With a bond energy of 946 kJ.mol-1, the triple bond of dinitrogen (N2) can be biologically broken only by nitrogenase, leading to the production of ammonium, in a reaction reminiscent of the chemical Haber-Bosch process. Nitrogenase also reduces carbon monoxide (CO, bond energy: 1079 kJ.mol-1) in short hydrocarbons, analogous to the Fischer-Tropsch process; it is therefore a key enzyme for the biological production of fertilizer and biofuels.
Despite extended biophysical and biochemical studies, many aspects of the complex nitrogenase reaction pathway remain obscure. To shed light on the reaction course, we will investigate several models: notably, the molybdenum nitrogenase of the soil bacterium Azotobacter vinelandii, which is one of the best described nitrogenases so far; the molybdenum nitrogenase of Chlorobaculum tepidum, similar to the well-characterized nitrogenase of Clostridium pasteurianum, but active at temperatures neighbouring 50°C; and the molybdenum nitrogenases of methanogenic Archaea, which remain to be characterized. Some of these methanogenic Archaea grow at very high temperatures, suggesting intermediate states could be trapped when their enzymatic reactions are carried out in laboratory conditions. Nitrogenases will be produced in the native organisms and characterized via a large number of biochemical and biophysical methods, from activity assays based on colorimetry and gas chromatography to a wide range of spectroscopy techniques (UV-Vis, EPR, MCD, Mössbauer, XES, XAS, EXAFS…) Enzyme variants will be generated by mutagenesis in order to tune and increase enzymatic activity.
C. tepidum, a thermophilic green sulfur bacterium, grows photoautotrophically and on nitrogen-fixation conditions; optimally, at temperatures comprised between 45°C and 50°C. C. tepidum is a model organism for the study of photosynthesis, however its energy metabolism in regard to nitrogen fixation has not been thoroughly characterized so far. We’re studying C. tepidum growth and nitrogen fixation activity on various substrates, and will complement these studies by transcriptomic analyses (cooperation) to identify the links between all its energy-harvesting pathways.
A crucial player in nitrogen fixation is the protein donating electrons to nitrogenase. In A. vinelandii, the nitrogenase physiological electron donors ferredoxin I and flavodoxin II have been thoroughly characterized; in Archaea, proteins playing this role remain to be identified.
We also aim to characterize the biosynthetic steps of cofactor maturation in methanogenic Archaea, including the specific processing of nitrogenase cofactor, but also the general iron-sulfur cluster biosynthesis system. At the heart of iron-sulfur cluster biosynthesis in Archaea, the SUF system is a key regulator of enzymatic reactions of energy conversion.
Proteins of interest will be either purified from the native host or heterologously produced in E. coli. Identification of complex components and interactants will be performed via methods to study protein-protein interactions (size-exclusion chromatography, pull-down) or iron-sulfur cluster transfer (iron staining after native PAGE, UV-vis spectroscopy, MCD, EPR).