Because the concentration of 2-oxobutyrate in plant cells is normally very low, plants must contain a form of acetohydroxyacid (acetolactate) synthase (AHAS) [EC 4.1.3.18] that prefers 2-oxobutyrate over pyruvate if they are to direct any carbon into isoleucine biosynthesis. AHAS with a high affinity for 2-oxobutyrate could cause preferential and uninterrupted flow of carbon to isoleucine. Feedback inhibition of threonine dehydratase (TD) [EC 4.2.1.16] by isoleucine would prevent this imbalance (Singh and Shaner, 1995).

As discussed by Aubert et al (1998) [and references cited therein], incubation of sycamore cells with sulfometuron methyl, a potent inhibitor of AHAS, leads to accumulation of 2-oxobutyrate and its transamination product alpha-aminobutyrate. When simulataneously supplied with homoserine and sulfometuron methyl, alpha-aminobutyrate accumulation is dramatically increased. Isoleucine prevents this accumulation. Incubation of sycamore cells with alpha-aminobutyrate results in dramatic accumulation of isoleucine. All these data suggests that the steps homoserine --> O-phosphohomoserine ---> threonine are relatively unregulated, as are the steps 2-oxobutyrate ---> isoleucine. Control appears to be exerted at the threonine dehydratase (TD) step which is feedback inhibited by isoleucine (Aubert et al, 1998). Consistent with this, mutants of Arabidopsis that contain an isoleucine-insensitive TD accumulate high levels of isoleucine (Singh and Shaner, 1995).

Leucine specifically regulates its own branch of the pathway by inhibiting isopropylmalate synthase [EC 4.1.3.12].

Valine and leucine feedback inhibit AHAS. Valine-resistant mutants of Arabidopsis and tobacco that grow in the presence of normally lethal concentrations of valine or valine + leucine have AHAS activities with decreased sensitivity to valine and leucine. In Arabidopsis valine resistance is due to a single dominant mutation in a nuclear gene that is tightly linked to the gene encoding AHAS (Singh and Shaner, 1995).

Aubert S, Curien G, Bligny R, Gout E, Douce R 1998 Transport, compartmentation, and metabolism of homoserine in higher plant cells. Plant Physiol. 116: 547-557.

Miflin BJ 1977 Modification controls in time and space. In (H Smith ed) "Regulation of Enzyme Synthesis and Activity in Higher Plants", Academic Press, New York, pp. 23-40.

Singh BK, Shaner DL 1995 Biosynthesis of branched chain amino acids: From test tube to field. Plant Cell 7: 935-944.