Describe the coupled redox reactions that lead to fixation of N2 into ammonia.
What will be an ideal response?
The coupled redox reactions that lead to N fixation are driven by the enzyme nitrogenase. Electrons flow through from e- donors (like NADH) to ferredoxin, which binds ATP, and transfers e- to the Fe-S cluster (Fd is oxidized and FeS is reduced - requiring 8 ATP). This reduces the FeMO protein (FeS is oxidized and FeMo is reduced). Finally, the electrons are passed to N2, which is reduced to NH3 (then - usually - protonated to NH4+). Nitrogen fixation requires four reduction cycles through nitrogenase:
1. Fe-S protein acquires 2e- from an electron transport protein such as ferredoxin, and then transfers them to the FeMo center.
2. The FeMo center binds 2H+, which is reduced to H2 gas.
3. N2 can now bind to the active site by displacing the H2.
4. Successive pairs of H+ and e- reduce N2 ? HN = NH ? H2N - NH2 ? 2NH3
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