Drug-Mediated Laser Photo Damage of Globular Proteins
Abstract
The research activity carried out as part of this funded project studied the mechanisms of interaction of two porphyrins (protoporphyrin IX (PPIX)) and tetra-sulfonatophenyl-porphyrin (TPPS)) with two globular proteins (Beta-lactoglobulin (BLG) and tubulin) and the effect that the irradiation of the porphyrins produce on the conformation of the two proteins. We established that both porphyrin can bind the proteins. However while PPIX binding to BLG is modulated by the pH of the solution, the binding of TPPS is not. In combination with computational data we have established that both porphyrins bind to superficial but separate sites which are controlled by different interactions (van der Waals for PPIX, purely Coulombian for TPPS). We found that binding by itself does not produce conformational changes in the proteins but that, in the case of BLG, TPPS increases the stability of the protein to denaturation. Finally we have established that irradiation of both porphyrin produces conformational changes in BLG. The changes are smaller (< 10%) for the BLG/PPIX complex and larger (> 15%) for the BLG/TPPS complex. They can be summarized as partial unfolding of the Beta-structure of the protein into an unstructured arrangement. The unfolding is not dependent on the presence of molecular oxygen and does not appear to be caused by a temperature change induced by the relaxation of the porphyrin. This leads us to conclude that the unfolding is likely caused by photo-induced electron transfer between the porphyrin and the protein that triggers subsequent unfolding of the polypeptide. This, in the case of TPPS, is accompanied by a chemical modification of one of the Trp residues of the protein, which confirms the charge-transfer mechanism.
Document Details
- Document Type
- Technical Report
- Publication Date
- Mar 10, 2009
- Accession Number
- ADA506492
Entities
People
- Lorenzo Brancaleon
Organizations
- University of Texas at San Antonio