The enzyme dihydroxy-acid dehydratase (EC 4.2.1.9) catalyzes the chemical reaction

2,3-dihydroxy-3-methylbutanoate ${\displaystyle \rightleftharpoons }$ 3-methyl-2-oxobutanoate + H₂O

This enzyme participates in valine, leucine and isoleucine biosynthesis and pantothenate and coenzyme A (CoA) biosynthesis.

This enzyme belongs to the family of lyases, specifically the hydro-lyases, which cleave carbon-oxygen bonds. The systematic name of this enzyme class is 2,3-dihydroxy-3-methylbutanoate hydro-lyase (3-methyl-2-oxobutanoate-forming). Other names in common use include

• acetohydroxyacid dehydratase,
• α,β-dihydroxyacid dehydratase,
• 2,3-dihydroxyisovalerate dehydratase,
• α,β-dihydroxyisovalerate dehydratase,
• dihydroxy acid dehydrase,
• DHAD,
• and 2,3-dihydroxy-acid hydro-lyase.