Torque generation mechanism in Fo motor of ATP synthase elucidated by free-energy and Coulomb-energy landscapes along the c-ring rotation

Mechanism of proton-powered c-ring rotation in a mitochondrial ATP synthase

Proton-powered c-ring rotation in mitochondrial ATP synthase is crucial to convert the transmembrane protonmotive force into torque to drive the synthesis of adenosine triphosphate (ATP). Capitalizing on recent cryo-EM structures, we aim at a structural and energetic understanding of how functional directional rotation is achieved. We performed multi-microsecond atomistic simulations to determine the free energy profiles along the c-ring rotation angle before and after the arrival of a new proton. Our results reveal that rotation proceeds by dynamic sliding of the ring over the a-subunit surface, during which interactions with conserved polar residues stabilize distinct intermediates. Ordered water chains line up for a Grotthuss-type proton transfer in one of these intermediates. After proton transfer, a high barrier prevents backward rotation and an overall drop in free energy favors forward rotation, ensuring the directionality of c-ring rotation required for the thermodynamically disfavored ATP synthesis. The essential arginine of the a-subunit stabilizes the rotated configuration through a salt bridge with the c-ring. Overall, we describe a complete mechanism for the rotation step of the ATP synthase rotor, thereby illuminating a process critical to all life at atomic resolution.

Electrostatic Ratchet for Successive Peptide Synthesis in Nonribosomal Molecular Machine RimK

A nonribosomal peptide-synthesizing molecular machine, RimK, adds l-glutamic acids to the C-terminus of ribosomal protein S6 (RpsF) in vivo and synthesizes poly-α-glutamates in vitro. However, the mechanism of the successive glutamate addition, which is fueled by ATP, remains unclear. Here, we investigate the successive peptide-synthesizing mechanism of RimK via the molecular dynamics (MD) simulation of glutamate binding. We first show that RimK adopts three stable structural states with respect to the ATP-binding loop and the triphosphate chain of the bound ATP. We then show that a glutamate in solution preferentially binds to a positively charged belt-like region of RimK and the bound glutamate exhibits Brownian motion along the belt. The binding-energy landscape shows that the open-to-closed transition of the ATP-binding loop and the bent-to-straight transition of the triphosphate chain of ATP can function as an electrostatic ratchet that guides the bound glutamate to the active site. We then show the binding site of the second glutamate, which allows us to infer the ligation mechanism. Consistent with MD results, the crystal structure of RimK we obtained in the presence of RpsF presents an electron density that is presumed to correspond to the C-terminus of RpsF. We finally propose a mechanism for the successive peptide synthesis by RimK and discuss its similarity to other molecular machines.

The Intrinsic Radius as a Key Parameter in the Generalized Born Model to Adjust Protein-Protein Electrostatic Interaction

The generalized Born (GB) model is an extension of the continuum dielectric theory of Born solvation energy and is a powerful method for accelerating the molecular dynamic (MD) simulations of charged biological molecules in water. While the effective dielectric constant of water that varies as a function of the separation distance between solute molecules is incorporated into the GB model, adjustment of the parameters is indispensable for accurate calculation of the Coulomb (electrostatic) energy. One of the key parameters is the lower limit of the spatial integral of the energy density of the electric field around a charged atom, known as the intrinsic radius ρ. Although ad hoc adjustment of ρ has been conducted to improve the Coulombic (ionic) bond stability, the physical mechanism by which ρ affects the Coulomb energy remains unclear. Via energetic analysis of three differently sized systems, here, we clarify that the Coulomb bond stability increases with increasing ρ and that the increased stability is caused by the interaction energy term, not by the self-energy (desolvation energy) term, as was supposed previously. Our results suggest that the use of larger values for the intrinsic radii of hydrogen and oxygen atoms, together with the use of a relatively small value for the spatial integration cutoff in the GB model, can better reproduce the Coulombic attraction between protein molecules.