CaATP inhibition of the MgATP-dependent proton pump (H+-ATPase) in bacterial photosynthetic membranes with a mechanism of alternative substrate inhibition |
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Authors: | Rita Casadio B A Melandri |
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Institution: | (1) Laboratory of Biochemistry and Biophysics, Department of Biology, Via Irnerio 42, I-40126 Bologna, Italy, IT |
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Abstract: | The membrane-bound F1 sector of the H+–ATPase complex (F-type ATPase) in dark-adapted photosynthetic chromatophores is endowed with MgATP- and CaATP-dependent ATPase
activities, both sensitive to inhibitors such as oligomycin and venturicidin. Because of contatamination of free Mg2
+ and Ca2+ ions in chromatophore preparations, kinetic characterization of the two hydrolitic reactions can be performed only in the
presence of both substrates, using a model for two alternative substrates. The two activities are characterized by similar
maximal rates and affinity constants VMgATP and VCaATP: 13±1 and 10±1 nmol s–1 ATP hydrolyzed (μmol BChl)–1; KMgATP and KCaATP: 0.22±0.06 and 0.20±0.05 mm]. However, only the MgATP-dependent ATPase is coupled to Δ*H
+ generation. In this process CaATP acts as an alternative substrate and a competitive inhibitor of the proton pump, with a
KI coincident with KCaATP for the hydrolytic activity. This finding highlights the central role that the coordination chemistry of the ion-nucleotide
complex plays in determining the proton gating mechanism at the catalytic site(s) of the enzyme complex. These results are
discussed on the basis of the coordination properties of the ions and of the available information on the protein structure.
Received: 5 December 1995 / Accepted: 7 March 1996 |
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Keywords: | H+ ATPase Enzyme kinetics Alternative substrate inhibition Proton pump Energy coupling |
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