scholarly journals A ‘proton ratchet’ for coupling the membrane potential to protein transport

2019 ◽  
Author(s):  
William J. Allen ◽  
Robin A. Corey ◽  
Daniel W. Watkins ◽  
Gonçalo C. Pereira ◽  
A. Sofia F. Oliveira ◽  
...  
Keyword(s):  
Membrane Potential ◽  
Protein Transport ◽  
Negative Charge ◽  
Protein A ◽  
Proton Motive Force ◽  
Electrochemical Gradient ◽  
Mitochondrial Import ◽  
Ratchet Mechanism ◽  
Energy Conserving ◽  
Sec System

AbstractThe proton-motive force (PMF) – the electrochemical gradient of protons across energy-conserving membranes – powers protein transport in bacteria, mitochondria and chloroplasts. Here, we propose a ‘proton ratchet’ mechanism for this process. In the Sec system of bacteria, protons are stripped from lysine side chains of the pre-protein at the cytosolic face of the plasma membrane, then replaced on the exterior, aided by the pH component of the PMF (ΔpH; acidic outside). This gives the translocating region of pre-protein a net negative charge, promoting electrophoretic diffusion across the membrane driven by membrane-potential (ΔΨ; positive outside). For mitochondrial import (through the TIM23 complex) the proton ratchet acts in the opposite direction, with negatively charged residues protonated for passage across the inner membrane into the negative matrix. The proton ratchet is an elegant solution for coupling the PMF to transport, likely to be used by a range of other transporters of charged molecules.

scholarly journals Membrane potential of mitochondria in intact lymphocytes during early mitogenic stimulation

1984 ◽  
Vol 217 (2) ◽  
pp. 453-459 ◽  
Author(s):  
M D Brand ◽  
S M Felber
Keyword(s):  
Membrane Potential ◽  
Mitochondrial Membrane Potential ◽  
Concanavalin A ◽  
Mitochondrial Membrane ◽  
Ph Gradient ◽  
Electrochemical Gradient ◽  
Inner Membrane ◽  
Mitogenic Stimulation ◽  
Ion Movements

The mitochondrial membrane potential (delta psi m) in intact lymphocytes was calculated by measuring the distribution of radiolabelled methyltriphenylphosphonium cation. The value obtained was 120 mV. The pH gradient across the mitochondrial membrane in situ (delta pH m) was estimated to be 73 mV (1.2 pH units). Thus the electrochemical gradient of protons was about 190 mV. Addition of the mitogen concanavalin A did not alter delta psi m, showing that, if movement of Ca2+ across the inner membrane of lymphocyte mitochondria occurs when concanavalin A is added, it is accompanied by charge-compensating ion movements.

The generation of electric currents in cardiac fibers by Na/Ca exchange

1979 ◽  
Vol 236 (3) ◽  
pp. C103-C110 ◽  
Author(s):  
L. J. Mullins
Keyword(s):  
Membrane Potential ◽  
Duty Cycle ◽  
Action Potentials ◽  
Reversal Potential ◽  
Resting Membrane Potential ◽  
Electrochemical Gradient ◽  
Ca Current ◽  
Cardiac Action ◽  
Cardiac Fiber ◽  
Cardiac Fibers

The presence of a detectable Ca current during the excitation of a cardiac fiber implies that the Ca lost during the resting interval of the duty cycle must also be detectable. Ca outward movement appears to be effected by Na/Ca exchange when more Na enters than Ca leaves per cycle, thus making the mechanism electrogenic. Since Na/Ca exchange can move Ca either inward or outward depending on the direction of the electrochemical gradient for Na, a potential exists where there is no electric current generated by the Na/Ca exchange mechanism, i.e., a reversal potential ER. Cardiac fibers appear to have a reversal potential that is about midway between their resting membrane potential and their plateau. Carrier currents both inward and outward are therefore generated during cardiac action potentials. The implications of the conditions stated above are explored.

Total contents and net movements of magnesium in the rat uterus

1971 ◽  
Vol 220 (6) ◽  
pp. 2067-2067
Author(s):  
A. H. Moawad ◽  
E. E. Daniel
Keyword(s):  
Membrane Potential ◽  
Active Transport ◽  
Extracellular Space ◽  
Transport Process ◽  
Electrochemical Gradient ◽  
Rat Uterus ◽  
Active Transport Process

Page 75: A. H. Moawad and E. E. Daniel. "Total contents and net movements of magnesium in the rat uterus." Page 80, column 2, line 44, involving the calculation of Vm the answer to the equation, –0.067 V, should read, "–0.012 V." Page 80, column 2, lines 49–54 should read, "The calculated magnesium equilibrum potential is less than the observed membrane potential, which is about 0.050 V. Therefore, some of the tissue magnesium may be excluded by an active transport process against an electrochemical gradient or by loose binding in the extracellular space."

Proton motive force as a function of the pH at which Methanobacterium bryantii is grown

1985 ◽  
Vol 31 (11) ◽  
pp. 1031-1034 ◽  
Author(s):  
G. Dennis Sprott ◽  
Sharon E. Bird ◽  
Ian J. McDonald
Keyword(s):  
Membrane Potential ◽  
Proton Motive Force ◽  
Motive Force ◽  
Ph Gradient ◽  
Alkaline Media ◽  
Cytoplasmic Ph ◽  
Acidic Media ◽  
Generation Times ◽  
Optimum Ph ◽  
Ph Range

Methanobacterium bryantii was grown on CO2 and H2 over a pH range between the extremes of 5.0 and 8.1. Generation times were shortest between pH 6.6 and 7.1. Cells grown at optimum pH had a proton motive force consisting predominantly of the membrane potential but those grown at nonoptimal pH generated a transmembrane pH gradient as well. This pH gradient was, however, insufficient to maintain a constant cytoplasmic pH during growth in very acidic or basic media. The results suggest that in acidic media growth may be limited by the cytoplasmic pH and that in alkaline media it may be limited by the cytoplasmic pH and (or) by the magnitude of the proton motive force.

Acute reductions in PO2 depolarize pulmonary artery endothelial cells and decrease [Ca2+]i

1994 ◽  
Vol 266 (4) ◽  
pp. H1416-H1421 ◽  
Author(s):  
T. Stevens ◽  
D. N. Cornfield ◽  
I. F. McMurtry ◽  
D. M. Rodman
Keyword(s):  
Endothelial Cells ◽  
Membrane Potential ◽  
Pulmonary Artery ◽  
Driving Force ◽  
Primary Cultures ◽  
K Channel ◽  
Channel Blockers ◽  
Electrochemical Gradient ◽  
Similar Reduction ◽  
Fura 2

Whereas pulmonary artery endothelial cells (PAECs) are sensitive to oxygen, neither the effect of an acute reduction in PO2 on PAEC membrane potential nor its effect on intracellular free Ca2+ ([Ca2+]i) is known. We hypothesized that in confluent primary cultures of PAECs, an acute decrease in PO2 would depolarize the cell membrane, inhibit Ca2+ influx, and reduce [Ca2+]i. To test this hypothesis, the membrane-sensitive fluorophore bis (1,3-dibutylbarbituric acid) trimethine oxonol (DiBAC4, 1 microM) and [Ca2+]i-sensitive probe fura 2 (3 microM) were used. A decrease in PO2 from 125 to 35 mmHg caused membrane depolarization and a 60 +/- 8% (data are means +/- SE) reduction in Ca2+ influx, estimated by manganese quenching of fura 2 fluorescence. While basal [Ca2+]i was 79 +/- 5 nM in normoxic cells, it decreased to 31 +/- 2 nM after 15 min of hypoxia. Decreasing the electrochemical gradient for Ca2+ entry with either low extracellular Ca2+, the K+ channel blockers tetraethylammonium or charybdotoxin, or blockade of Ca2+ entry with lanthanum decreased [Ca2+]i by 54-71% of that observed during an acute reduction in PO2. These results demonstrate that an acute reduction in PO2 1) depolarizes PAECs, 2) reduces Ca2+ influx, and 3) decreases [Ca2+]i, and that a similar reduction in [Ca2+]i was observed with interventions designed to reduce the electrochemical driving force for Ca2+ entry.

Mechanisms by which mitochondria transport calcium

1990 ◽  
Vol 258 (5) ◽  
pp. C755-C786 ◽  
Author(s):  
T. E. Gunter ◽  
D. R. Pfeiffer
Keyword(s):  
Membrane Potential ◽  
Mitochondrial Membrane ◽  
Permeability Transition ◽  
Electrochemical Gradient ◽  
Matrix Space ◽  
Available Energy ◽  
Outward Transport ◽  
Wide Range ◽  
The Matrix ◽  
Efflux Mechanism

It has been firmly established that the rapid uptake of Ca2+ by mitochondria from a wide range of sources is mediated by a uniporter which permits transport of the ion down its electrochemical gradient. Several mechanisms of Ca2+ efflux from mitochondria have also been extensively discussed in the literature. Energized mitochondria must expend a significant amount of energy to transport Ca2+ against its electrochemical gradient from the matrix space to the external space. Two separate mechanisms have been found to mediate this outward transport: a Ca2+/nNa+ exchanger and a Na(+)-independent efflux mechanism. These efflux mechanisms are considered from the perspective of available energy. In addition, a reversible Ca2(+)-induced increase in inner membrane permeability can also occur. The induction of this permeability transition is characterized by swelling of the mitochondria, leakiness to small ions such as K+, Mg2+, and Ca2+, and loss of the mitochondrial membrane potential. It has been suggested that the permeability transition and its reversal may also function as a mitochondrial Ca2+ efflux mechanism under some conditions. The characteristics of each of these mechanisms are discussed, as well as their possible physiological functions.

scholarly journals Is Myelin a Mitochondrion?

2012 ◽  
Vol 33 (1) ◽  
pp. 33-36 ◽  
Author(s):  
Julia J Harris ◽  
David Attwell
Keyword(s):  
Membrane Potential ◽  
Respiratory Chain ◽  
Atp Synthase ◽  
Proton Motive Force ◽  
Motive Force ◽  
Myelin Membrane ◽  
Considerable Doubt

It has been hypothesized that myelin acts like a mitochondrion, generating ATP across the membranes of its sheath. By calculating the proton motive force across the myelin membrane based on known values for the pH and membrane potential of the oligodendrocyte, we find that insufficient energy could be harvested from proton flow across the myelin membrane to synthesize ATP. In fact, if the respiratory chain were present in the myelin membrane, then the ATP synthase would function in reverse, hydrolyzing rather than synthesizing ATP. This calculation places the hypothesis of an energy-producing role for myelin in considerable doubt.

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