pKa of the essential Glu54 and backbone conformation for subunit c from the H+-coupled F1F0 ATP synthase from an alkaliphilic Bacillus

The conformation of the ATP synthase e-subunit and the pK(a) of its essential E54 residue were characterized in alkaliphilic Bacillus pseudofirmus OF4. The c-subunit folds as a helix-loop-helix, with inter-helical contacts demonstrated by paramagnetic relaxation effects. The E54 pK(a) of 7.7 is significantly higher than in non-alkaliphiles, which likely prevents proton loss from the e-rotor at high pH. The E54 pK(a) was unchanged in a mutant, cP51A, that has a severe ATP synthesis defect at high pH only. cP51 must have some structural role that accounts for the mutant defect, such as different subunit-subunit interactions at high pH. (C) 2004 Federation of European Biochemical Societies. Published by Elsevier B.V. All rights reserved.