Evidence for proton transfer from Glu-46 to the chromophore during the photocycle of photoactive yellow protein

Abstract

Photoactive yellow protein (PYP) belongs to the novel group of eubacterial photoreceptor proteins. To fully understand its light signal transduction mechanisms, elucidation of the intramolecular pathway of the internal proton is indispensable because it closely correlates with the changes in the hydrogen-bonding network, which is likely to induce the conformational changes. For this purpose, the vibrational modes of PYP and its photoproduct were studied by Fourier transform infra-red spectroscopy at -40 °C. The vibrational modes characteristic for the anionic p-coumaryl chromophore (Kim, M., Mathies, R. A., Hoff, W. D. and Hellingwerf, K. J. (1995) Biochemistry 34, 12669-12672) were observed at 1482, 1437, and 1163 cm-1 for PYP. However, the bands corresponding to these modes were not observed for PYP(M), the blue-shifted intermediate, but the 1175 cm-1 band characteristic of the neutral p-coumaryl chromophore was observed, indicating that the phenolic oxygen of the chromophore is protonated in PYP(M). A 1736 cm-1 band was observed for PYP, but the corresponding band for PYP(M) was not. Because it disappeared in the Glu-46 → Gln mutant of PYP, this band was assigned to the C=O stretching mode of the COOH group of Glu-46. These results strongly suggest that the proton at Glu-46 is transferred to the chromophore during the photoconversion from PYP to PYP(M).