Structure and Catalytic Mechanism of N5,N10-Methenyltetrahydromethanopterin Cyclohydrolase

Methenyltetrahydromethanopterin (methenyl-H4MPT+) cyclohydrolase (Mch) catalyzes the interconversion of methenyl-H4MPT+ and formyl-H4MPT in the one-carbon energy metabolism of methanogenic, methanotrophic, and sulfate-reducing archaea and of methylotrophic bacteria. To understand the catalytic mechanism of this reaction, we kinetically characterized site-specific variants of Mch from Archaeoglobus fulgidus (aMch) and determined the X-ray structures of the substrate-free aMch(E186Q), the aMch:H4MPT complex, and the aMch(E186Q):formyl-H4MPT complex. (Formyl-)H4MPT is embedded inside a largely preformed, interdomain pocket of the homotrimeric enzyme with the pterin and benzyl rings being oriented nearly perpendicular to each other. The active site is primarily built up by the segment 93:95, Arg183 and Glu186 that either interact with the catalytic water attacking methenyl-H4MPT+ or with the formyl oxygen of formyl-H4MPT. The catalytic function of the strictly conserved Arg183 and Glu186 was substantiated by ...