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AAK_AK-HSDH: Amino Acid Kinase Superfamily (AAK), AK-HSDH; this CD includes the N-terminal catalytic domain of aspartokinase (AK) of the bifunctional enzyme AK - homoserine dehydrogenase (HSDH). These aspartokinases are found in bacteria (E. coli AKI-HSDHI, ThrA and E. coli AKII-HSDHII, MetL) and higher plants (Z. mays AK-HSDH). AK and HSDH are the first and third enzymes in the biosynthetic pathway of the aspartate family of amino acids. AK catalyzes the phosphorylation of Asp to P-aspartyl phosphate. HSDH catalyzes the NADPH-dependent conversion of Asp 3-semialdehyde to homoserine. ThrA and MetL are involved in threonine and methionine biosynthesis, respectively. In E. coli, ThrA is subject to allosteric regulation by the end product L-threonine and the native enzyme is reported to be tetrameric. As with bacteria, plant AK and HSDH are feedback inhibited by pathway end products. Maize AK-HSDH is a Thr-sensitive 180-kD enzyme. Arabidopsis AK-HSDH is an alanine-activated, threonine-sensitive enzyme whose ACT domains, located C-terminal to the AK catalytic domain, were shown to be involved in allosteric activation.
Jiyao W et al.(2023). "The conserved domain database in 2023.",