H⁺-transporting NADH/NADP Transhydrogenase, PntA1 (α1)/PntA2 (α2)/PntB (β).  A 3-d structure is available (Leung et al. 2015). Another structre (2.2 Å resolution) revealed conformational changes of helix positions from the previous structure solved at pH 8.5, and  internal water molecules interacting with residues implicated in proton translocation. Water flows across a narrow pore and a hydrophobic "dry" region in the middle of the membrane channel, with key residues His42alpha2 (chain A) being protonated and Thr214beta (chain B) displaying a conformational change, respectively, to gate the channel access to both cytoplasmic and periplasmic chambers. Mutation of Thr214beta to Ala deactivated the enzyme (Padayatti et al. 2017).