Structure of the photoactive yellow protein reconstituted with caffeic acid at 1.16 Å resolution

A structural study is described of the photoactive yellow protein (PYP) reconstituted with the chromophore derivative 3,4-dihydroxycinnamic acid. The crystal structure of PYP reconstituted with this chromophore at 1.16 Å resolution is reported in space group P6₅. This is the first high-resolution structure of a photoreceptor containing a modified chromophore. The introduction of an extra hydroxyl group in the native chromophore (i.e. p-coumaric acid) appears to perturb the structure of the hybrid yellow protein only slightly. The chromophore is bound by the protein in two different conformations, separated by a rotation of 180° of the catechol ring. In combination with available spectroscopic data, it is concluded that the caffeic acid chromophore binds to the protein in a strained conformation, which leads to a faster ejection from the chromophore-binding pocket upon pB formation.