[111]. Phytochelatins are glutathione-derived peptides (poly -glutamylcystein followed by a C-terminal
glycine) [112].
PYROPHOSPHATASE Another primary proton pump, an H-PPiase (H-pyrophosphatase), func-
tions in parallel with the V-ATPase to create a proton gradient across the tonoplast [113]. The reaction is
H 4 P 2 O 7 2H 2 O←PPiase→2H 3 PO 4 (H)o(OH)iThis enzyme has not been identified in endomembranes of animal cells. The molecular mass of the H-
PPiase is 81 kDa, and it appears to comprise a single polypeptide [32]. The substrate of this enzyme is
MgHPPi or MgPPi, and it is stimulated by additional free Mg^2 [114]. The optimal pH for the H-PPi-
ase is 8.5–9.0, depending on the Mg^2 concentration [115]. The enzyme is not inhibited by vanadate or
NO 3 [116]. The proton transport by both the PPiase and the V-ATPase is regulated by the Hacross
the tonoplast. The PPiase exhibits an almost absolute requirement for Kon its cytoplasmic face [117].
On the basis of this and additional data [117], it was proposed that the PPiase serves to catalyze the co-
ordinated translocation of both Hand Kfrom the cytosol into the vacuole [113]. Under chill or hypoxic
stress, the transcript level and activity of the PPiase can increase to counter the impaired activity of the
V-ATPase as ATP levels drop and the latter enzyme dissociates [118].
- Cotransport
Cotransport is the secondary active transport [12] of a specific solute. It is coupled to transmembrane ion
gradients formed by primary active transport. Plant cells mostly employ the proton motive force as the
energy source for active solute cotransport. The general tenet of this transport is similar to that of a pul-
ley: protons are carried “downhill” (passively) across the membrane, while the other solute is carried “up-
hill” (actively). The direction of passive proton transport is from the free space or vacuole to the cyto-
plasm. The cotransport of protons and the other solute in the same direction is called symport; when
protons and the other solute are cotransported in opposite directions, the phenomenon is termed antiport
[7].
Cotransporters are perceived as membrane-embedded transport proteins [119]. Conformation
changes are supposed to expose the solute binding sites alternatively to the inside or outside. Two princi-
pal modes of cotransport were proposed [120]. According to the simultaneous model of Jauch and Läger,
a proton binds to a proton binding site when the site is exposed to the medium, where the chemical po-
tential of protons is high. This supposedly induces a conformation change, resulting in increased affinity
of the binding site for the cotransported solute. The increased affinity facilitates binding of the cotrans-
ported solute, even when its chemical potential is low. A symported solute binds on the same side of the
membrane as the proton, and an antiported one binds on the opposite side. Binding of the cotransported
solute is supposed to induce another conformation change that exposes the proton to the inside. The pro-
ton is then released inside, where the proton electrochemical potential is lower. A cotransported solute is
now also exposed to the inside and an antiported solute to the outside. The release of the proton decreases
the affinity of the binding site for the cotransported solute and it is also released, at the side of its higher
electrochemical potential (inside and outside for symport and antiport, respectively).
The stoichiometry of protons and cotransported solute differs in the various cases and is not always
known. In some cotransport systems, the number of cotransported protons equals the negative charges of
symported anions or positive charges of antiported cations. This results in electroneutral transport, ex-
clusively driven by the pH component of the proton motive force. In other cotransport systems, an ex-
cess of protons is cotransported and electrogenic transport results. The latter kind of transport is driven
by both components of the proton motive force, namely the pH and the EM.
For a known accumulation ratio of a specific cotransported ion, the minimal required number of co-
transported protons per ion can be estimated when both EMandpH are known [10]. At the steady state,
the following relation between the electrochemical potential difference of a solute jand the number (n)
of symported protons should apply [10]:
jnH or jnFpmf (18)Substitution of Eq. (2) for j, and substitution of Eq. (7) for pmf at 30°C gives
2.3RTlogaa
joij
zjF(io)nF(EM 60 pH)MINERAL NUTRIENT TRANSPORT IN PLANTS 347