An iron-sulfur hourglass: The trick of methanogens to fix CO2
Methane is an important greenhouse gas and also an important component of biogas. Methane is synthesized by a group of microorganisms called methanogenic archaea. Because of its function as a greenhouse gas and in biogas, there is considerable interests in understanding how methane is synthesized by methanogenic archaea.
To understand how methane is formed from CO2, the scientists focused on the enzyme that catalyzes the first step in methane formation that involves a challenging ATP-independent CO2 fixation process. This reaction is driven by the formyl-methanofuran dehydrogenase (Fwd) which fixes CO2 on the methanofuran cofactor to produce formyl-methanofuran. To understand how Fwd is able to fix CO2 without any energy requirement Dr. Tristan Wagner in the research group of Seigo Shima at the Max Planck Institute in Marburg solved the structure of the native complex in collaboration with Ulrich Ermler at the Max Planck Institute of Biophysics.
The X-ray structure led to unexpected results: Fwd is organized in a tetramer of heterohexamer, Fwd(ABCDFG)4, that has the shape of an hourglass and contains an outstanding 46 [4Fe-4S] clusters. The classic CO2 fixation (for instance in the Calvin cycle) starts with CO2 fixation as a carboxy group followed by a reduction. However, Fwd uses a different strategy where CO2 is firstly reduced to formate, which is then bound to the methanofuran to generate formyl-methanofuran. In more details, FwdB and FwdD subunits compose the formate dehydrogenase part, which funnels the CO2 to a tungstopterin active site via a specific hydrophobic channel. Then, electrons from ferredoxin are transferred by the numerous [4Fe-4S] clusters to the tungstopterin for the CO2 reduction to formate. The formate formed is then sent to the next active site via a hydrated internal cavity. The FwdA subunit is an amidohydrolase similar to urease and connects the formate to the methanofuran. The formyl-methanofuran is provided for the next steps in methane formation, which provides energy and biomass. One of the most astonishing features of the 800-kDa complex is a gigantic electronic wire that consists of 46 iron-sulfur clusters. All clusters are electronically connected for electron transfer. This raises the question of why nature has evolved such a so complicated relay? Is it only to perform long distance electron transfer? Or is it an electron-storage system or a communication device to synchronize the four tungstopterin active sites? We aim to answer to these questions in the near future.
Wagner, T., Ermler, U. & Shima, S. (2016)
The methanogenic CO2 reducing-and-fixing enzyme is bifunctional and contains 46 [4Fe-4S] clusters. Science 354, 114-117.