pH-dependent transition between delocalized and trapped valence states of a CuA center and its possible role in proton-coupled electron transfer

Hee Jung Hwang, Yi Lu

Research output: Contribution to journalArticle

Abstract

A pH-dependent transition between delocalized and trapped mixed valence states of an engineered CuA center in azurin has been investigated by UV-visible absorption and electron paramagnetic resonance spectroscopic techniques. At pH 7.0, the CuA azurin displays a typical delocalized mixed valence dinuclear [Cu(1.5)⋯Cu(1.5)] spectra with optical absorptions at 485, 530, and 760 nm, and with a seven-line EPR hyperfine. Upon lowering of the pH from 7.0 to 4.0, the absorption at 760 nm shifted to lower energy toward 810 nm, and a four-line EPR hyperfine, typical of a trapped valence, was observed. The pH-dependent transition is reversible because increasing the pH restores all delocalized spectral features. Lowering the pH resulted in not only a trapped valence state, but also a dramatically increased reduction potential of the Cu center (from 160 mV to 340 mV). Mutation of the titratable residues around the metal-binding site ruled out Glu-114 and identified the C-terminal histidine ligand (His-120) as a site of protonation, because the His120Ala mutation abolished the above pH-dependent transition. The corresponding histidine in cytochrome c oxidases is along a major electron transfer pathway from CUA center to heme a. Because the protonation of this histidine can result in an increased reduction potential that will prevent electron flow from the CuA to heme a, the CuA and the histidine may play an important role in regulating proton-coupled electron transfer.

Original languageEnglish (US)
Pages (from-to)12842-12847
Number of pages6
JournalProceedings of the National Academy of Sciences of the United States of America
Volume101
Issue number35
DOIs
StatePublished - Aug 31 2004

ASJC Scopus subject areas

  • General

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