Beschreibung:
<jats:p>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.</jats:p>