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The structure of bovine F1-ATPase, crystallized in the presence of AMP-PNP and ADP, but in the absence of azide, has been determined at 1.9 Å resolution. This structure has been compared with the previously described structure of bovine F1-ATPase determined at 1.95 Å resolution with crystals grown under the same conditions but in the presence of azide. The two structures are extremely similar, but they differ in the nucleotides that are bound to the catalytic site in the βDP-subunit. In the present structure, the nucleotide binding sites in the βDP- and βTP-subunits are both occupied by AMP- PNP, whereas in the earlier structure, the βTP site was occupied by AMP-PNP and the βDP site by ADP, where its binding is enhanced by a bound azide ion. Also, the conformation of the side chain of the catalytically important residue, αArg-373 differs in the βDP- and βTP-subunits. Thus, the structure with bound azide represents the ADP inhibited state of the enzyme, and the new structure represents a ground state intermediate in the active catalytic cycle of ATP hydrolysis. [ABSTRACT FROM AUTHOR]