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. 1992 Dec;100(4):1698–1705. doi: 10.1104/pp.100.4.1698

Kinetics of the Vacuolar H+-Pyrophosphatase 1

The Roles of Magnesium, Pyrophosphate, and their Complexes as Substrates, Activators, and Inhibitors

Roger A Leigh 1,2, Andrew J Pope 1,2,2, Ian R Jennings 1,2, Dale Sanders 1,2
PMCID: PMC1075853  PMID: 16653186

Abstract

The responses of the vacuolar membrane (tonoplast) proton-pumping inorganic pyrophosphatase (H+-PPase) from oat (Avena sativa L.) roots to changes in Mg2+ and pyrophosphate (PPi) concentrations have been characterized. The kinetics were complex, and reaction kinetic models were used to determine which of the various PPi complexes were responsible for the observed responses. The results indicate that the substrate for the oat root vacuolar H+-PPase is Mg2PPi and that this complex is also a non-competitive inhibitor. In addition, the enzyme is activated by free Mg2+ and competitively inhibited by free PPi. This conclusion differs from that reached in previous studies, in which it was proposed that MgPPi is the substrate for plant vacuolar H+-PPases. However, models incorporating MgPPi as a substrate were unable to describe the kinetics of the oat H+-PPase. It is demonstrated that models incorporating Mg2PPi as the substrate can describe some of the published kinetics of the Kalanchoë daigremontiana vacuolar H+-PPase. Calculations of the likely concentrations of Mg2PPi in plant cytoplasm suggest that the substrate binding site of the oat vacuolar H+-PPase would be about 70% saturated in vivo.

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Selected References

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