Photoreaction of the blue light sensor protein PixD: Changes in the hydrogen-bond network involving a tyrosine residue near flavin

Abstract

PixD from Thermosynechococcus elongates is a flavin-binding BLUF protein relevant to phototaxis. In the X-ray structure of PixD, Gln50 and Tyr8 form a H-bond network from the flavin C4=O group, which was proposed to exhibit a large structural change upon photoreaction. The role of this H-bond network in the PixD function, however, has yet to be clarified. In this study, we have examined the relevance of Tyr8 to the reaction mechanism of PixD by means of FTIR spectroscopy. In the light-induced FTIR spectrum of PixD, the CO and COH bands of Tyr were identified using the PixD in which Tyr residues are replaced with 4-13C-Tyr. Theoretical analyses revealed that the OH of Tyr8 is H-bonded to the C=O of Gln50 and this H-bond is strengthened upon photoreaction. This result suggests that the H-bond network from flavin C4=O to Gln50-Tyr8 plays an important role in the signal transduction mechanism of PixD.