Kavitha Mannem, Swamy Musti J
School of Chemistry, University of Hyderabad, Hyderabad, India.
IUBMB Life. 2006 Dec;58(12):720-30. doi: 10.1080/15216540601069761.
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 x 10(4) to 8.0 x 10(4) M(-1). Both cationic and anionic porphyrins were found to bind to PSL with comparable affinity. Presence of 0.1 M methyl-alpha-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: delthaH degree = -95.4 to -33.9 kJ x mol(-1) and deltaS degree = -237.2 to -32.2 J x mol(-1) x 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.
由于卟啉作为光动力疗法治疗癌症的光敏剂的应用，以及一些凝集素优先识别肿瘤细胞的能力，卟啉与凝集素相互作用的研究备受关注。在此我们报告了几种游离碱和金属卟啉与豌豆（Pisum sativum）凝集素（PSL）相互作用的热力学研究。通过监测卟啉Soret带的变化，采用吸收滴定法获得了缔合常数（Ka），不同温度下各种卟啉的Ka值在1.0×10⁴至8.0×10⁴ M⁻¹范围内。发现阳离子和阴离子卟啉均以相当的亲和力与PSL结合。0.1 M甲基-α-D-甘露吡喃糖苷（一种被PSL特异性识别的碳水化合物配体）的存在对结合没有显著影响，这表明卟啉和糖在凝集素的不同位点结合。根据Ka值对温度的依赖性，估算了与结合过程相关的热力学参数、焓变和熵变。这些值在以下范围内：ΔH° = -95.4至-33.9 kJ·mol⁻¹，ΔS° = -237.2至-32.2 J·mol⁻¹·K⁻¹，表明卟啉与豌豆凝集素的结合主要由焓力驱动，熵的贡献为负。在不同卟啉与PSL的相互作用中观察到了焓-熵补偿，但中位-四-（4-磺基苯基）卟啉锌（II）除外，这强调了水结构在整个结合过程中的作用。圆二色性和差示扫描量热研究表明，虽然卟啉结合不会引起凝集素结构和热稳定性的显著变化，但碳水化合物结合会引起蛋白质三级结构的适度变化，还会提高其热解折叠温度和解折叠转变的焓。
