Datensatz

Zusammenfassung

Tetrahydromethanopterin (H₄ MPT) is a tetrahydrofolate analogue involved as a C₁ carrier in the metabolism of various groups of microorganisms. How H₄MPT is bound to the respective C₁ unit converting enzymes remained elusive. We describe here the structure of the homopentameric formaldehyde-activating enzyme (Fae) from Methylobacterium extorquens AM1 established at 2.0 angstrom without and at 1.9 angstrom with methylene-H₄MPT bound. Methylene-H₄MPT is bound in an "S"-shaped conformation into the cleft formed between two adjacent subunits. Coenzyme binding is accompanied by side chain rearrangements up to 5 angstrom and leads to a rigidification of the C-terminal arm, a formation of a new hydrophobic cluster, and an inversion of the amide side chain of Gln⁸⁸. Methylene-H₄MPT in Fae shows a characteristic kink between the tetrahydropyrazine and the imidazolidine rings of 70 degrees that is more pronounced than that reported for free methylene-H₄MPT in solution (50 degrees). Fae is an essential enzyme for energy metabolism and formaldehyde detoxification of this bacterium and catalyzes the formation of methylene-H₄MPT from H₄MPT and formaldehyde. The molecular mechanism ofthis reaction involving His²² as acid catalyst is discussed.