Electron-transfer-pathway state

high-resolution terminology - matching measurements at high-resolution

Electron-transfer-pathway state

Description

Pathway control states (or substrate control states) are obtained in mitochondrial preparations (isolated mitochondria, permeabilized cells, permeabilized tissues, tissue homogenate) by depletion of endogenous substrates and addition of specific ETS substrate types to the mitochondrial respiration medium. Mitochondrial pathway control states have to be defined complementary to mitochondrial coupling control states. Coupling states (LEAK, OXPHOS, ETS) require electron transfer system competent substrate states, including oxygen supply. Categories of SUIT protocols are defined according to ETS substrate types. » MiPNet article

Reference: Gnaiger 2009 Int J Biochem Cell Biol, Gnaiger 2014 MitoPathways, Categories of SUIT protocols

MitoPedia concepts: MiP concept, Respiratory state, SUIT concept 

MitoPedia methods: Respirometry 

Pathway control states

Gnaiger E (2016) Pathway control states. Mitochondr Physiol Network 2016-08-12.

OROBOROS (2016) MiPNet

Abstract: Pathway control states are defined complementary to coupling control states in mitochondrial physiology.

• O2k-Network Lab: AT Innsbruck Gnaiger E

Labels:

Preparation: Permeabilized cells, Permeabilized tissue, Homogenate, Isolated mitochondria, SMP 

Coupling state: LEAK, OXPHOS, ETS"ETS" is not in the list (LEAK, ROUTINE, OXPHOS, ET) of allowed values for the "Coupling states" property. 

HRR: Theory 

ETS competent pathway control states

Coupling states (LEAK, OXPHOS, ETS) require electron transfer system (ETS) competent pathway states based on external substrate supply, including sufficient oxygen supply. ETS competence of external substrates requires (i) transport of substrates across the inner mt-membrane or oxidation by dehydrogenases located on the outer face of the inner mt-membrane (e.g. glycerophosphate dehydrogenase complex, CGpDH), (ii) oxidation in the mt-matrix (TCA cycle dehydrogenases and other matrix dehydrogenases, e.g. mtGDH) or on the inner face of the inner mt-membrane (succinate dehydrogenase), (iii) oxidation of substrates without accumulation of inhibitory endproducts (e.g. oxaloacetate inhibiting succinate dehydrogenase; NADH and oxaloacetate inhibiting malate dehydrogenase), and (iv) electron transfer through the membrane-bound ETS (mETS). Endproducts must be either easily exported from the matrix across the inner mt-membrane (e.g. malate formed from succinate via fumarate), or metabolized in the TCA cycle (e.g. malate-derived oxaloacetate forming citrate in the presence of external pyruvate&malate).

Single substrate type control states

Single substrate type control states are pathway control states for selective entry of electron transfer into the Q-junction through one particular respiratory Complex; for instance N-respiration through CI (PM; GM; PGM with or without malonic acid: Gnaiger 2014 MitoPathways Chapter 3), S-respiration) through CII, CIV (Tm: MiPNet06.06_ChemicalBackground).

Further details, see Categories of SUIT protocols.

Multiple pathway control states

Multiple pathway control states are pathway control states obtained in intact cells respiring on endogenous substrates or in media with physiological exogenous substrates, or designed for reconstitution of TCA cycle function in isolated mitochondria, permeabilized cells or permeabilized tissues. In all cases, electron flow converges at the Q-junction with multiple entry sites of NS-electron transfer through CI&II, NFS through CI&II&FAO, NSGp through CI&II&GpDH.

Further details » Categories of SUIT protocols

Pathway versus kinetic substrate control

Control by substrate type: pathway control states

A: Intact cells

1. Endogenous pathway control: In intact cells, endogenous organic carbon substrates are mobilized in the cytosol as intermediary metabolites transported across the inner mitochondrial membrane and thus exerting control over mitochondrial respiration. If no organic carbon substrates are supplied in the incubation medium, then substrate control is entirely endogenous. Long-term incubation under such conditions leads to progressive depletion of endogenous substrates.
2. Exogenous pathway control: Cells are grown in complex culture media with a variety of organic carbon substrates, and different exogenous pathway control states are achieved by variation of these substrates. Long-term incubation in closed systems without exchange of culture medium leads to progressive depletion of exogenous substrates. Incubation of cells in simple media allows for sequential titration of specific carbon substrates (e.g. glucose or fructose; lactate or glutamate; fatty acids) for the study of exogenous pathway control of respiration.

B: Mitochondrial preparations

Specific substrate-inhibitor combinations are selected to establish pathway states for (i) stimulating defined segments of the electron transfer system, or (ii) reconstitution of TCA cycle function.

1. Pathway control states with electron gating: Specific substrate-inhibitor combinations are applied for selectively stimulating electron entry from N-type substrates through CI, S through CII, or other substrates feeding additional branches converging at the Q-junction, particularly F-type (fatty acid oxidation and Gp (glycerophosphate). The most commonly applied pathway states select for N-electron input through Complex I (pyruvate&malate, PM; glutamate&malate, GM), S-electron input (Complex II: succinate and rotenone, SRot), or Complex IV electron input (CIV: ascorbate&TMPD(Ama)).
2. Physiological pathway control states: Reconstitution of TCA cycle function requires NS-substrate combinations, such as PMS, GMS, GS, PGMS, or PGS, applied simultaneously without inhibitor of any respiratory complexes.

Control by substrate concentration: kinetic control states

1. Kinetic substrate or adenylate control: Kinetic studies with variation of a specific substrate (reduced substrate supplying electrons to the ETS; ADP, Pi; O₂; cytochrome c) are analyzed by kinetic functions (e.g. hyperbolic), yielding kinetic parameters, such as J_max, K_m', c₅₀ [µM], or p₅₀ [kPa].
2. Kinetic inhibitor control: Kinetic studies with variation of a specific inhibitor yield apparent kinetic constants, such as the K_I'.