The crystal structure of methenyltetrahydromethanopterin cyclohydrolase from the hyperthermophilic archaeon Methanopyrus kandleri

Background: The reduction of carbon dioxide to methane in methanogenic arch aea involves the tetrahydrofolate analogue tetrahydromethanopterin (H4MPT) as a C-1 unit carrier. In the third step of this reaction sequence, N-5-for myl-H4MPT is converted to methenyl-H4MPT+ by the enzyme methenyltetrahydrom ethanopterin cyclohydrolase. The cyclohydrolase from the hyperthermophilic archaeon Methanopyrus kandleri (Mch) is extremely thermostable and adapted to a high intracellular concentration of lyotropic salts. Results: Mch was crystallized and its structure solved at 2.0 Angstrom reso lution using a combination of the single isomorphous replacement (SIR) and multiple anomalous dispersion (MAD) techniques. The structure of the homotr imeric enzyme reveals a new alpha/beta fold that is composed of two domains forming a large sequence-conserved pocket between them. Two phosphate ions were found in and adjacent to this pocket, respectively; the latter is dis placed by the phosphate moiety of the substrate formyl-H4MPT according to a hypothetical model of the substrate binding. Conclusions: Although the exact position of the substrate is not yet known, the residues lining the active site of Mch could be tentatively assigned. Comparison of Mch with the tetrahydrofolate-specific cyclohydrolase/dehydro genase reveals similarities in domain arrangement and in some active-site r esidues, whereas the fold appears to be different. The adaptation of Mch to high salt concentrations and high temperatures is reflected by the excess of acidic residues at the trimer surface and by the higher oligomerization state of Mch compared with its mesophilic counterparts.