A Key Factor for Ultrafast Rates of Photoinduced Electron Transfer among Five Flavin Mononucleotide Binding Proteins : Effect of Negative, Positive, and Neutral Charges at Residue 13 on the Rate

Crystal structures of E13K [G1u13 with negative charge in wild type (WT) was replaced by Lys with positive charge] and E13R (G1u13 was replaced by Arg with positive charge) of flavin mononucleotide-binding protein (FMN-bp) from Desulfovibrio vulgaris (Miyazaki F) were determined by X-ray diffraction method. Ultrafast fluorescence dynamics of FMN in these proteins were measured by fluorescence up-conversion. The average lifetimes of E13K and E13R were 0.198 and 0.186 ps, which are compared to the reported lifetimes of WT, E13T (G1u13 was replaced by Thr with neutral charge) and E13Q (G1u13 was replaced by Gln with neutral charge) FMN-bp, 0.220, 0.872, and 1.10 ps, respectively. These ultrashort lifetimes are ascribed to photoinduced electron transfer (ET) from Trp32, Tyr35, and Trp106 to the excited isoalloxazine. The observed lifetimes of the five FMN-bp isoforms were simultaneously analyzed with both Marcus and Hush, and Kakitani and Mataga ET theories, in order to characterize the ET mechanisms. It was concluded that the electrostatic (ES) energy between the photoproducts and ionic charges inside the proteins is a key factor for the ET rate in the flavoproteins.