Complex II ambiguities

high-resolution terminology - matching measurements at high-resolution

Complex II ambiguities

Description

The Succinate pathway is represented in the literature in some cases with a surprising confusion which warrants an analysis of Complex II ambiguities. The CII ambiguity has its roots in the narrative that reduced coenzymes (NADH and FADH₂) feed electrons from the Krebs cycle into the membrane-bound electron transfer system. In corresponding ambiguous graphical representations, CII in the canonical ('forward') Krebs cycle is shown to reduce FAD to FADH₂ (correct), yet CII in the membrane-bound electron transfer system is paradoxically represented as the site of oxidation of FADH₂ to FAD. For clarification: "The substrate of CII is succinate, which is oxidized forming fumarate while reducing flavin adenine dinucleotide FAD to FADH₂, with further electron transfer to the quinone pool. Whereas reduced NADH is a substrate of Complex I linked to dehydrogenases of the TCA cycle and mt-matrix upstream of CI, reduced FADH₂ is a product of Complex II with downstream electron flow from CII to Q" (quote from Gnaiger 2020).

Abbreviation: CII ambiguities

Reference: Gnaiger E (2020) Mitochondrial pathways and respiratory control. An introduction to OXPHOS analysis. 5th ed. Bioenerg Commun 2020.2. https://doi.org/10.26124/bec:2020-0002

FADH₂ and S-pathway

• FADH₂ appears in several publications as the substrate of CII in the electron transfer system. This commonly found error requires correction. For clarification, see page 48 in Gnaiger (2020).

• Arnold 2022 J Biol Chem
• Martinez-Reyes 2020 Nat Commun
• Yepez 2018 PLOS One
• DeBerardinis 2016 Sci Adv
• Sanchez 2001 Br J Pharmacol

CII and fatty acid oxidation

"Since mitochondrial Complex II also participates in the oxidation of fatty acids (6), .." (quote from [ Lemmi et al 1990]).

• Ref 6: Tzagoloff A (1982) Mitochondria. Plenum, New York.

MitoPedia concepts: MiP concept 

MitoPedia topics: Enzyme, Substrate and metabolite