InterPro: IPR005106 Aspartate/homoserine dehydrogenase, NAD-binding

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 [1].

Homoserine dehydrogenase (EC:1.1.1.3) catalyses the third step in the aspartate pathway; theNAD(P)-dependent reduction of aspartate beta-semialdehyde into homoserine [2, 3]. Homoserine is an intermediate in the biosynthesis of threonine, isoleucine, and methionine. The enzyme can be found in a monofunctional form, in some bacteria and yeast, or a bifunctional form consisting of an N-terminal aspartokinase domain and a C-terminal homoserine dehydrogenase domain, as found in bacteria such as Escherichia coli and in plants. Structural analysis of the yeast monofunctional enzyme (P31116) indicates that the enzyme is a dimer composed of three distinct regions; an N-terminal nucleotide-binding domain, a short central dimerisation region, and a C-terminal catalytic domain [4]. The N-terminal domain forms a modified Rossman fold, while the catalytic domain forms a novel alpha-beta mixed sheet.

This entry represents the NAD(P)-binding domain of aspartate and homoserine dehydrogenase. Asparate dehydrogenase (EC:1.4.1.21) is strictly specific for L-aspartate as substrate and catalyses the first step in NAD biosynthesis from aspartate. The enzyme has a higher affinity for NAD+ than NADP+ [5].

Note that the C terminus of the protein contributes a helix to this domain that is not covered by this model.