Water occupancy in the Acinetobacter baumannii F-ATP synthase c-ring and its implications as a novel inhibitor target

The Acinetobacter baumannii F₁F_O-ATP synthase is essential for the opportunistic human pathogen. Its membrane-embedded F_O domain consists of the c-ring and subunit a. The c-ring translocates protons via a conserved carboxylate across the membrane via two half-channels in subunit a, and its revolution enables the F₁ domain to carry out ATP formation. Here, we used molecular dynamics simulations, free energy calculations, and in vivo mutational experiments to assess the likely existence of water molecules in the binding site of the A. baumannii c-ring. We first predicted its binding site structure in the ion-locked conformation and extrapolated the presence of two water molecules in the ion-binding site. Based on our predictions, amino acid point mutations confirmed the critical role of key residues involved in the water-binding site upon ATP synthesis ability and cell growth. We discuss the implications of our findings in the context of rational drug design to target the A. baumannii F_O domain.