A recurring regulatory domain. The regulatory domain formed by two subunits of 3-phosphoglycerate dehydrogenase is structurally related to the single-chain regulatory domain of threonine deaminase. Notice that both structures have four α helices and eight β strands in similar locations. Sequence analyses have revealed this amino acid-binding regulatory domain to be present in other enzymes as well.
[Drawn from 1PSD and 1TDJ.pdb.]