PCD_DCoH: The bifunctional protein pterin-4alpha-carbinolamine dehydratase (PCD), also known as DCoH (dimerization cofactor of hepatocyte nuclear factor-1), is both a transcription activator and a metabolic enzyme. DCoH stimulates gene expression by associating with specific DNA binding proteins such as HNF-1alpha (hepatocyte nuclear factor-1) and Xenopus enhancer of rudimentary homologue (XERH). DCoH also catalyzes the dehydration of 4alpha- hydroxy- tetrahydrobiopterin (4alpha-OH-BH4) to quinoiddihydrobiopterin, a percursor of the phenylalanine hydroxylase cofactor BH4 (tetrahydrobiopterin). The DCoH homodimer has a saddle-shaped structure similar to that of TBP (TATA binding protein).