Characterization of sodium and proton flows in sub-bacterial particles of Halobacterium halobium in terms of nonequilibrium thermodynamics

Abstract

A thermodynamic characterization of the Na +-H + exchange system in Halobacterium halobium was carried out by evaluating the relevant phenomenological parameters derived from potential-jump measurements. The experiments were performed with sub-bacterial particles devoid of the purple membrane, in 1 M NaCl, 2 M KCl, and at pH 6.5–7.0. Jumps in either pH or pNa were brought about in the external medium, at zero electric potential difference across the membrane, and the resulting relaxation kinetics of protons and sodium flows were measured. It was found that the relaxation kinetics of the proton flow caused by a pH-jump follow a single exponential decay, and that the relaxation kinetics of both the proton and the sodium flows caused by a pNa-jump also follow single exponential decay patterns. In addition, it was found that the decay constants for the proton flow caused by a pH-jump and a pNa-jump have the same numerical value. The physical meaning of the decay constants has been elucidated in terms of the phenomenological coefficients (mobilities) and the buffering capacities of the system. The phenomenological coefficients for the Na +-H + flows were determined as differential quantities. The value obtained for the total proton permeability through the particle membrane via all available channels, L H = ( ∂J H + ∂Δ pH) Δψ,Δ pNa , was in the range of 850–1150 nmol H +·(mg protein) −1·h −1·(pH unit) −1 for four different preparations; for the total Na + permeability, L Na = ( ∂J Na + ∂Δ pNa) Δψ,Δ pH , it was 1620–2500 nmol Na +·(mg protein) −1·h −1·(pNa unit) −1; and for the proton ‘cross-permeability’, L HNa = ( ∂J H + ∂Δ pNa) Δψ,Δ pH , it was 220–580 nmol H +·(mg protein) −1·h −1·(pNa unit) −1, for different preparations. From the above phenomenological parameters, the following quantities have been calculated: the degree of coupling ( q), the maximal efficiency of Na +-H + exchange ( η max), the flow and force efficacies (ϵ) of the above exchange, and the admissible range for the values of the molecular stoichiometry parameter ( r). We found q ⩽ 0.4; η max ⩽ 5%; 0.36 ⩽ r ⩽ 2; ϵ J Na + ⩽ 1.3 · 10 5 μmol · ( RT unit) −1 at J Na = 1 μmol Na + · ( mg protein) −1 · h −1; and ϵ ΔpNa ⩽ 5 · 10 4 ΔpNa · (mg protein) · h · (RT unit) −1 at ΔpNa = 1 unit, for different preparations.