 The N-terminal domain forms a modified Rossmann fold, while the catalytic domain forms a novel alpha-beta mixed sheet.The enzyme can also be found in 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.The bifunctional aspartokinase-homoserine dehydrogenase (AK-HSD) enzyme has a regulatory domain that consists of two subdomains with a common loop-alpha helix-loop-beta strand loop-beta strand motif.