ATP synthases in the year 2000: Defining the different levels of mechanismand getting a grip on each

ATP synthases are unusually complex molecules, which fractionate most readi ly into two major units, one a water soluble unit called Fl and the other a detergent soluble unit called F-0. In almost all known species the F-1 uni t consists of 5 subunit types in the stoichiometric ratio alpha (3)beta (3) gamma delta epsilon while the F-0 unit contains 3 subunit types (a, b, and c) in E. coli, and at least 10 subunit types (a, b, c, and others) in highe r animals. It is now believed by many investigators that during the synthes is of ATP protons derived from an electrochemical gradient generated by an electron transport chain are directed through the F-0 unit in such a way as to drive the rotation of the single gamma subunit: which extends from an o ligomeric ring of at least 10 c subunits in F-0 through the center of F1. I t is further believed by many that the rotating gamma subunit, by interacti ng sequentially with the 3 alpha beta pairs of F-1 (360 degrees cycle) in t he presence of ADP, P-i, and Mg++, brings about via "power strokes" conform ational/binding changes in these subunits that promote the synthesis of ATP and its release on each alpha beta pair. In support of these views, studie s in several laboratories either suggest or demonstrate that F-0 consists i n part of a proton gradient driven motor while F-1 consists of an ATP hydro lysis driven motor, and that the gamma subunit does rotate during P-i funct ion. Therefore, current implications are that during ATP synthesis the form er motor drives the latter in reverse via the gamma subunit. This would sug gest that the process of understanding the mechanism of ATP synthases can b e subdivided into three major levels, which include elucidating those chemi cal and/or biophysical events involved in (1) inducing rotation of the gamm a subunit, (2) coupling rotation of this subunit to conformational/binding changes in each of the 3 alpha beta pairs, and (3) forming ATP and water (f rom ADP, P-i, and Mg++) and then releasing these products from each of the 3 catalytic sites. Significantly, it is at the final level of mechanism whe re the bond breaking/making events of ATP synthesis occur in the transition state, with the former two levels of mechanism setting the stage for this critical payoff event. Nevertheless, in order to get a better grip in this new century on how ATP synthases make ATP and then release it, we must take on the difficult challenge of elucidating each of the three levels of mech anism.