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Exploring the conformational equilibrium of E. coli thioredoxin reductase: Characterization of two catalytically important states by ultrafast flavin fluorescence spectroscopy

¹ MicroSpectroscopy Centre, Laboratory of Biochemistry, Wageningen University, Wageningen, The Netherlands
² Department of Veterans Affairs Medical Center, and Department of Biological Chemistry, University of Michigan, Ann Arbor, Michigan 48105, USA
³ Faculty of Sciences, Division of Physics and Astronomy, Vrije Universiteit, Amsterdam, The Netherlands
⁴ Department of Structural Biology, Institute of Molecular Biological Sciences, Vrije Universiteit, Amsterdam, The Netherlands

Reprint requests to: Dr. A.J.W.G. Visser, MicroSpectroscopy Centre, Laboratory of Biochemistry, Wageningen University, Dreijenlaan 3, 6703 HA Wageningen, The Netherlands; e-mail: Ton.Visser{at}laser.bc.wau.nl; fax: 31-317-484801.

The conformational dynamics of wild-type Escherichia coli thioredoxin reductase (TrxR) and the mutant enzyme C138S were studied by ultrafast time-resolved fluorescence of the flavin cofactor in combination with circular dichroism (both in the flavin fingerprint and far-UV regions) and steady-state fluorescence and absorption spectroscopy. The spectroscopic data show two conformational states of the enzyme (named FO and FR), of which the physical characteristics differ considerably. Ultrafast fluorescence lifetime measurements make it possible to distinguish between the two different populations: Dominant picosecond lifetimes of 1 ps (contribution 75%) and 7 ps (8%) are associated with the FO species in TrxR C138S. Long-lived fluorescence with two time constants in the range of 0.2–1 ns (total contribution 17%) originates from enzyme molecules in the FR conformation. The near absence of fast lifetime components in oxidized wild-type TrxR supports the idea of this enzyme being predominantly in the FR conformation. The emission spectrum of the FO conformation is blue-shifted with respect to that of the FR conformation. Because of the large difference in fluorescence characteristics, fluorescence measurements on time scales longer than 100 ps are fully determined by the fraction of enzyme molecules in the FR conformation. Binding of the thiol reagent phenyl mercuric acetate to wild-type enzyme and TrxR C138S stabilizes the enzymes in the FR conformation. Specific binding of the NADPH-analog, AADP⁺, to the FR conformation resulted in dynamic fluorescence quenching in support of the multiple quenching sites model. Raising the temperature from 277K–323K resulted in a moderate shift to the FR conformation for TrxR C138S. High concentrations of the cosolvent glycerol triggered the domain rotation from the FO to the FR conformation.

Keywords: Flavin; conformation; protein dynamics; time-resolved fluorescence; thioredoxin reductase; fluorescence quenching

Abbreviations: AAD+, 3-aminopyridine adenine dinucleotide • AADP+, 3-aminopyridine adenine dinucleotide phosphate • CD, circular dichroism • DAS, decay-associated spectrum • DTNB, 5,5`-dithiobis(2-nitrobenzoic acid) • DTT, 1,4-dithiothreitol • FAD, flavin adenine dinucleotide • FWHM, full width at half maximum • NADPH, reduced nicotinamide adenine dinucleotide phosphate • PMA, phenylmercuric acetate • TCSPC, time-correlated single photon counting • TrxR, thioredoxin reductase • wt TrxR, wild-type thioredoxin reductase • TrxR C138S, thioredoxin reductase with Cys138 mutated to Ser • TrxR C138S-PMA, thioredoxin reductase C138S treated with phenylmercuric acetate • TrxR wt-PMA, wild-type thioredoxin reductase reduced with NADPH and subsequently treated with phenylmercuric acetate and reoxidized

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