Formylmethanofuran: Tetrahydromethanopterin formyltransferase from Methanopyrus kandleri - New insights into salt-dependence and thermostability

Background: Formylmethanofuran: tetrahydromethanopterin formyltransferase (Ftr) from the methanogenic Archaeon Methanopyrus kandleri (optimum growth temperature 98 degrees C) is a hyperthermophilic enzyme that is absolutely dependent on the presence of lyotropic salts for activity and thermostability. The enzyme is involved in the pathway of carbon dioxide reduction to methane and catalyzes the transfer of formyl from formylmethanofuran to tetrahydromethanopterin. Results: The crystal structure of Ftr, determined to a resolution of 1.73 Angstrom, reveals a homotetramer composed essentially of two dimers. Each subunit is subdivided into two tightly associated lobes both consisting of a predominantly antiparallel beta sheet flanked by a helices forming an alpha/beta sandwich structure. The approximate location of the active site was detected in a region close to the dimer interface. Conclusions: The adaptation of Ftr against high lyotropic salt concentrations is structurally reflected by a large number of negatively charged residues and their high local concentration on the surface of the protein. The salt-dependent thermostability of Ftr might be explained on a molecular basis by ionic interactions at the protein surface, involving both protein and inorganic salt ions, and the mainly hydrophobic interactions between the subunits and within the core.