ENERGY TRANSDUCTION ON THE NANOSECOND TIME-SCALE - EARLY STRUCTURAL EVENTS IN A XANTHOPSIN PHOTOCYCLE

Photoactive yellow protein (PYP) is a member of the xanthopsin family of eubacterial blue-light photoreceptors. On absorption of light, PYP enters a photocycle that ultimately transduces the energy contained in a light signal into an altered biological response, Nanosecond time-r esolved x-ray crystallography was used to determine the structure of t he short-lived, red-shifted, intermediate state denoted [pR], which de velops within 1 nanosecond after photoelectronic excitation of the chr omophore of PYP by absorption of light. The resulting structural model demonstrates that the [pR] state possesses the cis conformation of th e 4-hydroxyl cinnamic thioester chromophore, and that the process of t rans to cis isomerization is accompanied by the specific formation of new hydrogen bonds that replace those broken upon excitation of the ch romophore. Regions of flexibility that compose the chromophore-binding pocket serve to lower the activation energy barrier between the dark state, denoted pG, and [pR], and help initiate entrance into the photo cycle. Direct structural evidence is provided for the initial processe s of transduction of light energy, which ultimately translate into a p hysiological signal.