Single mutations at residues a218, a219 Of G245 WeTO Shown to have a considerable

impact on Fo-mediated proton conduction (144, 146, 170). When comparing amino acid

sequences of various mitochondria, chloroplast and bacteria, there appears to be an

instance of evolutionary covariation with these three amino acids (144). This suggests

that when a mutation occurred in one of the three residues, it was accompanied by a

second mutation to compensate for any loss in activity. This would cause the two

residues to pass through evolution as a hereditary unit. Based on this observation, double

mutants were constructed in the E. coli a subunit to imitate other lines of evolution (144,

171). Every double mutant studied resulted in functional F1Fo ATP synthase complexes

with considerably more activity than any of the single residue mutants. Due to the

functional relationship, it is possible that these three amino acids are in close proximity to

each other.

 A few other strongly conserved amino acid residues located on the fourth and fifth

transmembrane helices are worth mentioning. Residues aasp214 and agln252 were both

strongly conserve but found nonessential, with the effects of mutations at these residues

varied widely (146, 170, 172, 173). Models of the a subunit have these residues lining a

water-filled proton channel. Recently, the aqueous accessibility of residues along

transmembrane helices 2 and 5 has been shown to extend to both sides of the membrane

(174). Also, a mutation at residue 217, aala217,ar,, blocked proton conduction and

inhibited F1Fo ATP synthase activity (167).

The 8 subunit

 The E. coli 6 subunit is one of the F1 subunits. It is discussed here because it is an

essential part of the F1Fo ATP synthase stator stalk. The simplest stator stalks occur in