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Valine catabolism shares common initial steps in leucine and isoleucine catabolism. First, branched-chain 2-oxoacids are synthesized by aminotransferases. Various isoforms of aminotransferases have been identifies in rice and arabidopsis localized in different subcellular compartments. Mitochondrial isoform is involved in catalysis of this step. The oxidative decarboxylation of ?-ketoacids producing acyl-CoAs is catalyzed by the branched-chain ?-ketoacid dehydrogenase complex, which is functionally and structurally similar to pyruvate dehydrogenase and 2-oxoglutarate dehydrogenase (Hildebrandt et al., 2015). Subsequently, isovaleryl-CoA dehydrogenase oxidizes isovaleryl-CoA derived from leucine catabolism, but also 2-methylbutanoyl-CoA and 2-methylpropanoly-CoA from isoleucine and valine degradation (Däschner et al., 2001; Hildebrandt et al., 2015). enoyl-CoAs are converted to hydroxyacyl-CoAs by enoyl-CoA hydratase. 3-Hydroxyisobutyryl-CoA is hydrolyzed to 3-hydroxyisobutyrate followed by an oxidation step. At final step, methylmalonate-semialdehyde dehydrogenase catalyzes the oxidative decarboxylation of methylmalonate semialdehyde yielding propionyl-CoA. Branched-chain amino acid catabolism is very complex and all further steps are not well know in plants. It is still a matter of debate whether the entire BCAA degradation pathway is localized in the mitochondria (Binder, 2010; Kochevenko et al., 2012). Some reaction steps are also required for peroxisomal fatty acid ? oxidation, and localization of the involved enzyme isoforms is unclear (Hildebrandt et al., 2015).
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