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::::* See [[Complex_II_ambiguities#CII_and_fatty_acid_oxidation]]
== Selected quotes and comments ==
[[Gnaiger E]] (2023-09-07)
::::* VLCAD is a homodimer containing a FAD cofactor that is the first step in the mitochondrial matrix for oxidation of the acyl-CoA substrate.
::::* The FAO-generated NADH and QH<sub>2</sub> are potentially exposed to oxidation in the reactive environment of the mitochondrial matrix and rely on safe transfer of electron-reducing equivalents from NADH and QH<sub>2</sub> to ETC to generate ATP.
::::::''Comment: If electron flavoprotein dehydrogenase (ETFDH) is recognized as a mtIM-bound respiratory Complex (CETFDH), then it is clear that QH<sub>2</sub> is generated within the membrane-bound electron transfer system (mETS), ETF but not QH<sub>2</sub> transfers reducing equivalents to the mETS, and QH<sub>2</sub> in the mtIM is not exposed to the mt-matrix.''
::::* Thus, a suitable physical interaction is required to ensure the safe transfer of electron equivalents from FAO to ETC.
::::::''Comment: It is helpful to distinguish FAO and beta-oxidation, since FAO includes (quote) 'transport of substrates into mitochondria through carnitine palmitoyltransferases I and II (CPTI and CPTII) linked by a carnitine-acylcarnitine translocase', beta-oxidation, and downstream transfer of reducing equivalents to O<sub>2</sub> as the final electron acceptor.''
::::* .. the FAO enzymes physically and functionally interact with the ETC supercomplexes, albeit more weakly than the interaction between ETC complexes in supercomplexes.
::::* ETFDH and Com III are predicted to contact each other, presumably in close approximation to the core II CoQ-binding subunit.
::::* TFP is linked both with the complex I NADH-binding domain and VLCAD, likely on opposite sides of the TFP molecule, whereas ETFDH interacts with complex III at its matrix side.
::::* For steps 1–3, long-chain acyl-CoA substrates are transferred into mitochondria as acylcarnitines, which cross from the intermembrane space into VLCAD through CPTII in the inner membrane. VLCAD then accepts and catalyzes the released long-chain acyl-CoA substrate to its enoyl–CoA product with reduction of ETF. The protein complex promotes metabolite channeling for all these reactions. For steps 4 and 5, reduced ETF is released from VLCAD into the mitochondrial matrix, where it is free to find its redox partner, ETFDH, and shuttle its reducing equivalents (QH2) to ETC complex III. Alternatively, for high catalytic efficiency of transfer of electrons from FAO to ETC, the ETF may remain associated with the macromolecular FAO–ETC complex and instead slide down the membrane-associated proteins to more efficiently contact ETFDH.
::::* The interaction between TFP/complex I and ETFDH/complex III can ensure safe transfer of electron equivalents to ETC and ultimate ATP generation.
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