one of the catalytic sites was empty and in the latter, all three active sites were occupied with nucleotides (20, 21). Since the occupancy state of the three catalytic sites has been of considerable debate, a more accurate depiction of the Fl moiety, which would give some insight about the mechanism of ATP synthesis, must be attained from crystals obtained under physiological conditions. A more accurate depiction of the structures and roles of each individual subunits of the Fl ATPase follows. The up hexamer Homology. The a subunit of the E. coli F1Fo ATP synthase, product of the uncA gene, is the largest subunit consisting of 513 amino acids with a deduced molecular weight of 55,3 13 Da. The P subunit, a product of the uncD gene, is a 459 amino acid subunit, with a molecular weight of 50,325 Da. Based on the primary sequences, the a and p subunits of E coli Fl have the most obvious homologies in the chlororplast and mitochondrial enzymes (30). The highest conserved subunit from the E. coli F1Fo ATP synthase is the p subunit with approximately 70% homology with the chloroplast and mitochondria equivalents (31). The a subunits exhibit roughly 50% homology (31). A total of 6 nucleotide binding sites are housed at the up interfaces, three catalytic contributed primarily by the P subunit and three noncatalytic housed primarily by the a subunit (32, 33). The nucleotide binding regions have sequence homologies with other proteins that bind nucleotide or phosphate, including secA protein, N-ethylmaleimide sensitive fusion protein, herpes simplex virus UL15, Ca2+-ATPase, H /K+ ATPase and Na /K+ ATPase (34-37). Furthermore, the nucleotide binding motif, GXXXXGKT/S, known as the Walker A motif, which was first identified in the a and P sequences ofF1, has been found to be conserved in the high-resolution structures of other proteins