InterPro domain: IPR042199

This superfamily represents the N-terminal catalytic domain found in aspartokinase and in the bifunctional enzyme aspartokinase/homoserine dehydrogenase. It has a key role in amino acid binding.

The bifunctional enzyme aspartokinase/homoserine dehydrogenase (AK-HSDH), found in bacteria and plant chloroplasts, catalyses the first and third steps of the aspartate pathway. Homoserine dehydrogenase ( 1.1.1.3 ) catalyses the conversion of L-homoserine to L-aspartate-4-semialdehyde using NAD(P), while aspartate kinase ( 2.7.2.4 ) catalyses the phosphorylation of L-aspartate to 4-phospho-L-aspartate. There are two genes encoding different isoforms of this bifunctional enzymes; one isoform is threonine-sensitive, while the other is methionine-sensitive [ 1 ].

Aspartokinase is the first enzyme in the aspartate metabolic pathway and catalyzes the conversion of aspartate and ATP to aspartylphosphate and ADP. In Bacillus subtilis, YclM is reported to be a single polypeptide of 50 kD. The B. subtilis 168 AKIII is induced by lysine and repressed by threonine, and it is synergistically inhibited by lysine and threonine [ 2 , 3 ].

1. Mechanism of control of Arabidopsis thaliana aspartate kinase-homoserine dehydrogenase by threonine. J. Biol. Chem. 278, 5361-6
2. Characterization of aspartate kinase III of Bacillus subtilis. Biosci. Biotechnol. Biochem. 65, 1391-4
3. Aspartokinase III, a new isozyme in Bacillus subtilis 168. J. Bacteriol. 172, 218-23