Photoisomerization and Proton Transfer in Photoactive Yellow Protein

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• 作者：Michael J. Thompson ; Donald Bashford ; Louis Noodleman ; and Elizabeth D. Getzoff
• 刊名：Journal of the American Chemical Society
• 出版年：2003
• 出版时间：July 9, 2003
• 年：2003
• 卷：125
• 期：27
• 页码：8186 - 8194
• 全文大小：247K
• 年卷期：v.125,no.27(July 9, 2003)
• ISSN：1520-5126

文摘

The photoactive yellow protein (PYP) is a bacterial photosensor containing a para-coumarylthioester chromophore that absorbs blue light, initiating a photocycle involving a series of conformationalchanges. Here, we present computational studies to resolve uncertainties and controversies concerningthe correspondence between atomic structures and spectroscopic measurements on early photocycleintermediates. The initial nanoseconds of the PYP photocycle are examined using time-dependent densityfunctional theory (TDDFT) to calculate the energy profiles for chromophore photoisomerization and protontransfer, and to calculate excitation energies to identify photocycle intermediates. The calculated potentialenergy surface for photoisomerization matches key, experimentally determined, spectral parameters. Thecalculated excitation energy of the photocycle intermediate cryogenically trapped in a crystal structure byGenick et al. [Genick, U. K.; Soltis, S. M.; Kuhn, P.; Canestrelli, I. L.; Getzoff, E. D. Nature 1998, 392,206-209] supports its assignment to the PYP_B (I₀) intermediate. Differences between the time-resolvedroom temperature (298 K) spectrum of the PYP_B intermediate and its low temperature (77 K) absorbanceare attributed to a predominantly deprotonated chromophore in the former and protonated chromophore inthe latter. This contrasts with the widely held belief that chromophore protonation does not occur until afterthe PYP_L (I₁ or pR) intermediate. The structure of the chromophore in the PYP_L intermediate is determinedcomputationally and shown to be deprotonated, in agreement with experiment. Calculations based on ourPYP_B and PYP_L models lead to insights concerning the PYP_BL intermediate, observed only at lowtemperature. The results suggest that the proton is more mobile between Glu46 and the chromophorethan previously realized. The findings presented here provide an example of the insights that theoreticalstudies can contribute to a unified analysis of experimental structures and spectra.