Catalysis and energy coupling of H⁺-ATPase (ATP synthase): Molecular biological approaches

The molecular biological approach has provided important information for understanding the F_oF₁ H⁺-ATPase. This article focuses on our recent results on the catalytic site in the β subunit, and the roles of α β subunit interaction and amino/carboxyl terminal interaction of the γ subunit in energy coupling. Extensive mutagenesis of the β subunit revealed that βLys-155, βThr-156, βGlu-181 and βArg-182 are essential catalytic residues. βGlu-185 is not absolutely essential, but a carboxyl residue may be necessary at this position. A pseudo-revertant analysis positioned βGly-172, βSer-174, βGlu-192 and βVal-198 in the proximity of βGly-149. The finding of the roles of βGly-149, βLys-155, and βThr-156 emphasized the importance of the glycine-rich sequence (Gly-X-X-X-X-Gly-Lys-Thr/Ser, E. coli β residues between βGly-149 and βThr-156) conserved in many nucleotide binding proteins. The A subunits of vacuolar type ATPases may have a similar catalytic mechanism because they have conserved glycine-rich and Gly-Glu-Arg (corresponding to βGly-180-βArg-182) sequences. The results of these mutational studies are consistent with the labeling of βLys-155 and βLys-201 with AP3-PL, and of βGlu-192 with DCCD [15]. The DCCD-binding residue of a thermophilic Bacillus corresponds to βGlu-181, an essential catalytic residue discussed above. The defective coupling of the βSer-174 → Phe mutant was suppressed by the second mutation αArg-296 → Cys, indicating the importance of α β interaction in energy coupling. The γ subunit, especially its amino/carboxyl interaction, seems to be essential for energy coupling between catalysis and transport judging from studies on γMet-23 → Lys or Arg mutation and second-site mutations which suppressed the γLys-23 mutation. Thus the conserved γMet-23 is not absolutely essential but is located in the important region for amino/carboxyl interaction for energy coupling.