Spectral studies on aluminum ion binding to the ligands with phenolic group(s): implications for the differences between N- and C-terminal binding sites of human serum apotransferrin.
Both the binding and releasing of ferric ions in C-, and N-terminal binding sites of human serum transferrin are different. To understand the difference here the interactions of aluminum with the ligands containing phenolic group(s), including 8-hydroxyquinoline, salicylic acid, N,N'-di(2-hydroxybenzyl)ethylenediamine-N,N'-diacetic acid, N,N'-ethylenebis[2-(o-hydroxyphenolic)glycine], and human serum apotransferrin, respectively, are investigated by using UV difference and fluorescence spectra methods in 0.1 M N-2-hydroxyethylpiperazine-N-2-ethanesulfonic acid at pH 7.4. Aluminum binding produces a UV difference peak near 235 nm that is characteristic of phenolic groups binding to aluminum. The peak at 235 nm has been used to determine conditional binding constants of log K(Al-HBED)=8.88+/-0.74 and log K(Al-EHPG)=9.38+/-0.03. However, the effects of aluminum binding on the fluorescence intensity of N,N'-ethylenebis[2-(o-hydroxyphenolic)glycine], salicylic acid and N,N'-di(2-hydroxybenzyl) ethylenediamine-N,N'-diacetic acid, 8-hydroxyquinoline are disparate, the former showing a decrease and the latter an increase. At pH 7.4, there is N cdots, three dots, centered H-O type intramolecular hydrogen bond in 8-hydroxyquinoline, N,N'-di(2-hydroxybenzyl) ethylenediamine-N,N'-diacetic acid and O cdots, three dots, centered H-O type intramolecular hydrogen bond in salicylic acid, N,N'-ethylenebis[2-(o-hydroxyphenolic)glycine]. The effects of salts on the fluorescence intensity of the ligands containing phenolic group(s) show that fluorescence emission increases with the breaking of an N cdots, three dots, centered H-O type intramolecular hydrogen bond and fluorescence emission decreases with the breaking of an O cdots, three dots, centered H-O type intramolecular hydrogen bond. Fluorescence titrations of apotransferrin and both forms of monoferric transferrin with aluminum indicated that there is O cdots, three dots, centered H-O type intramolecular hydrogen bonds for the phenolic groups of Tyr426 and Tyr517 in the C-terminal binding site. While N cdots, three dots, centered H-O type intramolecular hydrogen bonds are found for the phenolic groups of Tyr95 and Tyr188 in the N-terminal binding site.
Yang
Binsheng
B
Institute of Molecular Science, Shanxi University, No. 36 Wucheng Road, 030006, Taiyuan, China. yangbs@sxu.edu.cn
Feng
Jiuying
J
Li
Yingqi
Y
Gao
Fei
F
Zhao
Yaqing
Y
Wang
Jinling
J
D000535:Aluminum / Q000737:chemistry* / Q000378:metabolism; D001059:Apoproteins / Q000737:chemistry* / Q000378:metabolism; D001665:Binding Sites; D006801:Humans; D008024:Ligands; D015125:Oxyquinoline / Q000737:chemistry; D010636:Phenols / Q000737:chemistry; D020156:Salicylic Acid / Q000737:chemistry; D013057:Spectrum Analysis; D014168:Transferrin / Q000737:chemistry* / Q000378:metabolism
D001059:Apoproteins; D008024:Ligands; D010636:Phenols; D014168:Transferrin; C028284:apotransferrin; D015125:Oxyquinoline; D000535:Aluminum; D020156:Salicylic Acid