Three-color Fo¨rster resonance energy transfer within single

Catalytic activities of enzymes are associated with elastic conformational changes of the protein backbone. Fo¨rster-type resonance energy transfer, commonly referred to as FRET, is required in order to observe the dynamics of relative movements within the protein. Fo¨rster-type resonance energy transfer between two specifically attached fluorophores provides a ruler with subnanometer resolution between 3 and 8 nm, submillisecond time resolution for time trajectories of conformational changes, and single-molecule sensitivity to overcome the need for synchronization of various conformations. -ATP synthase is a rotary molecular machine which catalyzes the formation of adenosine triphosphate (ATP). The enzyme comprises a proton driven 10 stepped rotary motor connected to a 3-stepped motor, where ATP is synthesized. This mismatch of step sizes will result in elastic deformations within the rotor parts. We present a new single-molecule FRET approach to observe both rotary motors simultaneously in a single -ATP synthase at work. We labeled this enzyme with three fluorophores, specifically at the stator part and at the two rotors. Duty cycle-optimized with alternating laser excitation, referred to as DCO-ALEX, allowed to control enzyme activity and to unravel associated transient twisting within the rotors of a single enzyme during ATP hydrolysis and ATP synthesis. Monte Carlo simulations revealed that the rotor twisting is larger than 36 deg.