Studies on the Acid Induced Unfolding of Human Serum Albumin
Abstract:The acid induced unfolding of HSA (Human Serum Albumin) was studied using UV-difference spectroscopy, fluorescence spectroscopy and far-UV CD spectroscopy. In UV-difference spectroscopy, the molar extinction coefficient decreased from N state to F state. Partially buried Tyr residues are transferred from a medium of high polarizability (native N state ) to a medium of low polarizability (F state). This is followed by loss of two electrostatic interactions Lys 205- Glu 465 and Arg218-Asp451 or one electrostatic interaction Lys205-Glu 465 / Arg218-Asp451 and one buried carboxyl group of acidic amino acid. Similarly, UV-difference spectroscopy showed a decrease in absorbance in F&8652;E transition due to exposure of completely buried tyrosine residues from medium of high polarizability (F state) to a medium of low polarizability (E state). This is also followed by loss of one electrostatic interaction out of three electrostatic interactions namely, Asp187-Lys432, Asp187-Lys521 and Lys190-Glu425. The tryptophanyl fluorescence spectra showed that the N&8652;F transition is accompanied by a decrease in fluorescence intensity. This implies that there is partial exposure of Trp214 to aqueous environment. Consequently, there is a loss of two electrostatic interactions Lys 205-Glu 465 and Arg218-Asp451 or one electrostatic interaction Lys205-Glu 465 / Arg218-Asp451 and one buried carboxyl group of acidic amino acid in N&8652;F transition. The tryptophanyl fluorescence spectroscopy also showed that partially exposed Trp214 residue becomes nearly completely exposed in F&8652;E transition. This is also followed by loss of two electrostatic interactions out of three Asp 187-Lys432,Asp187-Arg521 and Lys190-Glu425 in F&8652;E transition. Taken together, these results showed that in N&8652;F and F&8652;E transitions a different number of electrostatic interaction is detected by different techniques. Secondly, in both N&8652;F and F&8652;E transitions, chromophoric groups are exposed first to aqueous environment and this is followed by loss of electrostatic interactions.
Document Type: Research Article
Publication date: March 1, 2009
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