Recording of blue light-induced energy and volume changes within the wild-type and mutated Phot-LOV1 domain from Chlamydomonas reinhardtii

The time-resolved thermodynamics of the flavin mononucleotide (FMN)-binding LOV1 domain of Chlamydomonas reinhardtii phot (phototropin homolog) was studied by means of laser-induced optoacoustic spectroscopy. In the wild-type protein the early red-shifted intermediate LOV715 exhibits a small volume contraction, DeltaV(715)=-1.50 ml/mol, with respect to the parent state. LOV715 decays within few mus into the covalent FMN-Cys-57 adduct LOV390, that shows a larger contraction, DeltaV(390)=-8.8 ml/mol, suggesting a loss of entropy and conformational flexibility. The high energy content of LOV390, E-390=180 kJ/mol, ensures the driving force for the completion of the photocycle and points to a strained photoreceptor conformation. In the LOV-C57S mutated protein the photoadduct is not formed and DeltaV(390) is undetected. Large effects on the measured DeltaVs are observed in the photochemically competent R58K and R58K/D31Q mutated proteins, with DeltaV(390)=-2.0 and -1.9 ml/mol, respectively, and DeltaV(715)approximate to0. The D31Q and D31N substitutions exhibit smaller but well-detectable effects. These results show that the photo-induced volume changes involve the protein region comprising Arg-58, which tightly interacts with the FMN phosphate group.