Mg2+ is an essential activator of hydrolytic activity of membrane-bound pyrophosphatase of Rhodospirillum rubrum.
The substrate for the hydrolytic activity of membrane-bound pyrophosphatase is the PP(i)-Mg2+ complex. The enzyme has no activity when the free Mg2+ concentration is lower than 10 microM (at 0.5 mM-PP(i)-Mg2+), and therefore free Mg2+ is an essential activator of the hydrolytic activity. The Km for the substrate changes in response to variation in free Mg2+ concentration, from 10.25 to 0.6 mM when free Mg2+ is increased from 0.03 to 1.0 mM respectively. The Km for Mg2+ depends on the substrate concentration: the Km decreases from 0.52 to 0.14 mM from 0.25 to 0.75 mM-PP(i)-Mg2+ respectively. The extrapolated Km for Mg2+ in the absence of the substrate is 0.73 mM. Imidodiphosphate-Mg2+ and free Ca2+ were used as competitive inhibitors of substrate and activator respectively. The equilibrium binding kinetics suggest an ordered mechanism for the activator and the substrate: Mg2+ ions bind the enzyme before PP(i)-Mg2+ in the formation of the catalytic complex, membrane-bound pyrophosphatase-(Mg2+)-(PP(i)-Mg2+).
Sosa
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A
Departamento de Bioenergética, Universidad Nacional Autóoma de México, D.F.
Ordaz
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H
Romero
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I
Celis
H
H
D020130:Bacterial Chromatophores / Q000201:enzymology; D004789:Enzyme Activation; D043564:Inorganic Pyrophosphatase; D007700:Kinetics; D008274:Magnesium / Q000494:pharmacology*; D011755:Pyrophosphatases / Q000378:metabolism*; D012247:Rhodospirillum rubrum / Q000201:enzymology*
D011755:Pyrophosphatases; D043564:Inorganic Pyrophosphatase; D008274:Magnesium