Structural Change of Site-Directed Mutants of PYP: New Dynamics during pR State

Abstract

The energetics, protein dynamics, and diffusion coefficients of three mutants of photoactive yellow protein, R52Q, P68A, and W119G, were studied by the transient grating and pulsed laser-induced photoacoustic method. We observed a new dynamics with a lifetime of ∼1 μs in the transient grating signal, which is silent by the light absorption technique. This fact indicates that, after the structure change around the chromophore is completed (pR 1), the protein part located far from the chromophore is still moving to finally create another pR (pR 2) species, which can transform to the next intermediate, pB. Although the kinetics of pR 2→pB→pG are very different depending on the mutants, the enthalpies of the first long-lived (in microseconds, 100- μs range) intermediate species (pR 2) are similar and very high for all mutants. The diffusion coefficients of the parent (pG) and pB species of the mutants are also similar to that of the wild-type photoactive yellow protein. From the temperature dependence of the volume change, the difference in the thermal expansion coefficients taken as indicator of the flexibility of the structure between pG and pR 2 is measured. They are also similar to that of the wild-type photoactive yellow protein. These results suggest that the protein structures of pR 2 and pB in these mutants are globally different from that of pG, and this structural change is not altered so much by the single amino acid residue mutation. This is consistent with the partially unfolded nature of these intermediate species. On the other hand, the volume changes during pR 1→pR 2 are sensitive to the mutations, which may suggest that the volume change reflects a rather local character of the structure, such as the chromophore-protein interaction.