Regulation of Photocycle Kinetics of Photoactive Yellow Protein by Modulating Flexibility of the β-Turn.

The role of the significant flexibility of the β-turn in photoactive yellow protein (PYP) due to Gly115 was studied. G115A and G115P mutations were observed to accelerate the photocycle and shift the equilibrium between the late photocycle intermediate (pB) and its precursor (pR) toward pR. Thermodynamic analysis of dark-state recovery from pB demonstrated that the transition state (pB) has a negative change in transition heat capacity, suggesting that an exposed hydrophobic surface of pB is buried in pB. Fourier transform infrared spectroscopy showed that the structural ensemble of pB is populated by the compact structure in G115P. Taken together, the rigid structure induced by mutation of Gly115 facilitates its transition to pB, which adopts a substantially more compact structure as opposed to the ensemble-averaged structure of pB. The photocycle kinetics of PYP may be fine-tuned by modulating the flexibility of the 115 loop to activate an appropriate number of transducer proteins.