| Primary Identifier | IPR000506 | Type | Domain |
| Short Name | KARI_C |
| description | Ketol-acid reductoisomerase (KARI; ()), also known as acetohydroxyacid isomeroreductase (AHIR or AHAIR), catalyzes the conversion ofacetohydroxy acids into dihydroxy valerates in the second step of thebiosynthetic pathway for the essential branched-chain amino acids valine,leucine, and isoleucine. KARI catalyzes an unusual two-step reactionconsisting 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-dependentreduction to give 2,3-dihydroxy-3-isovalerate or 2,3-dihydroxy-3-methylvalerate respectively [, , , , , ].KARI is present only in bacteria, fungi, and plants, but not in animals. KARIsare divided into two classes on the basis of sequence length andoligomerization state. Class I KARIs are ~340 amino acid residues in lengthand include all fungal KARIs, whereas class II KARIs are ~490 residues longand include all plant KARIs. Bacterial KARIs can be either class I or classII. KARIs are composed of two types of domains, an N-terminal Rossmann folddomain and one or two C-terminal knotted domains. Two intertwinned knotteddomains are required for function, and in the short-chain or class I KARIs,each polypeptide chain has one knotted domain. As a result, dimerization oftwo monomers forms two complete KARI active sites. In the long-chain or classII KARIs, a duplication of the knotted domain has occurred and, as a result,the protein does not require dimerization to complete its active site[, , , , , ].The α-helical KARI C-terminal knotted domain can be described as a six-helix core in which helices coil like cable threads around each other, thusforming a bundle [, , , , ]. |
