Ketol-acid reductoisomerase (KARI; EC:1.1.1.86), also known as acetohydroxy acid isomeroreductase (AHIR or AHAIR), catalyses the conversion of acetohydroxy acids into dihydroxy valerates in the second step of the biosynthetic pathway for the essent ...
Ketol-acid reductoisomerase (KARI; EC:1.1.1.86), also known as acetohydroxy acid isomeroreductase (AHIR or AHAIR), catalyses 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 [1, 2, 3, 4, 5, 6]. 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 oligomerisation 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 dimerisation to complete its active site [1, 2, 3, 4, 5, 6]. 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 [1, 2, 3, 4, 5].
