4.7 Article

α-Lactalbumin is unfolded by all classes of surfactants but by different mechanisms

Journal

JOURNAL OF COLLOID AND INTERFACE SCIENCE
Volume 329, Issue 2, Pages 273-283

Publisher

ACADEMIC PRESS INC ELSEVIER SCIENCE
DOI: 10.1016/j.jcis.2008.10.021

Keywords

Denaturation; Apo-state; Anionic surfactants; Zwitterionic surfactants; Kinetics; Isothermal titration calorimetry

Funding

  1. Danish Ministry of Science, Innovation and Technology (D.E.O. and P.S.)
  2. Villum Kann Rasmussen Foundation
  3. Danish Biotechnological Instrument Centre (DABIC)
  4. Japan Society for the Promotion of Science
  5. Carlsberg Foundation
  6. Danish Research Foundation and the Danish Research Agency [26-02-0160, 21-04-0087]

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We show that all four classes of surfactants (anionic, cationic, non-ionic, and zwitterionic) denature alpha-lactalbumin (alpha LA), making alpha LA an excellent model system to compare their denaturation mechanisms. This involves at least two steps in all surfactants but is more complex in charged surfactants due to their strong binding properties. At very low concentrations, charged surfactants bind specifically as monomers, but the first denaturation process only sets in when 4-10 surfactant molecules are bound to form clusters on the protein surface and is followed by a second loss of structure as 20-25 surfactant molecules are bound. Sub-micellar interactions can be modeled as simple independent binding at multiple sites which does not achieve saturation before micelle formation sets in. In contrast, no specific sub-micellar surfactant binding is detected by calorimetry in the presence of zwitterionic and non-ionic surfactants, and denaturation only Occurs around the cmc. Unfolding rates are very rapid in charged surfactants and reach a similar plateau level around the cmc, indicating that monomers and micelles operate on a mutually exclusive basis. In contrast, unfolding occurs slowly in zwitterionic and non-ionic surfactants; and the rate increases with the cmc, suggesting that monomers cooperate with micelles in denaturation. (C) 2008 Elsevier Inc. All rights reserved.

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