Irradiation of the porphyrin causes unfolding of the protein in the protoporphyrin IX/beta-lactoglobulin noncovalent complex.
Porphyrins such as protoporphyrin IX (PPIX) are known to occasionally cause conformational changes in proteins for which they are specific ligands. It has also been established that irradiation of porphyrins noncovalently intercalated between bases or bound to one of the grooves can cause conformational effects on DNA. Conversely, there is no evidence reported in the literature of conformational changes caused by noncovalently bound PPIX to globular proteins for which the porphyrin is not a specific ligand. This study shows that the irradiation of the porphyrin in the PPIX/lactoglobulin noncovalent complex indeed causes a local and limited (approximately 7%) unfolding of the protein near the location of Trp19. This event causes the intrinsic fluorescence spectrum of the protein to shift to the red by 2 nm and the average decay lifetime to lengthen by approximately 0.5 ns. The unfolding of lactoglobulin occurs only at pH >7 because of the increased instability of the protein at alkaline pH. The photoinduced unfolding does not depend on the presence of O2 in solution; therefore, it is not mediated by formation of singlet oxygen and is likely the result of electron transfer between the porphyrin and amino acid residues.
Department of Physics and Astronomy, University of Texas at San Antonio, San Antonio, Texas, USA.
Nicholas F
NF
Fernandez
Samuel
S
Sansone
Alberto
A
Mazzini
Lorenzo
L
Brancaleon
D001665:Binding Sites; D002942:Circular Dichroism; D003198:Computer Simulation; D018360:Crystallography, X-Ray; D004591:Electrophoresis, Polyacrylamide Gel; D006863:Hydrogen-Ion Concentration; D007782:Lactoglobulins / Q000737:chemistry* / Q000378:metabolism*; D008958:Models, Molecular; D017510:Protein Folding; D017434:Protein Structure, Tertiary; D011524:Protoporphyrins / Q000737:chemistry*; D013053:Spectrophotometry
D007782:Lactoglobulins; D011524:Protoporphyrins; C028025:protoporphyrin IX