The iron (Fe) proteins of molybdenum (Mo) and vanadium (V) nitrogenases mimic carbon monoxide (CO) dehydrogenase in catalyzing the interconversion between CO and CO under ambient conditions. Catalytic reduction of CO to CO is achieved in vitro and in vivo upon redox changes of the Fe-protein-associated [FeS] clusters. These observations establish the Fe protein as a model for investigation of CO activation while suggesting its biotechnological adaptability for recycling the greenhouse gas into useful products.

Photosynthesis uses chlorophylls for the conversion of light into chemical energy, the driving force of life on Earth. During chlorophyll biosynthesis in photosynthetic bacteria, cyanobacteria, green algae and gymnosperms, dark-operative protochlorophyllide oxidoreductase (DPOR), a nitrogenase-like metalloenzyme, catalyzes the chemically challenging two-electron reduction of the fully conjugated ring system of protochlorophyllide a. The reduction of the C-17=C-18 double bond results in the characteristic ring architecture of all chlorophylls, thereby altering the absorption properties of the molecule and providing the basis for light-capturing and energytransduction processes of photosynthesis. We report the X-ray crystallographic structure of the substrate-bound, ADP-aluminium fluoride-stabilized (ADP·AlF 3 -stabilized) transition state complex between the DPOR components L 2 and (NB) 2 from the marine cyanobacterium Prochlorococcus marinus. Our analysis permits a thorough investigation of the dynamic interplay between L 2 and (NB) 2 . Upon complex formation, substantial ATP-dependent conformational rearrangements of L 2 trigger the protein-protein interactions with (NB) 2 as well as the electron transduction via redox-active [4Fe-4S] clusters. We also present the identification of artificial "small-molecule substrates" of DPOR in correlation with those of nitrogenase. The catalytic differences and similarities between DPOR and nitrogenase have broad implications for the energy transduction mechanism of related multiprotein complexes that are involved in the reduction of chemically stable double and/or triple bonds.dynamic switch protein | electron transfer T he biosynthesis of chlorophylls is essential for the capture of global energy. This complex, multienzymatic process generates chlorophyllide a (Chlide) through the stereospecific reduction of the C-17=C-18 double bond of ring D in protochlorophyllide a (Pchlide) (Fig. 1A). Two completely unrelated enzymes have evolved for Pchlide reduction: a monomeric, lightdependent system (1), found in angiosperms and cyanobacteria, and the dark-operative protochlorophyllide oxidoreductase (DPOR), found in anoxygenic photosynthetic bacteria, cyanobacteria, algae, and gymnosperms (2). DPOR is a two-component metalloprotein comprising an ATP-dependent reductase (L 2 ) and a catalytic unit [(NB) 2 ], both sharing a substantial degree of structural and sequence identity with nitrogenase ( Fig. 1B) (3, 4). As in nitrogenase, both components of DPOR carry redox active metallocenters (5-8), which mediate the ATPdriven electron transfer from L 2 to the site of substrate reduction in (NB) 2 . L 2 and (NB) 2 are only transiently associated with each other during catalysis (9), and ATP hydrolysis triggers their association and dissociation, permitting control of the timing of the accompanying electron transfer process between the two proteins. Previously determined structures of L 2 and (NB) 2 (5-7) provided a static picture of DPOR catalysis. However, only the structural investi...

Methane (CH4) is the most abundant organic compound in the atmosphere, largely originating from anthropogenic and natural biogenic sources1. Traditionally, biogenic CH4 has been regarded as the nal product of the anoxic decomposition of organic matter by methanogenic Archaea. However, plants2-4, fungi5, algae6,7 and cyanobacteria8 have recently been shown to produce CH4 in the presence of oxygen. While methanogens produce CH4 enzymatically during anaerobic energy metabolism9, the requirements and pathways for CH4 production by "non-methanogenic" cells are poorly understood. Here we demonstrate that CH4 formation by Bacillus subtilis is triggered by free iron species, enhanced by oxidative stress and restricted to metabolically active life-cycle stages. We also show that other model organisms from Bacteria and Eukarya including a human cell line release CH4 and respond to inducers of oxidative stress by enhanced CH4 formation. Our results imply that all living cells possess a common mechanism of CH4 formation without the need for speci c enzymes. We propose that CH4 formation is a conserved feature of living systems which is coupled to metabolic activity and the concomitant generation of reactive oxygen species. Our ndings open new perspectives for our understanding of environmental CH4 cycling, oxidative stress responses and the search for extraterrestrial life. BackgroundMethane is a highly potent greenhouse gas that affects Earth's climate. Around 70 % of all emissions to the atmosphere derive from biogenic sources 10 . Biological CH 4 formation has long been considered to occur only under strictly anoxic conditions in organisms belonging to the domain Archaea. To generate the cellular fuel ATP, methanogenic archaea convert simple compounds, such as CO 2 , H 2 or acetate, into CH 4 . This process of methanogenesis depends on reactions that are catalysed by unique sets of enzymes and co-enzymes 9 . In addition, small amounts of CH 4 can be formed via 'mini-methanogenesis' in several sulphate-reducing bacteria which contain the enzyme carbon monoxide dehydrogenase 11,12 . However, during the past 15 years, evidence has been accumulating that other organisms produce CH 4 under aerobic conditions. These include both multicellular organisms, such as plants 2-4 and saprotrophic fungi 5 , and unicellular organisms, including marine and freshwater algae 6,13 and cyanobacteria 8 . These organisms generate energy via photosynthesis and/or respiration, and it is unclear why and how they release CH 4 . Multiple marine and freshwater bacteria harbouring the C-P lyase pathway have been reported to generate CH 4 from methylphosphonate [14][15][16][17][18] . Several bacteria and archaea have also been shown to possess alternative nitrogenases 19,20 or nitrogenase-like reductases 21 , which can produce CH 4 and other hydrocarbons. In addition -as we will demonstrate here -living systems can form CH 4 without the need for speci c enzymes, and such pathways could drive CH 4 formation in all cells.In plants, CH 4 formation is enha...

The Mo- and V-nitrogenases are two homologous enzymes with distinct structural and catalytic features. Previously, we demonstrated that the V-nitrogenase was nearly 700 times more active than its Mo-counterpart in reducing CO to hydrocarbons. Here, we report a similar discrepancy between the two nitrogenases in the reaction of CO2 reduction, with the V-nitrogenase capable of reducing CO2 to CO, CD4, C2D4 and C2D6, and its Mo-counterpart only capable of reducing CO2 to CO. Further, we show that V-nitrogenase may route the formation of CD4 in part via CO2-derived CO, but it does not catalyze the formation of C2D4 and C2D6 via this route. The exciting observation of C-C coupling by V-nitrogenase from CO2 adds another interesting reaction to the catalytic repertoire of this unique enzyme system; whereas the differential activities of V- and Mo-nitrogenases in CO2 reduction provide an important framework for systematic investigations of this reaction in the future.