Site-specific mutagenesis replacing Glu309 and Glu771 with glutamine resulted in unusually stable ADP-insensitive phosphoenzyme intermediates, indicating their central role in energy transduction.
Glu309 and Glu771 are central to the energy-transducing conformational changes and/or activation of phosphoenzyme hydrolysis in the sarcoplasmic reticulum Ca(2+)-ATPase.
Site-specific mutagenesis was used to replace Glu309, Glu771, and Asp800 in the Ca(2+)-ATPase of rabbit fast twitch muscle sarcoplasmic reticulum with their corresponding amides. These residues are predicted to lie in the transmembrane domain and have been suggested as oxygen ligands for Ca2+ binding at high affinity sites (Clarke, D. M., Loo, T. W., Inesi, G., and MacLennan, D. H. (1989) Nature 339, 476-478). The Glu309----Gln and Asp800----Asn mutants were unable to form a phosphoenzyme from ATP at the Ca2+ concentrations examined (up to 12.5 mM), whereas the Glu771----Gln mutant phosphorylated from ATP at 2.5 mM Ca2+. In all three mutants, Ca2+ at concentrations well below 12.5 mM prevented or inhibited phosphorylation with Pi, suggesting that at least one calcium-binding site was functioning in each mutant. In the mutants Glu309----Gln and Glu771----Gln, the ADP-insensitive phosphoenzyme intermediate was unusually stable, as indicated by a very low rate of dephosphorylation observed in kinetic experiments and by an increased apparent affinity for Pi determined in equilibrium phosphorylation experiments. These data indicate a central role of Glu309 and Glu771 in the energy-transducing conformational changes and/or in the activation of phosphoenzyme hydrolysis.
Andersen et al. (1992) studied this question. Site-specific mutagenesis (Glu309, Glu771, Asp800 to amides) was evaluated on Phosphorylation from ATP and Pi, and dephosphorylation rates. Site-specific mutagenesis replacing Glu309 and Glu771 with glutamine resulted in unusually stable ADP-insensitive phosphoenzyme intermediates, indicating their central role in energy transduction.