Moore 1991 Plant Physiol
| Moore AL, Dry IB, Wiskich JT (1991) Regulation of electron transport in plant mitochondria under state 4 conditions. Plant Physiol 95:34-40. |
Moore AL, Dry IB, Wiskich JT (1991) Plant Physiol
Abstract: The regulation of electron transport in pea (Pisum sativum L.) leaf mitochondria under state 4 conditions has been investigated by simultaneously monitoring oxygen uptake, the steady-state reduction level of ubiquinone, and membrane potential. Membrane potentials were measured using a methyltriphenylphosphonium electrode while a voltametric technique was used to monitor changes in the steady-state reduction levels of quinone. It was found that the addition of glycine to mitochondria oxidising malate in state 4 led to a marked increase in the rate of O2 uptake and increased both the membrane potential and reduction level of the quinone pool. Increases in the state 4 respiratory rate were attributed to both an increase in driving flux, due to increased Q-pool reduction, and in membrane potential. Due to the nonohmic behavior of the inner membrane, under these conditions, an increase in potential would result in a considerable rise in proton conductance. Measurement of dual substrate oxidation, in the presence of n-propylgallate, revealed that the increase in respiratory activity was not mediated by the alternative oxidase. Similar increases in membrane potential and the level of Q-pool reduction were observed even in the presence of rotenone suggesting that the rotenone-insensitive pathway is a constitutive feature of plant mitochondria and may play a role in facilitating rapid state 4 rates even in the presence of a high energy charge.
• Bioblast editor: Gnaiger E
- Energy charge - a different definition.
Selected quotes and comments
- "The transition to state 4 conditions increases the protonmotive force which exerts a back pressure on the respiratory chain restricting the rate of electron flow and hence overall oxygen uptake."
- Comment: Protonmotive force exerts a 'back force', whereas protonmotive pressure (Gnaiger 2020 BEC MitoPathways) creates a 'back pressure'.
- "The control of respiration in plant mitochondria is somewhat more complicated than in mammalian tissues since the majority of plant mitochondria possess, albeit to varying extents, a cyanide-insensitive alternative oxidase, and a rotenone-insensitive bypass of Complex I (10). Since electron flux via these pathways is not linked to proton extrusion (21) their engagement could make a considerable contribution to the overall respiratory rate under state 4 conditions. The degree to which the antimycin-insensitive alternative oxidase contributes to ADP-limited respiration has been generally assessed from the effect of inhibitors of this pathway on respiratory control indices (16). Inhibition of the pathway results in a marked increase in control suggesting it is engaged under state 4 conditions. More recently it has been demonstrated that the degree to which this pathway is engaged is dependent, in a nonlinear fashion, upon the level of reduction of the quinone pool (12)."
- "The redox state of Q-2 was measured voltametrically using a glassy carbon working electrode and a platinum electrode connected to Ag/AgCl reference electrode. The working electrode was poised at 360 mV with respect to the reference as previously described (22)."
- Comment: Current is converted into a voltage. In this sense, the amperometric principle of the measurement of the redox state of Q-2 using the electrochemical sensor may be considered as a "voltametric" approach, although we do not encourage to use the term "potentiometric" in this context. - Compare: O2k signals and output
- matrix volume of 1.4 pL/mg protein (26).
- "Fully oxidized Q was taken as the base of the trace following addition of 1 µM Q-2 and the appropriate quantity of mitochondrial protein, and fully reduced as that in the presence of a bc1 inhibitor or upon anaerobiosis."
- "It can be seen from Figure 1 that, following transition to state 4, the reduction of the Q-pool was more pronounced than in state 2 (41 % versus 31 %) but the oxygen uptake rate was slower (68 versus 74 nmol/min/mg). This difference in rate has been attributed to the ATPase acting as an ion-influx channel, because state 2 conditions favor the release of the inhibitor protein IF, from its inhibitory site on F, facilitating increased H+-conductance (30)."
- Comment: Note that "state 2" as used in the context of Figure 1 is opposite to "state 2" defined originally by Chance and Williams (1955). - See BEC 2020.1 doi10.26124bec2020-0001.v1
- "It should be noted that since n-propylgallate (and, indeed, SHAM and disulfiram) interact with the quinone electrode, it was not possible to simultaneously monitor steady-state redox levels of the Q-pool."
- "Figures 1 and 2 suggest that a comparable membrane potential is generated by either NAD+-linked substrates or by succinate, under state 4 conditions, and yet they maintain differing levels of Q-pool reduction, confirming the idea that electron flux is limited by the quinone redox poise."
- Question: Are the terms 'redox poise', 'redox state', and 'redox eqilibrium' clearly defined, and how do the respective definitions differ?
Cited by
- Komlódi T, Cardoso LHD, Doerrier C, Moore AL, Rich PR, Gnaiger E (2021) Coupling and pathway control of coenzyme Q redox state and respiration in isolated mitochondria. Bioenerg Commun 2021.3. https://doi.org/10.26124/bec:2021-0003
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Regulation: Q-junction effect
MitoFit 2021 CoQ
