Binding of Alkyl Polyglucoside Surfactants to Bacteriorhodopsin and its Relation to Protein Stability
Maria Gabriella Santonicola 1, Abraham M. Lenhoff 1 and Eric W Kaler 1*
1 University of Delaware
* To whom correspondence should be addressed. E-mail: eric.kaler{at}stonybrook.edu.
Submitted on May 19, 2007
Revised on June 17, 2007
Accepted on 24 October 2007
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Abstract |
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The binding of alkyl polyglucoside surfactants to the integral membrane protein bacteriorhodopsin (BR) and the formation of protein-detergent complexes (PDCs) are investigated by sedimentation equilibrium via analytical ultracentrifugation and by small-angle neutron scattering (SANS). Contrast variation techniques in SANS enable measurement of the composition of the protein-surfactant complexes and determination of the thickness of the surfactant shell bound to the protein. The results indicate that alkyl polyglucosides can bind to bacteriorhodopsin as single surfactant layers or as a thicker shell. The thickness of the surfactant shell increases with increasing surfactant tail length, and it is generally unrelated to the aggregation number of the micelles even for a small and predominantly hydrophobic membrane protein such as BR. The aggregation numbers determined by sedimentation equilibrium methods match those measured by SANS, which also allows reconstruction of the shape of the PDC. When the surfactant is present as a single layer, the bacteriorhodopsin loses activity, as measured by absorption spectroscopy, more quickly than it does when the surfactant forms a thicker shell.
Key Words:
Bacteriorhodopsin, Protein Detergent Complex, Surfactants
