Sharma S, Bharadwaj S, Surolia A, Podder S K
Department of Biochemistry, Indian Institute of Science, Bangalore 560012, India.
Biochem J. 1998 Aug 1;333 ( Pt 3)(Pt 3):539-42. doi: 10.1042/bj3330539.
High-sensitivity isothermal titration calorimetry has been used to investigate the thermodynamics of binding of Ricinus communis agglutinin to galactose, lactose and their derivatives in the temperature range 280.5-298 K. The present study unequivocally establishes the carbohydrate-binding stoichiometry of the tetrameric agglutinin from castor bean as two, i.e. the (As-sB)2-type tetramer of the agglutinin has two equivalent sites that are non-interacting and independent. The site binding constants range from 2.2x10(3) M-1 at 282 K for galactose to 4.84x10(4) M-1 at 281 K for N-acetyl-lactosamine. The binding enthalpies range from -21.9 kJ. mol-1 at 293 K for 4-methylumbelliferyl-beta-galactoside to -50.2 kJ. mol-1 at 292.9 K for thiodigalactoside. The observation of limited entropy-enthalpy compensation for binding of the sugars to the lectin indicates that reorganization of water molecules plays an important role in binding. As the slope of the compensation plot is greater than unity, the reactions are largely enthalpically driven. These studies show that the stronger binding of N-acetyl-lactosamine than lactose is due to a favourable interaction between the acetamido group of the reducing-end N-acetylglucosamine of the former and the corresponding loci in the agglutinin molecule. Preferential binding of methyl-beta-galactoside over methyl-alpha-galactoside also indicates the apolar nature of the interaction with the methyl group of the former sugar.
高灵敏度等温滴定量热法已用于研究蓖麻凝集素在280.5 - 298 K温度范围内与半乳糖、乳糖及其衍生物结合的热力学。本研究明确确定了来自蓖麻子的四聚体凝集素的碳水化合物结合化学计量为两个，即凝集素的(As - sB)2型四聚体有两个等效的、非相互作用且独立的位点。位点结合常数范围从282 K时半乳糖的2.2×10³ M⁻¹到281 K时N - 乙酰乳糖胺的4.84×10⁴ M⁻¹。结合焓范围从293 K时4 - 甲基伞形酮基 - β - 半乳糖苷的 - 21.9 kJ·mol⁻¹到292.9 K时硫代二半乳糖苷的 - 50.2 kJ·mol⁻¹。对糖与凝集素结合的有限熵 - 焓补偿的观察表明，水分子的重排在结合中起重要作用。由于补偿图的斜率大于1，反应在很大程度上由焓驱动。这些研究表明，N - 乙酰乳糖胺比乳糖结合更强是由于前者还原端N - 乙酰葡糖胺的乙酰氨基与凝集素分子中相应位点之间的有利相互作用。甲基 - β - 半乳糖苷比甲基 - α - 半乳糖苷的优先结合也表明与前一种糖的甲基相互作用的非极性性质。
