from the cala24 and cile28 Of the first helix (122). With the exception of Casp61 in the second

helix, both helices consist entirely of nonpolar amino acids. The first helix is greatly

enriched in glycines and alanines, which led to a smaller diameter. The a-helical

structure of the second helix is interrupted around casp61 due to disrupted hydrogen bonds

around cpro64 which cause the angle of the helical packing to change direction from there

to the carboxyl terminus (122). A recent study, using parallax analysis of fluorescence

quenching, the proton binding site casp61 WAS found to be deeply embedded in the

membrane at about 1.8 A+ from the center of the bilayer (13 1).

 Stoichiometry. The stoichiometry of c subunits would be valuable in determining

the number of protons transported per ATP synthesized and will directly relate to the P/O

ratio of oxidative phosphorylation. However, the number of c subunits in Fo had been a

matter of controversy for many years. The number of c subunits in an F1Fo ATP synthase

complex was suggested to be between 9 and 14, but whether this number fluctuated based

on the species or environmental conditions or whether it was a fixed number were the

two prevailing arguments until just a few years prior. Based on a related family of

vacuolar (V-type) ATPases, in which the proposed subunit c had evolved into a fused

dimer of four transmembrane helices with a single proton-transporting glutamate in the

center of the fourth helix, Fillingame et al. set out to genetically fuse the E. coli c subunit

by introducing a flexible loop of similar length (123). The generated c-c dimers and c-c-c

trimers resulted in functional enzyme complexes. In combination with crosslinking

studies and normalization to the a/p content of the membranes, the favored stoichiometry

was fixed to 12 c subunits per F1Fo ATP synthase complex (132). More recently, the

experiment was revised to include only trimers and tetramers of the c subunit (105).