Specific interaction of 7-hydroxyquinoline with Trp-214 in the drug-binding site IIA of human serum albumin

Osama K. Abou-Zied, Najla Al-Lawatia

Research output: Chapter in Book/Report/Conference proceedingConference contribution

1 Citation (Scopus)

Abstract

The drug-binding site subdomain IIA of human serum albumin (HSA) was characterized by absorption and fluorescence spectroscopy using 7-hydroxyquinoline (7-HQ) as a local reporter. The spectra of 7-HQ in solution indicate that a ztitterionic tautomer is stabilized by water in the ground state and produces a unique absorption peak at 400 nm and a fluorescence peak at 510 nm. By examining the spectral change in binary mixtures of water and 1,4-dioxane, three water molecules were estimated to stabilize this tautomer through direct interactions with the polar regions of the molecule. When 7-HQ is mixed with HSA, a reduction in the absorbance of the zwitterionic tautomer was observed which indicates a less polar environment around the molecule. The 7-HQ molecule is found to specifically bind in subdomain IIA of HSA and causes a reduction in the fluorescence intensity of the Trp-214 residue which is located in the same binding site. The reduction in the fluorescence of Trp-214 is due to energy transfer from the Trp-214 residue to the 7- HQ probe. The distance between Trp-214 and the probe was calculated using Förster theory for energy transfer to be 1.95 nm. This distance and the calculated quenching rate constant using a Stern-Valmer plot (kq = 3.04 x 1012 M-1s-1) both point to a static quenching mechanism. The binding constant and the number of binding sites of the complex were also estimated and the calculations show that the 7-HQ probe binds only in subdomain IIA. The change in the fluorescence intensity of HSA in the presence of the probe indicates that the 7-HQ molecule selectively interacts with the Trp-214 residue which results in partial unmasking of the fluorescence due to the Tyr-263 residue (located in the same site). A much stronger fluorescence from Tyr-263 is observed when HSA is chemically unfolded by 6.0 M GdnHCl. 7- HQ is found to still bind in subdomain IIA in the unfolded state of HSA and causes a reduction in the fluorescence intensities of both Trp-214 and Tyr-263. The present results propose 7-HQ as a useful photophysical probe in studying binding sites in proteins and exploring their hydrophobic environment.

Original languageEnglish
Title of host publicationReporters, Markers, Dyes, Nanoparticles, and Molecular Probes for Biomedical Applications II
DOIs
Publication statusPublished - 2010
Externally publishedYes
EventReporters, Markers, Dyes, Nanoparticles, and Molecular Probes for Biomedical Applications II - San Francisco, CA, United States
Duration: Jan 25 2010Jan 27 2010

Publication series

NameProgress in Biomedical Optics and Imaging - Proceedings of SPIE
Volume7576
ISSN (Print)1605-7422

Other

OtherReporters, Markers, Dyes, Nanoparticles, and Molecular Probes for Biomedical Applications II
Country/TerritoryUnited States
CitySan Francisco, CA
Period1/25/101/27/10

Keywords

  • 7-Hydroxyquinoline
  • Biological probes
  • Chemical unfolding
  • Drug binding sites
  • Fluorescence
  • Human serum albumin
  • Resonance energy transfer
  • Tautomerization

ASJC Scopus subject areas

  • Electronic, Optical and Magnetic Materials
  • Biomaterials
  • Atomic and Molecular Physics, and Optics
  • Radiology Nuclear Medicine and imaging

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