Guanosine triphosphate and formycin triphosphate (FTP) in the presence of excess Mg2+ can bind to empty non-catalytic sites of spinach chloroplast ATPase (CF1). This results in a greatly reduced capacity for ATP hydrolysis compared to the enzyme with non-catalytic sites filled with ATP. With two GTP bound at non-catalytic sites the inhibition is about 90%; with two FTP bound about 80% inhibition is obtained. Binding and release of the nucleotides from the non-catalytic sites are relatively slow processes. Exposure of CF1 with one or two empty non-catalytic sites to 5–10 μM FTP or GTP for 15 min suffices for about 50% of the maximum inhibition. Reactivation of CF1 after exposure to higher FTP or GTP concentrations requires long exposure to 2 μM EDTA. The findings show that, contrary to previous assumptions, GTP can bind tightly to non-catalytic sites of CF1. They suggest that the presence of adenine nucleotides at non-catalytic sites might be essential for high catalytic capacity of the F1 ATPases.
