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BIOENERGETICS AND PHOTOSYNTHESIS

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

Departamento de Bioquímica Médica, Instituto de Ciências Biomédicas, Universidade Federal do Rio de Janeiro, Rio de Janeiro, 21941–590, RJ, Brazil

The energy derived from pyrophosphate (PP_i) hydrolysis is used to pump protons across the tonoplast membrane, thus forming a proton gradient. In a plant's cytosol, the concentration of PP_i varies between 10 and 800 µM, and the PP_i 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 PP_i (Reaction 2) formed by Reaction 1, which is catalyzed by PP_i-dependent phosphofructokinase (PFP):

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 (PP_i) hydrolyzed. A proton gradient (pH) can be built up in tonoplast vesicles using PFP as a PP_i-regenerating system. The 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 PP_i available for the H⁺-pyrophosphatase. The maximal pH attained by the pyrophosphatase coupled to the PFP reaction can be maintained by PFP activities far below those found in higher plants tissues.

¹ This work was supported by Programa de Apoio a Núcleos de Excelência-PRONEX-Financiadora de Estudos e Projetos (grant), by Conselho Nacional de Desenvolvimento Científico e Tecnológico (grant and fellowship to A.C.d.S.), and by Fundação de Amparo à Pesquisa do Estado do Rio de Janeiro (grant and fellowship to W.S.d.-S.).

^* Corresponding author; e-mail galina{at}bioqmed.ufrj.br; fax 55–21–2270–8647.

Received May 8, 2003; returned for revision June 20, 2003; accepted June 27, 2003.

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