‘Backlash’ and the coupling between electron transport and proton translocation in bacterial chromatophores

Energization-induced redistribution of charge carriers near membranes

The electric field arising from proton pumping across a topologically closed biological membrane causes accumulation close to the membrane of ionic charges equivalent to the charge of the pumped protons, positive on the side towards which protons are pumped, negative on the other side. We shall call this the 'active surface charge'. We here use the Poisson-Boltzmann equation to evaluate the effects of zwitterionic buffer molecules and uncharged proteins in the aqueous phase bordering the membrane on the magnitude and ionic composition of the active surface charge. For the positive side of the membrane, the main results are: (1) If the membrane is freely accessible to bulk phase ions, pumped protons exchange with these ions, such that the active surface charge consists of salt cations. (2) If a significant fraction of the ions in bulk solution consists of buffer molecules, then some of the pumped protons will remain close to the membrane and constitute a major fraction of the active surface charge. (3) If a protein layer borders the membrane, a significant part of the transmembrane electric potential difference exists within that protein layer and protons inside this layer dominate the active surface charge. (4) On the negative side of the membrane the corresponding phenomena would occur. (5) All these effects are strictly dependent on the transmembrane electric potential difference arising from proton pumping and would come in addition to the well known effects of buffers and electrically charged proteins on the retention of scalar protons. (6) No additional proton diffusion barrier may be required to account for a deficit in number of protons observed in the aqueous bulk phase upon aeration-induced proton pumping.

Analysis of mechanisms of free-energy coupling and uncoupling by inhibitor titrations: theory, computer modeling and experiments

The rates of ATP synthesis and of ATP-driven NAD reduction have been measured in bovine heart submitochondrial particles as a function of the fraction of inhibited redox pumps (in titrations with either antimycin or rotenone) and of the fraction of inhibited ATPases (in titrations with DCCD). The flux control coefficients of the redox and ATPase proton pumps on the rates of ATP synthesis and of ATP-driven NAD reduction have been derived and found to be equal to 1 for both pumps; i.e., both pumps appear to be 'completely rate limiting'. A theoretical analysis of the inhibitor titration approach based on kinetic models of chemiosmotic coupling and on the theory of metabolic control is presented. This analysis (i) shows that the results of the single inhibitor titrations are incompatible with a delocalized chemiosmotic mechanism of energy coupling if the proton conductance of the membrane is sufficiently low with respect to the conductances of the pumps; and (ii) suggests an experimental approach based on the determination of the P/O and the respiratory control ratios at different degrees of inhibition of the proton pumps to establish the origin of the 'loose coupling' of submitochondrial particle preparations. Three independent types of observation show that the 'loose coupling' of the particle preparation is not mainly due to an increased membrane proton conductance. The same and other independent observations are consistent with the view that the loose coupling of submitochondrial particle preparation is due mainly to inhomogeneity, i.e. to the presence of a subpopulation of highly leaky non-phosphorylating vesicles respiring at maximal rate. The results as a whole together with the simulations and analysis presented lead to the conclusion that the mechanism of free-energy coupling in submitochondrial particles is not completely delocalized.