(discussed in "F1Fo ATP Synthase Mechanism" below) Dicyclohexylcarbodiimide

(DCCD) reacts specifically with casp61, blocking proton translocation, and this reaction is

blocked by the mutationS Cala24-ser Of Cile28-thr, Suggesting that the c subunit is folded in

such a way so that the asp61 of the second helix is in close vicinity to residues 24 and 28

of the first helix (109, 110). This model was further supported by the ability to move the

critical aspartate from residue 61 in the second helix to residue 24 in the first helix

without disruption of enzyme function (1 11). Also, only one subunit in the ring of 10 c

subunits need be modified by DCCD to inhibit activity, indicating that each one of the c

subunits is consecutively involved in proton translocation (112, 113). Furthermore,

modification of the c subunit by DCCD trapped the configuration of the a subunit

(discussed above), providing evidence for a connection between the c and a subunits (89).

 Intersubunit contacts made by the c subunit are evident from mutational data and

gives some insight to the topology. Mutations constructed in the polar loop region can

disrupt the binding of F1 to Fo (114-117). Three conserved amino acids, carg41-Cgln42-Cpro43,

lie at the apex of the polar loop region and are predicted to interact with the F1 s subunit

(98, 116). F1Fo ATP synthase complexes with the uncoupling mutation, Cgln42-glu, were

found to be recoupled with a second site suppressor mutation in the a subunit of F1,

Sglu31-gly, val, or cys (98) and was shortly followed by the observance of disulfide bridge

formation between the c subunit and the s and y subunits (99, 118). Also, switching the

essential aspartate from residue 61 in the second helix of the c subunit to residue 24 of

the first helix (discussed above) resulted in a functional F1Fo ATP synthase complex

though the cells were not as healthy compared to cells containing a wild-type enzyme

complex (111). Eighteen third-site suppressor mutants were found that helped to