Enzyme - Ketol-acid reductoisomerase (NADP(+))

Alternative Name(s)
  • Dihydroxyisovalerate dehydrogenase (isomerizing).
  • Alpha-keto-beta-hydroxylacyl reductoisomerase.
  • Acetohydroxy acid isomeroreductase.

Catalytic Activity

(2R)-2,3-dihydroxy-3-methylbutanoate + NADP(+) = (2S)-2-acetolactate + H(+) + NADPH


There are no Cofactors for this Enzyme

Reaction Mechanism

    Ketol-acid reductoisomerase (KARI) is a Mg(2+) -dependent enzyme in the branched-chain amino acid biosynthesis pathway. KARI catalyses two steps in the biosynthesis of branched-chain amino acids. The reaction involves an Mg(II) dependent alkyl migration followed by an NADPH-dependent reduction of the 2-keto group.

    The reaction is initiated by a proton abstraction (by the hydroxide ion bridging the two metal ions), followed by a concerted isomerisation and hydride transfer. The two protons needed to protonate the product are proposed to be donated from the bridging water ligand and a conserved lysine
    Catalytic Residues
    AA Uniprot Uniprot Resid PDB PDB Resid
    Asp Q01292 315 1yve 244
    Glu Q01292 319 1yve 248
    Lys Q01292 252 1yve 181
    Step Components

    overall reactant used, proton transfer, overall product formed, intramolecular rearrangement, native state of cofactor regenerated, native state of enzyme regenerated, inferred reaction step, hydride transfer

    Step 1.

    A magnesium activated water molecule abstracts the proton from the alcohol, initiating a 1,2-alkyl shift with concomitant deprotonation of the conserved Lys252 residue.

    Step 2.

    NADPH eliminates a hydride ion, that is added to the newly formed carbonyl group, which in turn abstracts the proton from the magnesium bound water molecule, regenerating the bound hydroxide ion.

    Step 3.

    In an inferred return step, water donates a proton to the lysine to regenerate the enzyme's active site.


    The products of the reaction.

Reaction Parameters

There are no kinetic parameters information for this Enzyme

Associated Proteins

Protein name Organism
Ketol-acid reductoisomerase (NADP(+)) 2 Bacillus cereus (strain ZK / E33L)
Ketol-acid reductoisomerase (NADP(+)) Escherichia coli (strain K12)
Ketol-acid reductoisomerase, chloroplastic Mouse-ear cress
Ketol-acid reductoisomerase (NADP(+)) 1 Bacillus cereus (strain ZK / E33L)
Putative ketol-acid reductoisomerase 3 Saccharolobus solfataricus (strain ATCC 35092 / DSM 1617 / JCM 11322 / P2)