In the spirit of both Christmas and recycling, I decided to build a Christmas tree by gluing together wine corks accumulated during this year. In addition, if you needed one, the intermediates of this process make rather cute models of polycyclic aromatic hydrocarbons.
The first step of biological methane formation from carbon dioxide is the reduction of CO2 to form N-formylmethanofuran from methanofuran. This reaction is catalysed by formylmethanofuran dehydrogenase (EC 1.2.99.5). There are two types of this enzyme in methanogenic archaea, a tungsten iron—sulphur protein (Fwd) and a molybdenum iron—sulphur protein (Fmd).
Wagner et al. [1] determined the X-ray structures of a Fwd enzyme from the thermophilic methanogenic archaeon Methanothermobacter wolfeii in several crystal forms [2—4]. To any bioinorganic chemist this metalloprotein should look like a treasure trove: every FwdABCDFG heterohexamer has got a mononuclear tungsten centre, a dinuclear zinc centre, and quite a few iron—sulphur clusters. The enzyme exists as either a dimer or a tetramer of the FwdABCDFG heterohexamers. The authors suggest that the 24-meric complex (FwdABCDFG)4 is a physiologically active form. It contains 46 (yes, forty-six) [Fe4S4] clusters which form an electron wire between the redox-active tungsten centres. The function of this wire remains unclear though.
