ABSTRACT
ATP is a high energy compound that living cells utilize for driving most of their endergonic reactions. Directly or indirectly, ATP yields energy through the splitting of its terminal pyrophosphate bond. In cells, the ATP synthase of energy transducing membranes is responsible for forming from ADP and phosphate most of the ATP that cells need for survival and reproduction. The question of how the enzyme catalyzes ATP synthesis has been addressed by numerous workers for over thirty years. A fundamental discovery was that the enzyme is localized in membranes, and that the energy for ATP formation derives from electrochemical gradients built up by enzymes that catalyze electron transfer and that are localized in those membranes. However, the molecular events that take place in the H+ -ATP synthase during the transformation of the energy of electrochemical gradients into the chemical energy of ATP have not been entirely unveiled. Studies of its structure have shown that the H+ -ATP synthase is one of the most complex enzymes discovered. It has a H+ conducting multisubunit pathway and a multisubunit complex where the catalytic events in ATP synthesis take place. Moreover, it is an enzyme that is regulated by numerous and different factors, i.e., adenine nucleotides, electrochemical H+ gradients and protein-protein interactions. Studies on the mechanisms of energy transduction have shown that synthesis of ATP at the catalytic site of the enzyme is a spontaneous process; this indicates that depending on the environment ATP may be a high or a low energy compound. Thus, even though the enzyme presents many unknowns, it continues to be a source of fundamental and unsuspected aspects of basic biochemistry.