title: Helical and expanded conformation of equine beta-lactoglobulin in the cold-denatured state.
abstract: The thermal unfolding transition of equine beta-lactoglobulin (ELG) was investigated by circular dichroism (CD) over a temperature range of -15 degrees C to 85 degrees C. In the presence of 2 M urea, a cooperative unfolding transition was observed both with increasing and decreasing temperature. The CD spectrum indicated that the heat and cold-denatured states of ELG have substantial secondary structures but lack persistent tertiary packing of the side-chains. In order to clarify the relation between the heat or cold-denatured state and the acid-denatured (A) state characterized previously, we have attempted to observe the temperature dependence of the CD spectrum at pH 1.5. The CD spectrum in the heat-denatured state is similar to that in the A state. The CD spectrum in the A state does not change cooperatively with increasing temperature. These results indicate that the heat-denatured state and the A state are the same structural state. On the other hand, the CD intensity at acid pH cooperatively increased with decreasing temperature. The CD spectrum at low temperature and acid pH is consistent with that in the cold-denatured state. Therefore, the cold-denatured state is distinguished from the heat-denatured state or the A state, and ELG assumes a larger amount of non-native alpha-helices in the cold-denatured state. Small angle X-ray scattering and analytical ultracentrifugation have indicated that ELG assumes an expanded chain-like conformation in the cold-denatured state in contrast to the compact globular conformation in the A state. The relation between the molecular size and the helical content in the partially folded states is discussed.
authors.affiliation: Department of Bioengineering, Graduate School of Engineering, Soka University, Hachioji, Tokyo 192-8577, Japan.
authors.forename: Yoshiteru
authors.initials: Y
authors.lastname: Yamada
authors.forename: Takeo
authors.initials: T
authors.lastname: Yajima
authors.forename: Kazuo
authors.initials: K
authors.lastname: Fujiwara
authors.forename: Munehito
authors.initials: M
authors.lastname: Arai
authors.forename: Kazuki
authors.initials: K
authors.lastname: Ito
authors.forename: Akio
authors.initials: A
authors.lastname: Shimizu
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Kihara
authors.forename: Kunihiro
authors.initials: K
authors.lastname: Kuwajima
authors.forename: Yoshiyuki
authors.initials: Y
authors.lastname: Amemiya
authors.forename: Masamichi
authors.initials: M
authors.lastname: Ikeguchi
mesh_terms: D000818:Animals; D002942:Circular Dichroism; D003080:Cold Temperature; D006736:Horses; D006863:Hydrogen-Ion Concentration; D007782:Lactoglobulins / Q000737:chemistry* / Q000378:metabolism*; D008892:Milk / Q000737:chemistry; D008970:Molecular Weight; D011487:Protein Conformation; D011489:Protein Denaturation; D017510:Protein Folding; D013816:Thermodynamics; D014508:Urea
chemical_list: D007782:Lactoglobulins; D014508:Urea