Thermodynamic studies on the interaction of water-soluble porphyrins with the glucose/mannose-specific lectin from garden pea (Pisum sativum)

Abstract

Due to the application of porphyrins as photosensitizers in photodynamic therapy to treat cancer, and the ability of some lectins to preferentially recognize tumor cells, studies on the interaction of porphyrins with lectins are of considerable interest. Here we report thermodynamic studies on the interaction of several free-base and metallo-porphyrins with pea (Pisum sativum) lectin (PSL). Association constants (Ka) were obtained by absorption titrations by monitoring changes in the Soret band of the porphyrins and the Ka values obtained for various porphyrins at different temperatures are in the range of 1.0 × 104 to 8.0 × 104 M-1. Both cationic and anionic porphyrins were found to bind to PSL with comparable affinity. Presence of 0.1 M methyl-α-D-mannopyranoside - a carbohydrate ligand that is specifically recognised by PSL - did not affect the binding significantly, suggesting that porphyrin and sugar bind at different sites on the lectin. From the temperature dependence of the Ka values, the thermodynamic parameters, change in enthalpy and change in entropy associated with the binding process were estimated. These values were found to be in the range: ΔH° = -95.4 to -33.9 kJ · mol-1 and ΔS° = -237.2 to -32.2 J · mol-1 · K-1, indicating that porphyrin binding to pea lectin is driven largely by enthalpic forces with the entropic contribution being negative. Enthalpy-entropy compensation was observed in the interaction of different porphyrins to PSL, with the exception of meso-tetra-(4-sulfonatophenyl)porphyrinato zinc(II), emphasizing the role of water structure in the overall binding process. Circular dichroism and differential scanning calorimetric studies indicate that while porphyrin binding does not induce significant changes in the lectin structure and thermal stability, carbohydrate binding induces moderate changes in the tertiary structure of the protein and also increases its thermal unfolding temperature and the enthalpy of the unfolding transition.