A1 Refereed original research article in a scientific journal
Mutual Effects of Cationic Ligands and Substrate on Activity of the Na(+)-Transporting Pyrophosphatase of Methanosarcina mazei
Authors: Malinen AM, Baykov AA, Lahti R
Publisher: AMER CHEMICAL SOC
Publication year: 2008
Journal: Biochemistry
Journal name in source: BIOCHEMISTRY
Journal acronym: BIOCHEMISTRY-US
Volume: 47
Issue: 50
First page : 13447
Last page: 13454
Number of pages: 8
ISSN: 0006-2960
DOI: https://doi.org/10.1021/bi801803b
Abstract
The PP(i)-driven sodium pump (membrane pyrophosphatase) of Methanosarcina mazei (Mm-PPase) absolutely requires Na(+) and Mg(2+) for activity and additionally employs K(+) as a modulating cation. Here we explore relationships among Na(+), K(+), Mg(2+), and PP(i) binding sites by analyzing the dependency of the Mm-PPase PP(i)-hydrolyzing function on these ligands and protection offered by the ligands against Mm-PPase inactivation by trypsin and the SH-reagent mersalyl. Steady-state kinetic analysis of PP(i) hydrolysis indicated that catalysis involves random order binding of two Mg(2+) ions and two Na(+) ions, and the binding was almost independent of substrate (Mg(2)PP(i) complex) attachment. Each pair of metal ions, however, binds in a positively cooperative (or ordered) manner. The apparent cooperativity is lost only when Na(+) binds to preformed enzyme-Mg(2+)-substrate complex. The binding of K(+) increases, by a factor of 2.5, the catalytic constant, the Michaelis constant, and the Mg(2+) binding affinity, and these effects may result from K(+) binding to either one of the Na(+) sites or to a separate site. The effects of ligands on Mm-PPase inactivation by mersalyl and trypsin are highly correlated and are strongly indicative of ligand-induced enzyme conformational changes. Importantly, Na(+) binding induces a conformational change only when completing formation of the catalytically competent enzyme-substrate complex or a similar complex with a diphosphonate substrate analog. These data indicate considerable flexibility in Mm-PPase structure and provide evidence for its cyclic change in the course of catalytic turnover.
The PP(i)-driven sodium pump (membrane pyrophosphatase) of Methanosarcina mazei (Mm-PPase) absolutely requires Na(+) and Mg(2+) for activity and additionally employs K(+) as a modulating cation. Here we explore relationships among Na(+), K(+), Mg(2+), and PP(i) binding sites by analyzing the dependency of the Mm-PPase PP(i)-hydrolyzing function on these ligands and protection offered by the ligands against Mm-PPase inactivation by trypsin and the SH-reagent mersalyl. Steady-state kinetic analysis of PP(i) hydrolysis indicated that catalysis involves random order binding of two Mg(2+) ions and two Na(+) ions, and the binding was almost independent of substrate (Mg(2)PP(i) complex) attachment. Each pair of metal ions, however, binds in a positively cooperative (or ordered) manner. The apparent cooperativity is lost only when Na(+) binds to preformed enzyme-Mg(2+)-substrate complex. The binding of K(+) increases, by a factor of 2.5, the catalytic constant, the Michaelis constant, and the Mg(2+) binding affinity, and these effects may result from K(+) binding to either one of the Na(+) sites or to a separate site. The effects of ligands on Mm-PPase inactivation by mersalyl and trypsin are highly correlated and are strongly indicative of ligand-induced enzyme conformational changes. Importantly, Na(+) binding induces a conformational change only when completing formation of the catalytically competent enzyme-substrate complex or a similar complex with a diphosphonate substrate analog. These data indicate considerable flexibility in Mm-PPase structure and provide evidence for its cyclic change in the course of catalytic turnover.
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