Catalytic and EPR studies of the beta E204Q mutant of the chloroplast F1-ATPase from Chlamydomonas reinhardtii.

Abstract

The mutation E204Q in the beta subunit of the chloroplast F1-ATPase was made by biolistic transformation of Chlamydomonas reinhardtii. The yield of chloroplast F1-ATPase (CF1) purified from thylakoids was unaltered, suggesting that the mutation did not affect protein assembly. However, photoautotrophic growth of Chlamydomonas strains containing beta E204Q was virtually abolished, and the effect of the mutation on the light-driven ATPsynthase activity catalyzed by purified thylakoids was comparable to the change in the photoautotrophic growth rate. The loss of ATPsynthase activity in the mutant was not the result of uncoupling. Addition of wild-type CF1 to mutant thylakoids depleted of CF1 reconstituted ATPsynthase activity indicating that the mutation did not affect assembly of F0. Furthermore, the mutant CF1F0 was capable of catalyzing ATPase-dependent proton pumping as measured by fluorescence quenching of 9-amino acridine. Although the mutation significantly affected the apparent kcat/K(m) of the Mg(2+)-ATPase activity of the purified CF1-ATPase, no significant effect on the apparent kcat was observed with the mutant compared to wild-type. No significant changes in the ability of Mg2+ or Mn2+ to serve either as a cofactor or as an inhibitor of ATPase activity were observed in the mutants relative to the wild-type CF1-ATPase. EPR spectra were also taken of VO2+ bound at catalytic site 3 in its latent form. In a large fraction of the latent enzyme, a carboxyl group has displaced the nucleotide-phosphate coordination to the metal which results in the free-metal inhibited form (M3). No significant effects on the gII and AII 51V hyperfine parameters were observed between wild-type and mutant. However, the mutation increased the abundance of the M3 form relative to the M3-N3 form (metal-nucleotide-coordinated form). On the basis of these results, beta E204 is not the carboxyl group that displaces the nucleotide phosphate as a ligand to form the free-metal inhibited enzyme form which predominates in site 3 in the latent state. Instead, the data are consistent with a role in which beta E204 is essential to protonate an inorganic phosphate-oxygen to make that oxygen a good leaving group to facilitate ATP synthesis and, via this role in H-bonding, increases the abundance of the functional metal-nucleotide complex bound to the catalytic site.