Location of the active site and proposed catalytic mechanism of pterin-4a-carbinolamine dehydratase.

Based on the recently solved three-dimensional structure of pterin-4a-carbinolamine dehydratase from rat/human liver the involvement of the proposed active-site residues Glu57, Asp60, His61, His62, Tyr69, His79, Arg87 and Asp88 was examined by site-directed mutagenesis. Most of the mutants showed reduced activity, and only the Glu57-->Ala mutant and the His61-->Ala, His62-->Ala double mutant were fully devoid of activity. The dissociation constants of quinonoid 6,6-dimethyl-7,8-dihydropterin were significantly increased for binding to the Glu57-->Ala, His61-->Ala, His62-->Ala single mutants and the His61-->Ala, His62-->Ala double mutant, confirming that His61 and His62 are essential for substrate binding and catalysis. The mechanism of dehydration is proposed to involve base catalysis at the N(5)-H group of the substrate by His61.