Figure 5.

Accumulation of intermediates of branched-chain amino acid catabolism in the ΔscdA mutant, TLMH17, incubated with Val. A. Detection of intermediates of Val, Ile, and Leu catabolism in culture supernatants of wild type and mutant (ΔscdA) strains of A. nidulans: 2-oxoisovaleric acid, 2-oxo-3-methylvaleric acid, 2-oxoisocaproic acid, and 3-hydroxyisobutyric acid. 2-oxocaproic acid was added to culture supernatants as an internal standard prior to sample lyophilization and derivatization. Shown are normalized GC/MS peak area measurements for wild type (A26, dark bars) and ΔscdA mutant (TLMH17, light bars). The 2-oxo acids were normalized using the peak areas of the 2-oxocaproic acid internal standard. The 3-hydroxyisobutyric acid peak areas were normalized using the 4-nitrophenol internal standard. B. Shown is the proposed Val catabolic pathway in A. nidulans, based on studies in other eukaryotes (Bachhawat et al., 1957; Robinson and Coon, 1957) and genetic and genomic data for A. nidulans (Maggio-Hall and Keller, 2004). Valine is converted to 2-oxoisovaleric acid by the action of an unidentified transaminase. 2-Oxoisovaleric acid is activated by the branched-chain α-keto acid dehydrogenase (BCKAD, Pettit et al., 1978) to yield a branched-chain acyl-CoA, isobutyryl-CoA. This is converted to 3-hydroxyisobutyryl-CoA by two enzymes of β-oxidation, acyl-CoA dehydrogenase and enoyl-CoA hydratase. After release of CoA, 3-hydroxyisobutyrate is expected to be converted to methylmalonate semialdehyde and then to propionyl-CoA and CO₂. Propionyl-CoA is metabolized for energy via the methylcitric acid cycle in A. nidulans (Brock et al., 2000).