ISSN:
1432-1424
Keywords:
Chara
;
Cl−
;
cotransport
;
reaction kinetic model
;
pH
;
kinetics
Source:
Springer Online Journal Archives 1860-2000
Topics:
Biology
,
Chemistry and Pharmacology
Notes:
Summary H+-coupled transport in plant and fungal cells is relatively insensitive to external pH (pH o ). H+-coupled Cl− transport at the plasma membrane ofChara corallina was studied to explore the phenomena responsible for this insensitivity. Raising pH o from a control value of 7.5 to 9.0 results in a modest (2.5-fold) decline inJ max and increase inK m . Further increase in pH o results in a selective increase inJ max, in accordance with predictions from a reaction kinetic model of the transport system (Sanders, D., Hansen, U.-P., 1981.J. Membrane Biol. 58:139–153). Increase in cytosolic Cl− concentration ([Cl−] c ) also results in a selective decrease inJ max at pH o =7.5. Quantitative kinetic modeling of the results is not possible if it is assumed that the sole effect of pH o isvia mass action on the binding of external H+ to a transport site. If, instead, the dependence of cytosolic pH (pH c ) on pH o (Smith, F.A., 1984,J. Exp. Bot. 35:1525–1536) is taken into account along with the dependence of Cl− influx on pH c (Sanders, D., 1980,J. Membrane Biol. 53:129–141), then the observed modest changes in Michaelis parameters can be accommodated by a reaction kinetic model. The quantitative parameters of the model yield respective pK a s of the internal and external H+-binding sites=7.85 and 7.2, respective dissociation constants of the internal and external Cl−-binding sites=160 and 40 μm, and an additional, kinetically transparent, H+-binding site with a pK a 〉8.0. The quantitative model independently predicts the response ofJ max andK m to acidic conditions. The results are discussed in terms of the general physiological requirement that fluxes through H+-coupled transport systems are relatively insensitive to environmental variation in pH o . It is proposed that (i) the weak (but finite) dependence of pH c on pH o , coupled with (ii) the strong dependence of H+-coupled transport on pH c are instrumental in endowing H+-coupled transport systems with a relative insensitivity to variation in pH o . This hypothesis might also explain why pH c in plants and fungi is not acutely controlled with respect to variation of pH o .
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF01871740
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