ACT domains of the bifunctional enzyme aspartokinase (AK) - homoserine dehydrogenase (HSDH)
This CD includes the first of two ACT domains of the bifunctional enzyme aspartokinase (AK) - homoserine dehydrogenase (HSDH). The ACT domains are positioned between the N-terminal catalytic domain of AK and the C-terminal HSDH domain found in bacteria (Escherichia coli (EC) ThrA) and higher plants (Zea 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. HSDH is the first committed reaction in the branch of the pathway that leads to Thr and Met. 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 were shown to be involved in allosteric activation. Also included in this CD is the first of two ACT domains of a tetrameric, monofunctional, threonine-sensitive, AK found in Methanococcus jannaschii and other related archaeal species. Members of this CD belong to the superfamily of ACT regulatory domains.
Feature 1:putative threonine allosteric regulatory site
Comment:potentially involved in the binding of Thr.
Comment:Kinetic experiments on Arabidopsis AK-HSDH showed that each regulatory (ACT) domain of the monomers contained two nonequivalent threonine binding sites, constituted in part by this conserved Gln residue of the ACT domain, and a conserved Gln residue lying outside this domain.
Comment:The interaction of threonine with this conserved Gln residue of the ACT in Arabidopsis AK-HSDH led to inhibition of AK activity and facilitated the binding of a second threonine on a conserved Gln residue lying outside this domain. Interaction of this second threonine with the latter led to inhibition of HSDH activity.
Comment:substitution of a Phe residue for a Ser immediately C-terminal to this ACT domain Gln revealed this Ser as a possible key residue involved with the allosteric regulation of both AKI and HSDHI activities.