Fig. 1

Structure and pathways. A Structure of BCAAs, including L-leucine, L-valine and L-isoleucine. B An overview of biosynthetic pathway of L-valine in bacteria, such as Corynebacterium glutamicum and Escherichia coli. C A schematic diagram for the complete metabolism of L-valine in animals. L-valine is catabolized to succinyl-CoA and subsequently enter the TCA cycle. The numbers correspond to the appropriate enzymes for main reactions. 1. Acetohydroxy acid synthase; 2. Acetohydroxy acid isomeroreductase; 3. Dihydroxy-acid dehydratase; 4. Branched chain amino acid transaminase; 5. Branched chain keto-acid dehydrogenase; 6. 2-Methylbutyryl-CoA dehydrogenase; 7. Enol-CoA dehydrogenase; 8. 3-Hydroxyisobutyryl-CoA deacylase; 9. 3-Hydroxyisobutyryl-CoA dehydrogenase; 10. Methylmalonic semialdehyde dehydrogenase; 11. Propionyl-CoA carboxylase; 12. Methylmalonyl-CoA mutase. Abbreviations: FAD+  = flavin adenine dinucleotide; GDP = guanosine diphosphate; GTP = guanosine triphosphate; NAD+  = nicotinamide adenine dinucleotide; TCA = tricarboxylic acid