Covalent modification of the catalytic sites of the H +-ATPase from chloroplasts, CF 0F 1, with 2-azido-[α- 32P]ADP: modification of the catalytic site 2 (loose) and the catalytic site 3 (open) impairs multi-site, but not uni-site catalysis of both ATP synthesis and ATP hydrolysis

Abstract

The H +-ATPase from chloroplasts, CF 0F 1, was isolated and purified. The enzyme contained one endogenous ADP at a catalytic site, and two endogenous ATP at non-catalytic sites. Incubation with 2-azido-[α- 32P]AD(T)P leads to a tight binding of the azido-nucleotides. Free nucleotides were removed by three consecutive passages through centrifugation columns, and after UV-irradiation, the label was covalently bound. The labelled enzyme was digested by trypsin, the peptides were separated by ion exchange chromatography into nitreno-AMP, nitreno-ADP and nitreno-ATP labelled peptides, and these were then separated by reversed phase chromatography. Amino acid sequence analysis was used to identify the type of the nucleotide binding site. After incubation with 2-azido-[α- 32P]ADP, the covalently bound label was found exclusively at β-Tyr-362, i.e. binding occurs only to catalytic sites. Incubation conditions with 2-azido-[α- 32P]ADP were varied, and conditions were found which allow selective binding of the label to different catalytic sites, either to catalytic site 2 or to catalytic site 3. For measurements of the degree of inhibition by covalent modification, CF 0F 1 was reconstituted into phosphatidylcholine liposomes, and the membranes were energised by an acid-base transition in the presence of a K +/valinomycin diffusion potential. The rate of ATP synthesis was 120 s −1, and the rate of ATP hydrolysis was 20 s −1, both measured under multi-site conditions. Covalent modification of either catalytic site 2 or catalytic site 3 inhibited both ATP synthesis and ATP hydrolysis, the degree of inhibition being proportional to the degree of modification. Extrapolation to complete inhibition indicates that modification of one catalytic site, either site 2 or site 3, is sufficient to completely block multi-site ATP synthesis and ATP hydrolysis. The rate of ATP synthesis and the rate of ATP hydrolysis were measured as a function of the substrate concentration from multi-site to uni-site conditions with covalently modified CF 0F 1 and with non-modified CF 0F 1. The result was that uni-site ATP synthesis and ATP hydrolysis were not inhibited by covalent modification of either catalytic site 2 or site 3. The results indicate cooperative interactions between catalytic nucleotide binding sites during multi-site catalysis, whereas neither uni-site ATP synthesis nor uni-site ATP hydrolysis require interaction with other sites.