Photoinduced unfolding of beta-lactoglobulin mediated by a water-soluble porphyrin.
We investigated the effects that the irradiation of a tetra-anionic porphyrin (mesotetrakis(sulfonatophenyl)porphyrin) noncovalently bound to beta-lactoglobulin (BLG) produces on the conformation of the protein. Although BLG is not a potential target for the biomedical applications of porphyrins, it is a useful model for investigating the effects of photoactive ligands on small globular proteins. We show in this paper that irradiation causes a large unfolding of the protein and that the conformational change is not mediated by the formation of reactive oxygen species. Instead, our data are consistent with an electron-transfer mechanism that is capable of triggering structural changes in the protein and causes the Trp19 residue to undergo chemical modifications to form a derivative of kynurenine. This demonstrates that protein unfolding is prompted by a type-III photosensitizing mechanisms. Type-III mechanisms have been suggested previously, but they have been largely neglected as useful mediators of biomolecular damage. Our study demonstrates that porphyrins can be used as mediators of localized protein conformational changes and that the biomedical applications as well as the mechanistic details of electron transfer between exogenous ligands and proteins merit further investigation.
Department of Physics and Astronomy, University of Texas at San Antonio, One UTSA Circle, San Antonio, Texas 78249, USA.
John
J
Belcher
Samuel
S
Sansone
Nicholas F
NF
Fernandez
William E
WE
Haskins
Lorenzo
L
Brancaleon
Lorenzo
L
Brancaleona
D002942:Circular Dichroism; D005453:Fluorescence; D007782:Lactoglobulins / Q000737:chemistry / Q000528:radiation effects*; D007834:Lasers; D010777:Photochemistry; D011166:Porphyrins / Q000737:chemistry* / Q000528:radiation effects; D017510:Protein Folding / Q000528:radiation effects*; D012995:Solubility; D014867:Water / Q000737:chemistry*
D007782:Lactoglobulins; D011166:Porphyrins; C541374:mesotetrakis(sulfonatophenyl)porphyrin; D014867:Water