en
Doctoral thesis
English

Cooperativity in biological nitrogen fixation by Mo-nitrogenase

Number of pages263
Imprimatur date2023
Defense date2023
Abstract

Nitrogen fixation consists in the activation of the inert triple bond of N2 and splitting it into ammonia, NH3, a bioavailable form of nitrogen. This chemical reaction represents a high energetic cost to achieve for both N-fixing micro-organisms and from an industrial chemistry aspect, via the Haber-Bosch process (HBP). In nature, the catalyst responsible for biological nitrogen fixation (BNF) is a complex multimeric metalloenzyme named nitrogenase that has evolved over the past three billion years to achieve efficiently this challenging reaction. Despite being considered a slow enzyme regarding the catalytic rate constant (kcat NH3 ~ 1.s-1 ) several orders of magnitude lower than other enzymes, it is a performant catalyst in term of kinetics, selectivity, and stability for this specific reaction. Yet, its origin, biosynthesis and mechanism at a molecular level are not fully elucidated to date. Nitrogenase undergoes N2 fixation at ambient conditions of pressure and temperature upon MgATP hydrolysis. Meanwhile the synthetic HBP, developed in the early 20th century achieves this reaction in comparatively harsh conditions of high pressure and temperature in the presence of heterogenous catalysts. One of the main drawbacks of HBP are the greenhouses gases (GHG) emissions associated, as virtually all dihydrogen (H2) feedstock originates from fossil fuels combustion. There is a real challenge for molecular chemist aiming to understand nitrogenase mechanism in order to develop more efficient bio-inspired catalysts. Nitrogenase contains peculiar metallic cofactors as it catalyses the transfer of electrons over a long distance across the protein shell for a reduction reaction. Another important aspect of nitrogenase resides in its dimeric structure, meaning that it possesses two active sites, yet the notion of cooperativity in nitrogenase was raised only a decade ago. Meanwhile it is well established that aerobic respiration by tetrameric haemoglobin is a cooperative process, there was no clear consensus regarding biological nitrogen fixation by nitrogenase at the time this work started. This thesis aimed to have a better understanding of cooperative effects in nitrogenase using in vitro reduction assays and surface immobilization studies. A resulting negative cooperativity was observed in standard N2 reduction assays, a key finding demonstrating the possible use of nitrogenase by electrochemistry for further mechanistic studies.

eng
Citation (ISO format)
CADOUX, Cécile Morgane. Cooperativity in biological nitrogen fixation by Mo-nitrogenase. 2023. doi: 10.13097/archive-ouverte/unige:174877
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