Dihydroxyisovaleric acid

Dihydroxyacid dehydratase is involved in the biosynthesis of valine and isoleucine. L-a,/3-dihydroxyisovaleric acid and L-aj3-dihydroxy-/3-methylvaleric acid are dehydrated to form the a-keto acid precursors of valine and isoleucine. 

Biosynthesis In microorganisms and plants from pyruvic acid 2 pyruvate- 2-acetolactic acid (acetolactate synthase, EC 4.1.3.18 coenzyme thiamin(e) diphosphate)- 2,3-dihydroxyisovaleric acid (2-acetolactate mutase, EC 5.4.99.3)- 2-oxoisovaleric acid (dihydroxy acid dehydratase, EC 4.2.1.9). This is finally am-inated by branched chain amino acid aminotransferase (EC 2.6.1.42). 2-Oxoisovaleric acid is also a precursor of Leu. 

The reaction mixture contained in a total volume of 1 mL 50 mM Tris-HC1 buffer (pH 8.0), 100 tnM MgG2, 3.8 mAf L-a,/3-dihydroxyisovaleric add (sodium salt), and crude extract containing 0.5 mg of protein. The reaction mixture was incubated for various times at 37°C, and the reaction was terminated by transferring 100 L of the assay mixture to a 200 fiL centrifuge tube containing 20 / L of 50% trichloroacetic acid. Predpitated proteins were removed by centrifugation. Then 50 fiL of the supemate was mixed with... 

The syntheses of valine, leucine, and isoleucine from pyruvate are illustrated in Figure 14.9. Valine and isoleucine are synthesized in parallel pathways with the same four enzymes. Valine synthesis begins with the condensation of pyruvate with hydroxyethyl-TPP (a decarboxylation product of a pyruvate-thiamine pyrophosphate intermediate) catalyzed by acetohydroxy acid synthase. The a-acetolactate product is then reduced to form a,/3-dihydroxyisovalerate followed by a dehydration to a-ketoisovalerate. Valine is produced in a subsequent transamination reaction. (a-Ketoisovalerate is also a precursor of leucine.) Isoleucine synthesis also involves hydroxyethyl-TPP, which condenses with a-ketobutyrate to form a-aceto-a-hydroxybutyrate. (a-Ketobutyrate is derived from L-threonine in a deamination reaction catalyzed by threonine deaminase.) a,/3-Dihydroxy-/3-methylvalerate, the reduced product of a-aceto-a-hydroxybutyrate, subsequently loses an HzO molecule, thus forming a-keto-/kmethylvalerate. Isoleucine is then produced during a transamination reaction. In the first step of leucine biosynthesis from a-ketoisovalerate, acetyl-CoA donates a two-carbon unit. Leucine is formed after isomerization, reduction, and transamination. 

Subsequently, the enzymic conversion of a-acetolactate to a,6-dihydroxyisovalerate and a-aceto-a-hydroxybutyrate to a,/3-dihydroxy-jS-methylvalerate was shown by Wagner et al. 166) with extracts of Neurospora and by Umbarger 167) with extracts of appropriately blocked E. coU mutants. These acids were then shown to be utUized for valine and isoleucine formation. 

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