 Each subdomain contains an ACT domain that allows for complex regulation of several different protein functions.The AK-HSD gene codes for aspartate kinase, an intermediate domain (coding for the linker region between the two enzymes in the bifunctional form), and finally the coding sequence for homoserine dehydrogenase.As of late 2007, 4 structures have been solved for this class of enzymes, with PDB accession codes , , , and .