title: The pH-dependent conformational transition of beta-lactoglobulin modulates the binding of protoporphyrin IX.
abstract: We have investigated the interaction between PPIX and beta-lactoglobulin (beta-lg) as a function of the pH of the solution. beta-lg is a small globular protein (MW approximately 18 kDa) with a very well characterized structure that reveals several possible binding sites for ligands. The interaction with beta-lg affects the photophysical properties of PPIX. The shift of PPIX emission maximum, excitation maximum and the increase of the fluorescence intensity is an indicator that binding between the porphyrin and beta-lg occurs. The binding constant appears to be modulated by the pH of the solution. Spectroscopic measurements do not reveal any significant energy transfer between the Trp residues of beta-lg and PPIX, however, fluorescence anisotropy decay measurements confirm the binding and the modulation introduced by the pH of the solution. Since beta-lg has been shown to be stable within the range of pH adopted in our experiments (5.0-9.0), the results suggest that PPIX binds a site affected by the pH of the solution. Because of the crystallographic evidence an obvious site is near the aperture of the interior beta-barrel however an alternative (or concurrent) binding site may still be present.
authors.affiliation: Department of Physics and Astronomy, University of Texas at San Antonio, 6900 N Loop 1604 W, San Antonio, TX 78249, USA.
authors.forename: Fang
authors.initials: F
authors.lastname: Tian
authors.forename: Katrina
authors.initials: K
authors.lastname: Johnson
authors.forename: Andrea E
authors.initials: AE
authors.lastname: Lesar
authors.forename: Harry
authors.initials: H
authors.lastname: Moseley
authors.forename: James
authors.initials: J
authors.lastname: Ferguson
authors.forename: Ifor D W
authors.initials: ID
authors.lastname: Samuel
authors.forename: Alberto
authors.initials: A
authors.lastname: Mazzini
authors.forename: Lorenzo
authors.initials: L
authors.lastname: Brancaleon
mesh_terms: D000818:Animals; D001665:Binding Sites; D002417:Cattle; D019281:Dimerization; D005454:Fluorescence Polarization; D006863:Hydrogen-Ion Concentration; D007782:Lactoglobulins / Q000737:chemistry* / Q000378:metabolism*; D011485:Protein Binding; D011487:Protein Conformation; D011524:Protoporphyrins / Q000378:metabolism*; D013050:Spectrometry, Fluorescence; D014364:Tryptophan
chemical_list: D007782:Lactoglobulins; D011524:Protoporphyrins; D014364:Tryptophan; C028025:protoporphyrin IX