Figure 4.

Multilevel regulation of AOX. AOX biosynthesis is regulated transcriptionally by citrate and by elevated levels of ROS and NO. AOX is modified post-translationally by the mitochondrial thioredoxins (Trx) h or o, which mediate the conversion between an inactive form with a disulfide bridge between the monomers and an active form with free thiol groups. Thioredoxin reductase regenerates Trx to the reduced form; NADPH for this reaction is supplied by NADP-dependent isocitrate dehydrogenase (NADP-ICDH). AOX is activated by oxo-acids including pyruvate (coming from the glycolysis and malic enzyme reactions), 2-OG and oxaloacetate (OAA; formed in the TCA cycle), and glyoxylate and hydroxypyruvate (from photorespiration). The post-translational binding of pyruvate and other oxo-acids to the reduced form further activates the enzyme. The increase in the concentration of the AOX substrate ubiquinol (UQH₂) in the inner membrane stimulates AOX activity. The individual biochemical processes are shown with the solid arrows; and the dotted lines indicate regulatory effects. Thick red arrows define different levels of AOX regulation. The figure was substantially modified from Møller et al. (2020a) and the Web Figure 12.3C of Taiz et al. (2015): http://6e.plantphys.net/topic12.03.html.