<p>Members of this group catalyse a step in tyrosine biosynthesis in the shikimate pathway, which is present only in bacteria, archaea, fungi, and plants [<cite idref="PUB00015637"/>]. They belong to the prephenate dehydrogenase (PDH) domain superfamily, for more information see <db_xref db="INTERPRO" dbkey="IPR008235"/>, <db_xref db="INTERPRO" dbkey="IPR008236"/>, <db_xref db="INTERPRO" dbkey="IPR008299"/>, and <db_xref db="INTERPRO" dbkey="IPR008244"/>).</p> <p>Many of the PDH enzymes are able to use the alternative intermediates of tyrosine biosynthesis, prephenate or L-arogenate, as substrates, having both prephenate dehydrogenase and arogenate dehydrogenase activities, and are sometimes collectively called cyclohexadienyl dehydrogenases [<cite idref="PUB00011046"/>, <cite idref="PUB00016002"/>]. Prephenate dehydrogenase (PDH) (<db_xref db="EC" dbkey="1.3.1.12"/>, <db_xref db="EC" dbkey="1.3.1.13"/>) catalyses oxidative decarboxylation of prephenate to 4-hydroxyphenylpyruvate. Arogenate dehydrogenase (<db_xref db="EC" dbkey="1.3.1.43"/>) catalyses oxidative decarboxylation of arogenate into tyrosine. The existence of different combinations for routing prephenate to phenylalanine or tyrosine means that the substrate specificity of a particular enzyme (arogenate or prephenate), as well as its susceptibility to feedback regulation by different metabolites is not easily predictable, and must be experimentally studied in detail [<cite idref="PUB00011044"/>].</p>