Description
State 3 respiration is the ADP stimulated respiration of isolated coupled mitochondria in the presence of high ADP and Pi concentrations, supported by a defined substrate or substrate combination at saturating oxygen levels (Chance and Williams, 1955). State 3 respiration can also be induced in permeabilized cells, including permeabilized tissue preparations and tissue homogenates. ADP concentrations applied in State 3 are not necessarily saturating, whereas OXPHOS capacity is measured at saturating concentrations of ADP and Pi (state P). For instance, non-saturating ADP concentrations are applied in State 3 in pulse titrations to determine the P/O ratio in State 3β4 (DβT) transitions, when saturating ADP concentrations would deplete the oxygen concentration in the closed oxygraph chamber before State 4 is obtained (Gnaiger et al 2000; Puchowicz et al 2004). Respiration in the OXPHOS state or in State 3 is partially coupled, and partially uncoupled (physiological) or partially dyscoupled (pathological). A high mt-membrane potential provides the driving force for oxidative phosphorylation, to phosphorylate ADP to ATP and to transport ADP and ATP across the inner mt-membrane through the adenine nucleotide translocase (ANT). The mt-membrane potential is reduced, however, in comparison to the LEAK state of respiration, whereas the cytochromes are in a more oxidized redox state.
Abbreviation: P
Reference: Chance 1955 JBC-III, Gnaiger 2014 MitoPathways
MitoPedia concepts:
Respiratory state
Maximum oxygen flux in State 3 ?
Oxygen flux is not necessarily 'maximum' in State 3 for different reasons:
- Concentrations of ADP and inorganic phosphate (Pi) are high but may not be saturating (kinetic limitation).
- The substrate or substrate combination may not support the physiological maximum flux through the electron transfer system (ETS), particularly due to electron gating of mitochondrial pathways converging at the Q-junction (limitation due to substrate control).
- OXPHOS capacity is less than ETS capacity in coupled mitochondria with limiting phosphorylation system capacity (limitation by the enzymatic capacity of utilizating the electrochemical proton gradient).