Involvement of Histidine Residues in the pH-Dependent 尾-Galactoside Binding Activity of Human Galectin-1

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• 作者：Hirotsugu Hiramatsu ; Katsuyuki Takeuchi ; Hideo Takeuchi
• 刊名：Biochemistry
• 出版年：2013
• 出版时间：April 2, 2013
• 年：2013
• 卷：52
• 期：13
• 页码：2371-2380
• 全文大小：462K
• 年卷期：v.52,no.13(April 2, 2013)
• ISSN：1520-4995

文摘

The pH dependence of the 尾-galactoside binding activity of human galectin-1 (hGal-1) was investigated by fluorescence spectroscopy using lactose as a ligand. The obtained binding constant K_b was 2.94 卤 0.10 mM^鈥? at pH 7.5. The K_b value decreased at acidic pH with a midpoint of transition at pH 6.0 卤 0.1. To elucidate the molecular mechanism of the pH dependence, we investigated the structures of hGal-1 and its two His mutants (H44Q and H52Q) using fluorescence, circular dichroism, UV absorption, and UV resonance Raman spectroscopy. Analysis of the spectra has shown that the pK_a values of His44 and His52 are 5.7 卤 0.2 and 6.3 卤 0.1, respectively. The protonation of His52 below pH 6.3 induces a small change in secondary structure and partly reduces the galactoside binding activity. On the other hand, the protonation of His44 below pH 5.7 exerts a cation鈭捪€ interaction with Trp68 and largely diminishes the galactoside binding activity. With reference to the literature X-ray structures at pH 7.0 and 5.6, protonated His52 is proposed to move slightly away from the galactoside-binding region with a partial unfolding of the 尾-strand containing His52. On the other hand, protonated His44 becomes unable to form a hydrogen bond with galactoside and additionally induces a reorientation and/or displacement of Trp68 through cation鈭捪€ interaction, leading to a loosening of the galactoside-binding pocket. These structural changes associated with His protonation are likely to be the origin of the pH dependence of the galactoside binding activity of hGal-1.