Metabolism of Valine, Leucine, Isoleucine, and Lysine

The amino acids valine, leucine, isoleucine, and lysine are essential for mammals and are synthesized primarily in plant and bacterial cells. None of these amino acids is known to play significant roles other than as protein constituents and as substrates for their own degradation.

Valine, leucine, and isoleucine - The synthetic pathway from threonine and pyruvate to valine, leucine and isoleucine is outlined in Figure 21.26. The last four reactions in the biosynthesis of valine and isoleucine are catalyzed by the same four enzymes. Threonine dehydratase, which catalyzes the first step in conversion of threonine to isoleucine, is inhibited by isoleucine. Leucine, isoleucine, and valine are all catabolized via transamination followed by oxidative decarboxylation of the respective keto-acids (see here) and oxidation. The oxidation is similar to fatty acid oxidation, except for a debranching reaction for each intermediate.

Lysine - Lysine has two distinct biosynthetic pathways, the diaminopimelic acid pathway and the -aminoadipic acid pathway. The diaminopimelic acid pathway operates in bacteria, some lower fungi, algae, and higher plants. It begins with condensation of pyruvate with aspartate-semialdehyde and ends with decarboxylation of diaminopimelate (see here). Diaminopimelate is an important constituent of bacterial cell walls. The aminoadipic acid pathway is less widespread and functions in other lower fungi, in higher fungi, and in the protist Euglena.

See also: Metabolism of Serine, Glycine, and Threonine, Essential Amino Acids

INTERNET LINKS:

> 1. Val, Leu, Ile Biosynthesis

> 2. Lysine Biosynthesis