Directed mutagenesis of the strongly conserved aspartate 242 in the beta-subunit of Escherichia coli proton-ATPase.

Abstract

Oligonucleotide-directed mutagenesis was used to substitute Asn or Val for residue Asp-242 in the beta-subunit of Escherichia coli F1-ATPase. Asp-242 is strongly conserved in beta-subunits of F1-ATPase enzymes, in a region of sequence which shows homology with numerous nucleotide-binding proteins. By analogy with adenylate kinase (Fry, D.C., Kuby, S.A., and Mildvan, A.S. (1986) Proc. Natl. Acad. Sci. U.S.A. 83, 907-911), beta-Asp-242 of F1-ATPase might participate in catalysis through electrostatic effects on the substrate Mg2+ or through hydrogen bonding to the substrate(s); an acid-base catalytic role is also plausible. The substitutions Asn and Val were chosen to affect the charge, hydrogen-bonding ability, and hydrophobicity of residue beta-Asp-242. Both mutations significantly impaired oxidative phosphorylation rates in vivo and membrane ATPase and ATP-driven proton-pumping activities in vitro. Asn-242 was more detrimental than Val-242. Purified soluble mutant F1-ATPases had normal molecular size and subunit composition, and displayed 7% (beta-Asn-242) and 17% (beta-Val-242) of normal specific Mg-ATPase activity. The relative MgATPase activities of both mutant enzymes showed similar pH dependence to normal. Relative MgATPase and CaATPase activities of normal and mutant enzymes were compared at widely varied pMg and pCa. The mutations had little effect on KM MgATP, but KM CaATP was reduced. The data showed that the carboxyl side-chain of beta-Asp-242 is not involved in catalysis either as a general acid-base catalyst or through direct involvement in any protonation/deprotonation-linked mechanism, nor is it likely to be directly involved in liganding to substrate Mg2+ during the reaction. Specificity constants (kcat/KM) for MgATP and CaATP were reduced in both mutant enzymes, showing that the mutations destabilized interactions between the catalytic nucleotide-binding domain and the transition state.