of ATP synthesis. This conformational switch of the a subunit was therefore suggested to

play a key role as a selective inhibitor of ATP hydrolysis and directional regulator of

rotational catalysis by acting as a ratchet (102).

 Movement of the two E a-helices was consistent with other observations. Changes

in the a subunit conformation due to nucleotide occupancy in the catalytic sites has been

observed in tryptic proteolysis experiments (89). Cysteine replacements in the carboxyl-

terminal a-helix (Su2) TOSulted in crosslinks with the a and P subunit (61, 103). More

importantly, treatment with a zero-length crosslinker, 1-ethyl-3 [3-dimethylamino]propyl

carbodiimide (EDC), resulted in a high yield of crosslinks between the a subunit and the

DELSEED (P380- P386) TegiOn of the P subunit (PDELSEED) following ATP hydrolysis in

the catalytic sites, but these interactions are disrupted upon the subsequent binding of

ATP. Also, in a composite structure of F1Fo ATP synthase incorporated with the E. coli

e-y complex as solved by Rodgers and Wilce, the s P-sandwich was at least 10 A+ away

from the PDELSEED TegiOn (Figure 1-3B) (101). The carboxyl-terminal Su2 prOduces

several points of interactions with the a, p and y as well as points of interactions with its

own P-sandwich domain a (61, 74, 88, 89, 92-99). In order for the Su2 to interact with

the PDELSEED, Ecul and the a subunit P-sandwich domain, it is clear from the structure that

the a subunit would be required to undergo large movements during the catalytic cycle.

 In E. coli, F1Fo ATP synthase can act in two functional directions. In the case of a

dissipated electrochemical gradient, the F1Fo complex acts primarily as an ATPase in

order to pump protons across the membrane to provide a gradient to drive various ion

transport activities in the cell. Under severe conditions where cellular ATP levels are

exceedingly low the enzyme acts predominantly in the direction of ATP synthesis.