Interaction of the catch-loop tyrosine beta Y317 with the metal at catalytic site 3 of Chlamydomonas chloroplast F1-ATPase

F1FO ATP synthase molecular motor mechanisms

The F-ATP synthase, consisting of F1 and FO motors connected by a central rotor and the stators, is the enzyme responsible for synthesizing the majority of ATP in all organisms. The F1 (αβ)3 ring stator contains three catalytic sites. Single-molecule F1 rotation studies revealed that ATP hydrolysis at each catalytic site (0°) precedes a power-stroke that rotates subunit-γ 120° with angular velocities that vary with rotational position. Catalytic site conformations vary relative to subunit-γ position (βE, empty; βD, ADP bound; βT, ATP-bound). During a power stroke, βE binds ATP (0°–60°) and βD releases ADP (60°–120°). Årrhenius analysis of the power stroke revealed that elastic energy powers rotation via unwinding the γ-subunit coiled-coil. Energy from ATP binding at 34° closes βE upon subunit-γ to drive rotation to 120° and forcing the subunit-γ to exchange its tether from βE to βD, which changes catalytic site conformations. In F1FO, the membrane-bound FO complex contains a ring of c-subunits that is attached to subunit-γ. This c-ring rotates relative to the subunit-a stator in response to transmembrane proton flow driven by a pH gradient, which drives subunit-γ rotation in the opposite direction to force ATP synthesis in F1. Single-molecule studies of F1FO embedded in lipid bilayer nanodisks showed that the c-ring transiently stopped F1-ATPase-driven rotation every 36° (at each c-subunit in the c10-ring of E. coli F1FO) and was able to rotate 11° in the direction of ATP synthesis. Protonation and deprotonation of the conserved carboxyl group on each c-subunit is facilitated by separate groups of subunit-a residues, which were determined to have different pKa’s. Mutations of any of any residue from either group changed both pKa values, which changed the occurrence of the 11° rotation proportionately. This supports a Grotthuss mechanism for proton translocation and indicates that proton translocation occurs during the 11° steps. This is consistent with a mechanism in which each 36° of rotation the c-ring during ATP synthesis involves a proton translocation-dependent 11° rotation of the c-ring, followed by a 25° rotation driven by electrostatic interaction of the negatively charged unprotonated carboxyl group to the positively charged essential arginine in subunit-a.

Interactions among gamma R268, gamma Q269, and the beta subunit catch loop of Escherichia coli F1-ATPase are important for catalytic activity

Removal of the ability to form a salt bridge or hydrogen bonds between the beta subunit catch loop (beta Y297-D305) and the gamma subunit of Escherichia coli F1Fo-ATP synthase significantly altered the ability of the enzyme to hydrolyze ATP and the bacteria to grow via oxidative phosphorylation. Residues beta T304, beta D305, beta D302, gamma Q269, and gamma R268 were found to be very important for ATP hydrolysis catalyzed by soluble F1-ATPase, and the latter four residues were also very important for oxidative phosphorylation. The greatest effects on catalytic activity were observed by the substitution of side chains that contribute to the shortest and/or multiple H-bonds as well as the salt bridge. Residue beta D305 would not tolerate substitution with Val or Ser and had extremely low activity as beta D305E, suggesting that this residue is particularly important for synthesis and hydrolysis activity. These results provide evidence that tight winding of the gamma subunit coiled-coil is important to the rate-limiting step in ATP hydrolysis and are consistent with an escapement mechanism for ATP synthesis in which alpha beta gamma intersubunit interactions provide a means to make substrate binding a prerequisite of proton gradient-driven gamma subunit rotation.