title: Proton transport in maize tonoplasts supported by fructose-1,6-bisphosphate cleavage. Pyrophosphate-dependent phosphofructokinase as a pyrophosphate-regenerating system.
abstract: 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 microm, and the PPi concentration needed for one-half maximal activity of the maize (Zea mays) root tonoplast H+-pyrophosphatase is 30 microm. 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): 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 (DeltapH) can be built up in tonoplast vesicles using PFP as a PPi-regenerating system. The Delta 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 Delta pH attained by the pyrophosphatase coupled to the PFP reaction can be maintained by PFP activities far below those found in higher plants tissues.
authors.affiliation: Departamento de Bioquímica Médica, Instituto de Ciências Biomédicas, Universidade Federal do Rio de Janeiro, Rio de Janeiro, RJ 21941-590, Brazil.
authors.forename: Anelise
authors.initials: A
authors.lastname: Costa dos Santos
authors.forename: Wagner
authors.initials: W
authors.lastname: Seixas da-Silva
authors.forename: Leopoldo
authors.initials: L
authors.lastname: de Meis
authors.forename: Antonio
authors.initials: A
authors.lastname: Galina
mesh_terms: D001692:Biological Transport; D011756:Diphosphates / Q000378:metabolism*; D007887:Fabaceae / Q000378:metabolism; D005635:Fructosediphosphates / Q000378:metabolism*; D007700:Kinetics; D025481:Phosphofructokinase-2 / Q000378:metabolism*; D011522:Protons; D003313:Zea mays / Q000378:metabolism*
chemical_list: D011756:Diphosphates; D005635:Fructosediphosphates; D011522:Protons; D025481:Phosphofructokinase-2; C029063:fructose-1,6-diphosphate