatics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.699
H-Index - 191
eISSN - 1097-0134
pISSN - 0887-3585
DOI - 10.1002/prot.20212
Subject(s) - chemistry , guanidine , circular dichroism , alpha lactalbumin , denaturation (fissile materials) , equilibrium unfolding , molten globule , lactalbumin , kinetics , crystallography , folding (dsp implementation) , protonation , protein folding , fluorescence , gibbs free energy , native state , thermodynamics , ion , chromatography , biochemistry , organic chemistry , physics , quantum mechanics , electrical engineering , nuclear chemistry , engineering
The equilibrium unfolding and the kinetic folding and unfolding of goat α‐lactalbumin (GLA) were studied by near‐ and far‐ultraviolet circular dichroism (CD) and by stopped‐flow fluorescence spectroscopy. Specifically, the influence of environmental conditions such as pH and Ca 2+ binding was examined. Compared to the apo‐form, the Ca 2+ ‐bound form was found to be strongly stabilized in equilibrium conditions at pH 7.5 and 25°C. The kinetics of the refolding of apo‐GLA show a major change of fluorescence intensity during the experimental dead‐time, but this unresolved effect is strongly diminished in holo‐GLA. In both cases, however, the chevron plots can adequately be fitted to a three‐state model. Moreover, double‐mix stopped‐flow experiments showed that the native state (N) is reached through one major pathway without the occurrence of alternative tracks. In contrast to the homologous bovine α‐lactalbumin (BLA), the compactness of GLA is strongly influenced by the presence of Ca 2+ ions. Unlike the two‐state transition observed in guanidine hydrochloride (GdnHCl)‐induced equilibrium denaturation experiments at higher pH, an equilibrium intermediate state (I) is involved in denaturation at pH 4.5. In the latter case, analysis of the kinetic data makes clear that the intermediate and the unfolded states (U) show practically no Gibbs free energy difference and that they are in rapid equilibrium with each other. A possible explanation for these variations in stability and in folding characteristics with pH could be the degree of protonation of His107 that directly influences non‐native interactions. Variation of environmental conditions and even small differences in sequence, therefore, can result in important effects on thermodynamic and folding parameters. Proteins 2004. © 2004 Wiley‐Liss, Inc.