Studies on the Acid Induced Unfolding of Human Serum Albumin

Parveen      Salahuddin

doi:10.2174/092986609787601750

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 FE 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 NF 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 NF transition. The tryptophanyl fluorescence spectroscopy also showed that partially exposed Trp214 residue becomes nearly completely exposed in FE transition. This is also followed by loss of two electrostatic interactions out of three Asp 187-Lys432,Asp187-Arg521 and Lys190-Glu425 in FE transition. Taken together, these results showed that in NF and FE transitions a different number of electrostatic interaction is detected by different techniques. Secondly, in both NF and FE transitions, chromophoric groups are exposed first to aqueous environment and this is followed by loss of electrostatic interactions.

Keywords: Acid induced unfolding, NF transition, FE transition, fluorescence spectroscopy, far-UV CD spectroscopy, UV-difference spectroscopy

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