Valine biosynthesis (Saccharomyces cerevisiae)

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The pathway of valine biosynthesis is a four-step pathway that shares all of its steps with the parallel pathway of isoleucine biosynthesis (https://www.wikipathways.org/pathways/WP250.html). These entwined pathways are part of the superpathway of branched chain amino acid biosynthesis, that generates not only isoleucine and valine, but also leucine.

As a consequence of having several of its component enzymes involved in the synthesis of three different amino acids, the pathway of isoleucine biosynthesis is subject to regulation by all three amino acids. The first step in the pathway is primarily inhibited by valine, along with inhibition by isoleucine and leucine. The potential disruption this might cause to the parallel isoleucine biosynthesis pathway step using the same enzymes is resolved by upregulation of an earlier step that is unique to isoleucine biosynthesis, as explained in the L-isoleucine biosynthesis I (from threonine) summary. In this way, valine biosynthesis can be regulated independently of isoleucine biosynthesis, despite all four valine biosynthesis enzymes also participating in isoleucine biosynthesis.

SOURCE: SGD pathways, http://pathway.yeastgenome.org/server.html