| Description | Bacteria, plants and fungi metabolise aspartic acid to produce four amino acids -lysine, threonine, methionine and isoleucine -in a series of reactions known as the aspartate pathway. Additionally, several important metabolic intermediates are produced by these reactions, such as diaminopimelic acid, an essential component of bacterial cell wall biosynthesis, and dipicolinic acid, which is involved in sporulation in Gram-positive bacteria. Members of the animal kingdom do not posses this pathway and must therefore acquire these essential amino acids through their diet. Research into improving the metabolic flux through this pathway has the potential to increase the yield of the essential amino acids in important crops, thus improving their nutritional value. Additionally, since the enzymes are not present in animals, inhibitors of them are promising targets for the development of novel antibiotics and herbicides. For more information see [ ].This entry represents the bifunctional enzyme aspartokinase/homoserine dehydrogenase (AK-HSDH) found in bacteria and plant chloroplasts, which catalyses the first and third steps of the aspartate pathway. Homoserine dehydrogenase ( ) catalyses the conversion of L-homoserine to L-aspartate-4-semialdehyde using NAD(P), while aspartate kinase ( ) 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 [ , ].Bifunctional enzymes that catalyse consecutive reactions offer the advantages of efficient channelling and protection of potentially reactive intermediates. AK-HSDH is unusual in its ability to catalyse two non-consecutive reactions. The enzyme that catalyses the intermediary step, aspartate semialdehyde dehydrogenase, is thought to provide a bridge to channel the intermediates between the non-consecutive reactions of AK-HSDH [ ].This entry also includes homoserine dehydrogenase from fungi, which catalyses the third step in the aspartate pathway and it is found in a monofunctional form in yeast. Structural analysis of this monofunctional form ( ) indicates that the enzyme is a dimer composed of an N-terminal nucleotide-binding domain that forms a modified Rossman fold, a short central dimerisation region, and a C-terminal catalytic domain which forms a novel α-β mixed sheet [ ]. | Name | Bifunctional aspartokinase/homoserine dehydrogenase |
| Short Name | Bifunc_aspartokin/hSer_DH | Type | Family |
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