PFAM accession number:PF01450
Interpro abstract (IPR000506):

Ketol-acid reductoisomerase (KARI; ( EC )), also known as acetohydroxy acid isomeroreductase (AHIR or AHAIR), catalyzes the conversion of acetohydroxy acids into dihydroxy valerates in the second step of the biosynthetic pathway for the essential branched-chain amino acids valine, leucine, and isoleucine. KARI catalyzes an unusual two-step reaction consisting of an alkyl migration in which the substrate, either 2-acetolactate (AL) or 2-aceto-2-hydroxybutarate (AHB), is converted to 3-hydoxy-3-methyl-2- oxobutyrate or 3-hydoxy-3-methyl-2-pentatonate, followed by a NADPH-dependent reduction to give 2,3-dihydroxy-3-isovalerate or 2,3-dihydroxy-3- methylvalerate respectively [ (PUBMED:9218783) (PUBMED:11352718) (PUBMED:12691757) (PUBMED:16322583) (PUBMED:25849365) (PUBMED:26644020) ].

KARI is present only in bacteria, fungi, and plants, but not in animals. KARIs are divided into two classes on the basis of sequence length and oligomerization state. Class I KARIs are ~340 amino acid residues in length and include all fungal KARIs, whereas class II KARIs are ~490 residues long and include all plant KARIs. Bacterial KARIs can be either class I or class II. KARIs are composed of two types of domains, an N-terminal Rossmann fold domain and one or two C-terminal knotted domains. Two intertwinned knotted domains are required for function, and in the short-chain or class I KARIs, each polypeptide chain has one knotted domain. As a result, dimerization of two monomers forms two complete KARI active sites. In the long-chain or class II KARIs, a duplication of the knotted domain has occurred and, as a result, the protein does not require dimerization to complete its active site [ (PUBMED:9218783) (PUBMED:11352718) (PUBMED:12691757) (PUBMED:16322583) (PUBMED:25849365) (PUBMED:26644020) ].

The alpha-helical KARI C-terminal knotted domain can be described as a six- helix core in which helices coil like cable threads around each other, thus forming a bundle [ (PUBMED:9218783) (PUBMED:11352718) (PUBMED:12691757) (PUBMED:16322583) (PUBMED:25849365) ].

GO process:branched-chain amino acid biosynthetic process (GO:0009082), oxidation-reduction process (GO:0055114)
GO function:ketol-acid reductoisomerase activity (GO:0004455)

This is a PFAM domain. For full annotation and more information, please see the PFAM entry IlvC