ACT domains C-terminal to the catalytic domain of aspartokinase (AK; 4-L-aspartate-4-phosphotransferase)This CD includes each of two ACT domains C-terminal to the catalytic domain of aspartokinase (AK; 4-L-aspartate-4-phosphotransferase). Typically, AK consists of two ACT domains in a tandem repeat, but the second ACT domain is inserted within the first, resulting in, what is normally the terminal beta strand of ACT2, formed from a region N-terminal of ACT1. AK catalyzes the conversion of aspartate and ATP to aspartylphosphate and ADP. Aspartokinase is the first enzyme in the pathway of the biosynthesis of the aspartate family of amino acids (lysine, threonine, methionine, and isoleucine) and the bacterial cell wall component, meso-diaminopimelate. One mechanism for the regulation of this pathway is by the production of several isoenzymes of aspartokinase with different repressors and allosteric inhibitors. Pairs of ACT domains are proposed to specifically bind amino acids leading to allosteric regulation of the enzyme. In Escherichia coli (EC), three different aspartokinase isoenzymes are regulated specifically by lysine, methionine, and threonine. AK-HSDHI (ThrA) and AK-HSDHII (MetL) are bifunctional enzymes that consist of an N-terminal AK and a C-terminal homoserine dehydrogenase (HSDH). ThrA and MetL are involved in threonine and methionine biosynthesis, respectively. The third isoenzyme, AKIII (LysC), is monofunctional and is involved in lysine synthesis. The three Bacillus subtilis (BS) isoenzymes, AKI (DapG), AKII (LysC), and AKIII (YclM), are feedback inhibited by meso-diaminopimelate, lysine, and lysine plus threonine, respectively. The E. coli lysine-sensitive AK is described as a homodimer, whereas, the B. subtilis lysine-sensitive AK is described as is a heterodimeric complex of alpha- and beta- subunits that are formed from two in-frame overlapping genes. A single AK enzyme type has been described in Pseudomonas, Amycolatopsis, and Corynebacterium, and apparently, unique to cyanobacteria, are aspartokinases with two tandem pairs of ACT domains, C-terminal to the catalytic domain. The fungal aspartate pathway is regulated at the AK step, with L-Thr being an allosteric inhibitor of the Saccharomyces cerevisiae AK (Hom3). At least two distinct AK isoenzymes can occur in higher plants, a monofunctional lysine-sensitive isoenzyme, which is involved in the overall regulation of the pathway and can be synergistically inhibited by S-adenosylmethionine. The other isoenzyme is a bifunctional, threonine-sensitive AK-HSDH protein. Also included in this AK family CD are the ACT domains of the Methylomicrobium alcaliphilum AK; the first enzyme of the ectoine biosynthetic pathway found in this bacterium and several other halophilic/halotolerant bacteria. Members of this CD belong to the superfamily of ACT regulatory domains.