FIGURE 2.

Amino acids can be used as electron donors to the mitochondrial electron transport chain. Protein degradation yields a range of amino acids that are further metabolized either into isovaleryl-CoA or HG. Isovaleryl-CoA can be produced by catabolism of the branched chain and aromatic amino acids and by both phytol and Lys degradation, whereas HG can be produced by aromatic amino acid degradation or from the Lys derivative L-pipecolate. The electrons generated are transferred to the respiratory chain through to the ubiquinol pool via an ETF/ETFQO system. Possible involvement of sulfur containing amino acids has also been implicated by the phenotype of ETHE1 knockdown plants. Some amino acids can facilitate energy production via the TCA cycle, either by conversion to pyruvic acid or acetyl-CoA or by direct conversion to TCA cycle intermediates, such as 2-OG, and direct electron supply to the ubiquinone pool of the mitochondrial electron transport chain in plants. Dotted arrows represent possible transport processes and multi enzymatic reactions. Abbreviations: BCAA, branched chain amino acids; D2HGDH, 2-D-hydroxyglutarate dehydrogenase; e–, electron; ETF, electron transfer flavoprotein; ETFQO, ETF:ubiquinone oxidoreductase; ETHE1, ethylmalonic encephalopathy protein1; HG, hydroxyglutarate; IVDH, isovaleryl-CoA dehydrogenase; 3-MC-CoA, 3-methylcrotonyl-CoA; 2-OG, 2-oxoglutarate; TCA cycle, tricarboxylic acid cycle; UQ, ubiquinone (Adapted from Araujo et al., 2011; Krüssel et al., 2014).