Dehydrogenases for the biosynthesis of L-tyrosine (Tyr) comprise prephenate dehydrogenase (EC 1.3.1.12) (PDH), arogenate dehydrogenase (EC 1.3.1.43) (ADH) and cyclohexadienyl dehydrogenase that can accept both prephenate and arogenate as substrate. These enzymes use a nicotinamide nucleotide as co-substrate and can be specific for NAD(+), for NADP(+) or can utilize either of these. Bacteria, archaea, plants and fungi possess one or more of these enzymes, which participate in two alternative pathways for the synthesis of Tyr. Prephenate, the common precursor, is formed from chorismate by chorismate mutase. PDH converts prephenate into p-hydroxyphenylpyruvate, which can yield Tyr according to pathway I. In the alternative pathway II, prephenate aminotransferase converts prephenate into arogenate (meaning 'giving rise to aromatics'). Conversion of arogenate can either yield Tyr by ADH, or phenylalanine (Phe) by arogenate dehydratase.

PDH in Tyr biosynthesis pathway I:

                 PDH                              Tyr aminotransferase
prephenate   ---------->   p-hydroxyphenylpyruvate    <==========>    tyrosine

ADH in Tyr biosynthesis pathway II:

          Prephenate aminotransferase                   ADH
prephenate   <===================>   arogenate   ----------------->   tyrosine

Both PDH and ADH contain a catalytic PDH/ADH domain, which has a length of ~180 residues. A potential NAD(+) binding motif is located in the N-terminal part [1,2]. A central conserved histidine is important for the catalytic activity in the Escherichia coli PDH [3]. Some PDH/ADH domain proteins have a C-terminal extension that may be involved in allosteric regulation [1,2,3,4].

Some proteins known to contain a prephenate/arogenate dehydrogenase domain:

• Bacterial T-protein encoded by the tyrA gene, a bifunctional enzyme of two catalytic domains, chorismate mutase (see <PDOC51167>) and PDH for biosynthesis of tyrosine (Tyr). Both enzyme activities are feedback inhibited by Tyr. This 'PDH' is NAD(+) specific and can utilize arogenate in vitro, thus technically it is a cyclohexadienyl dehydrogenase.
• Cyanobacterial arogenate dehydrogenase, which seems to be specific for NADP(+) and feedback inhibited by Tyr [2].
• Zymomonas mobilis tyrC, an NAD(+)-specific cyclohexadienyl dehydrogenase that can function as a PDH and as ADH, without feedback inhibition by Tyr.
• Yeast prephenate dehydrogenase, an NADP(+) specific PDH which is transcriptionally regulated by Phe.
• Arabidopsis thaliana arogenate dehydrogenases TyrAAT1 and TyrAAT2, which are monofunctional ADH specific for NADP(+), with strong Tyr feedback regulation. In plants, the arogenate pathway seems to be the dominant, or only, pathway for synthesis of both Phe and Tyr [2,4].

The profile we developed covers the entire prephenate/arogenate dehydrogenase domain.