Subpicosecond Protein Backbone Changes Detected during the Green-Absorbing Proteorhodopsin Primary Photoreaction

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• 作者：Jason J. Amsden ; Joel M. Kralj ; Logan R. Chieffo ; Xihua Wang ; Shyamsunder Erramilli ; Elena N. Spudich ; John L. Spudich ; Lawrence D. Ziegler ; Kenneth J. Rothschild
• 刊名：Journal of Physical Chemistry B
• 出版年：2007
• 出版时间：October 11, 2007
• 年：2007
• 卷：111
• 期：40
• 页码：11824 - 11831
• 全文大小：460K
• 年卷期：v.111,no.40(October 11, 2007)
• ISSN：1520-5207

文摘

Recent studies demonstrate that photoactive proteins can react within several picoseconds to photon absorptionby their chromophores. Faster subpicosecond protein responses have been suggested to occur in rhodopsin-like proteins where retinal photoisomerization may impulsively drive structural changes in nearby proteingroups. Here, we test this possibility by investigating the earliest protein structural changes occurring inproteorhodopsin (PR) using ultrafast transient infrared (TIR) spectroscopy with ~200 fs time resolutioncombined with nonperturbing isotope labeling. PR is a recently discovered microbial rhodopsin similar tobacteriorhodopsin (BR) found in marine proteobacteria and functions as a proton pump. Vibrational bands inthe retinal fingerprint (1175-1215 cm^-1) and ethylenic stretching (1500-1570 cm^-1) regions characteristicof all-trans to 13-cis chromophore isomerization and formation of a red-shifted photointermediate appearwith a 500-700 fs time constant after photoexcitation. Bands characteristic of partial return to the groundstate evolve with a 2.0-3.5 ps time constant. In addition, a negative band appears at 1548 cm^-1 with a timeconstant of 500-700 fs, which on the basis of total-¹⁵N and retinal C15D (retinal with a deuterium on carbon15) isotope labeling is assigned to an amide II peptide backbone mode that shifts to near 1538 cm^-1concomitantly with chromophore isomerization. Our results demonstrate that one or more peptide backbonegroups in PR respond with a time constant of 500-700 fs, almost coincident with the light-driven retinylidenechromophore isomerization. The protein changes we observe on a subpicosecond time scale may be involvedin storage of the absorbed photon energy subsequently utilized for proton transport.