Proton Transport in Maize Tonoplasts Supported by Fructose-1,6-Bisphosphate Cleavage. Pyrophosphate-Dependent Phosphofructokinase as a Pyrophosphate-Regenerating System1

AUTOR(ES)

Costa dos Santos, Anelise

FONTE

The American Society for Plant Biologists

RESUMO

The energy derived from pyrophosphate (PPi) hydrolysis is used to pump protons across the tonoplast membrane, thus forming a proton gradient. In a plant's cytosol, the concentration of PPi varies between 10 and 800 μm, and the PPi concentration needed for one-half maximal activity of the maize (Zea mays) root tonoplast H+-pyrophosphatase is 30 μm. In this report, we show that the H+-pyrophosphatase of maize root vacuoles is able to hydrolyze PPi (Reaction 2) formed by Reaction 1, which is catalyzed by PPi-dependent phosphofructokinase (PFP):…Table 1 Fructose-1,6-bisphosphate (F1,6BP) + Pi ↔ PPi +Fructose-6-phosphate (F6 P) (reaction 1) PPi → 2 Pi (reaction 2) H+cyt → H+vac (reaction 3) F1,6BP + H+cyt ↔ H+vac + F6P + Pi (reaction 4) During the steady state, one-half of the inorganic phosphate released (Reaction 4) is ultimately derived from F1,6BP, whereas PFP continuously regenerates the pyrophosphate (PPi) hydrolyzed. A proton gradient (ΔpH) can be built up in tonoplast vesicles using PFP as a PPi-regenerating system. The Δ pH formed by the H+-pyrophosphatase can be dissipated by addition of 20 mm F6P, which drives Reaction 1 to the left and decreases the PPi available for the H+-pyrophosphatase. The maximal Δ pH attained by the pyrophosphatase coupled to the PFP reaction can be maintained by PFP activities far below those found in higher plants tissues.

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