Involved in tetrahydrobiopterin biosynthesis (By similarity). Possesses pterin-4-alpha-carbinolamine dehydratase activity when expressed in a bacterial heterolgous system (PubMed:18245455). (4aS,6R)-4a-hydroxy-L-erythro-5,6,7,8-tetrahydrobiopterin = (6R)-L-erythro-6,7-dihydrobiopterin + H2O Belongs to the pterin-4-alpha-carbinolamine dehydratase family.