Biochemistry 2003, 42, 5877-5884
5877
Energy Transduction Optical Sensor in Skeletal Myosin† Thomas P. Burghardt,*,‡ Sungjo Park,‡ Wen-Ji Dong,§ Jun Xing,§ Herbert C. Cheung,§ and Katalin Ajtai‡ Department of Biochemistry and Molecular Biology, Mayo Foundation, 200 First Street Southwest, Rochester, Minnesota 55905, and Department of Biochemistry and Molecular Genetics, UniVersity of Alabama at Birmingham, 1530 3rd AVenue South, MCLM 490, Birmingham, Alabama 35294-2041 ReceiVed May 23, 2002; ReVised Manuscript ReceiVed January 29, 2003
ABSTRACT: The skeletal myosin cross-bridge in dynamic association with actin is the unitary energy transducer in muscle, converting free energy from ATP hydrolysis into contractile force. Myosin’s conserved ATP-sensitive tryptophan (AST) is an energy transduction optical sensor signaling transduction-related transient conformation change by modulating its fluorescence intensity amplitude and relaxation rate. Recently introduced techniques have provided the means of observing the time-resolved intensity decay from this single residue in the native protein to elucidate the mechanism of its ATP sensitivity. AST signal characteristics could be derived from local protein structure by a scenario involving interactions with excited-state tryptophan. This investigation suggests the very different possibility that hypochromism induced in the tryptophan absorption band, a ground-state effect, is a significant structural effector of optical transduction sensing. This possibility makes feasible the interpretation of the transient AST optical signal in terms of dynamical protein structure, thereby raising the empirical signal to the level of a structural determinant. Using the crystallographically based geometry from several myosin structures, the maximum calculated AST hypochromism is
