Fig. 1.

Schematic representation of lysine turnover (biosynthesis and degradation). Lysine is synthetized in the chloroplast using aspartate as a precursor. Dihydrodipicolinate synthase (DHDPS) is the first enzyme of lysine biosynthesis and it requires pyruvate export from the cytosol to the chloroplast. Under stress conditions, lysine is exported from the chloroplast to mitochondria to be degraded (trace arrows), and electrons are used as a donor for ATP synthesis in two ways: (i) lysine can be degraded by lysine-ketoglutarate reductase/saccharopine dehydrogenase (LKR/SDH) resulting in acetyl-CoA entering in the TCA cycle, or (ii) lysine can be degraded by D-2-hydroxyglutarate dehydrogenase (D2HGDH) resulting in 2-oxoglutarate (2-OG), which at the same time acts as an electron donor for the alternative respiration system mediated by electron transfer flavoprotein (ETF)–electron transfer flavoprotein:ubiquinone oxidoreductase (ETFQO). Thus there is a close relationship between chloroplasts and mitochondria in lysine metabolism.