748-Pos Structural Impact Of Myosin Methionine Oxidation Jennifer C. Klein, Nicole Piechowski, Margaret A. Titus, David D. Thomas. Univeristy of Minnesota, Minneapolis, MN, USA. We have examined the structural and functional consequences of methionine (Met) oxidation in Dictyostelium (Dicty) myosin II using a three-pronged approach that includes Met mutagenesis, site-directed spectroscopy, and molecular dynamics simulations. Protein oxidation by reactive oxygen species (ROS) is a critical element of cell function, but in the context of oxidative stress, has been implicated in disease progression and biological aging. Our goal is to bridge our understanding of protein oxidation and muscle dysfunction with molecularlevel insights into actomyosin interaction. A Cys-lite version of Dicty myosin II serves as our model system for examining site-specific Met oxidation. Peroxide treatment to mimic oxidative stress induced a two-fold decline in Vmax and KATPase for actin-activation, consistent with the decline in actomyosin interaction observed for biologically aged or peroxide-treated skeletal myosin. We tested the oxidation sensitivity of previously characterized myosin labeling sites in the force-producing region and actin-binding interface and found that spin label mobility and distance measurements in the actin-binding cleft are particularly sensitive to Met oxidation, but only in the presence of actin. Moreover, we conclude that the oxidation-induced structural change in myosin includes a redistribution of structural states involved in the weak to strong actin-binding transition, the step associated with muscle force production. Site-specific Met substitutions combined with functional measurements have allowed us to pinpoint which Met is responsible for the observed structural change. Lastly, we will examine Met oxidation in silico to gain mechanistic knowledge of how residue-specific oxidation translates into changes in both local and global myosin structural dynamics.