The hairpin extension controls solvent access to the chromophore binding pocket in a bacterial phytochrome: a UV–vis absorption spectroscopy study

Abstract Solvent access to the protein interior plays an important role in function of many proteins. Phytochromes contain a specific structural feature, hairpin extension that appears relay information from chromophore rest protein. The interacts with amino acids near chromophore, and hence shields surrounding solvent. We envision detachment surface allows solvent exchange reactions vicinity chromophore. This can facilitate for example, proton transfer processes between interior. To test this hypothesis, kinetics protonation state biliverdin Deinococcus radiodurans bacteriophytchrome, thus, pH solution, is determined. observed absorbance changes are related binding pocket, gated by extension. therefore propose model “open” (solvent-exposed, deprotonation-active on (sub)second time-scale) “closed” (solvent-gated, deprotonation inactive) state, where fluctuates slowly these conformations thereby controlling process minute time scale. When connection surroundings destabilized point mutation, amplitude phase increases considerably. In absence extension, deprotonates essentially without any “gating”. Hence, we introduce straightforward method study stability fluctuation phytochrome its photostationary state. approach be extended other chromophore-protein systems absorption reflect dynamic