Nitrogen fixation and assimilation are crucial processes in the nitrogen cycle. They convert atmospheric nitrogen into forms that living organisms can use, supporting plant growth and soil fertility. This topic explores how bacteria, plants, and industrial processes make nitrogen available for life.

The notes cover the enzymes involved in nitrogen fixation, like nitrogenase, and compare biological and industrial methods. They also explain how plants and microbes take up and incorporate nitrogen into organic compounds, highlighting key steps in nitrogen metabolism.

Nitrogen fixation: The process and its importance

Process of nitrogen fixation

• Nitrogen fixation converts atmospheric nitrogen (N₂) into biologically available forms (ammonia NH₃)
• Primarily carried out by prokaryotes called diazotrophs (certain bacteria and archaea)
• Requires significant energy input due to the strong triple bond in N₂ molecule
• Occurs through biological or industrial methods
  • Biological fixation happens at ambient temperature and pressure
  • Industrial fixation requires high temperatures and pressures (Haber-Bosch process)

Importance in the nitrogen cycle

• Crucial for maintaining soil fertility and supporting plant growth
• Most organisms cannot directly utilize atmospheric nitrogen
• Part of the larger nitrogen cycle involving various conversions
  • Nitrification converts ammonia to nitrite and nitrate
  • Denitrification returns nitrogen to the atmosphere
  • Ammonification breaks down organic nitrogen into ammonia
• Symbiotic relationships play vital role in agricultural ecosystems
  • Rhizobium-legume associations fix nitrogen in root nodules
  • Enhances soil fertility without synthetic fertilizers

Key enzymes in nitrogen fixation

Nitrogenase complex

• Primary enzyme responsible for biological nitrogen fixation
• Catalyzes the reduction of N₂ to NH₃
• Consists of two components:
  • Dinitrogenase reductase (iron protein)
  • Dinitrogenase (molybdenum-iron protein)
• Dinitrogenase reductase transfers electrons to dinitrogenase
• Dinitrogenase contains the active site for N₂ reduction
• Requires anaerobic conditions for optimal function

Supporting enzymes

• Hydrogenase enzymes recycle hydrogen produced during nitrogen fixation
  • Improves overall efficiency of the process
  • Helps conserve energy in nitrogen-fixing organisms
• Leghemoglobin maintains oxygen-free environment in legume root nodules
  • Binds oxygen to protect nitrogenase from inactivation
  • Similar function to hemoglobin in blood
• Glutamine synthetase assimilates fixed nitrogen
  • Catalyzes formation of glutamine from ammonia and glutamate
  • Key step in incorporating fixed nitrogen into organic compounds

Biological vs industrial nitrogen fixation

Process differences

• Biological fixation occurs in living organisms
  • Utilizes enzymes and cellular machinery
  • Happens at ambient temperature and pressure
• Industrial fixation purely chemical process (Haber-Bosch)
  • Uses iron-based catalysts
  • Requires high temperatures (400-450°C) and pressures (200-300 atm)
• Biological fixation utilizes renewable energy (ATP from metabolism)
• Industrial fixation consumes large amounts of fossil fuels
  • Contributes to greenhouse gas emissions
  • Energy-intensive process

Efficiency and environmental impact

• Industrial fixation has much higher rate of nitrogen fixation
  • Meets global demand for fixed nitrogen in agriculture
  • Produces about 450 million tons of nitrogen fertilizer annually
• Biological fixation more sustainable and environmentally friendly
  • Reduces need for synthetic fertilizers
  • Minimizes nitrogen runoff and water pollution
• Industrial fixation associated with significant environmental costs
  • Contributes to eutrophication of water bodies
  • Produces nitrous oxide, a potent greenhouse gas
• Biological fixation limited by natural constraints
  • Cannot meet current global agricultural demands alone
  • Supplemented by industrial fixation in modern agriculture

Nitrogen assimilation in plants and microorganisms

Uptake and reduction of inorganic nitrogen

• Begins with uptake of inorganic nitrogen compounds
  • Nitrate (NO₃⁻) or ammonium (NH₄⁺) absorbed from environment
• In plants, nitrate reduced to nitrite in cytoplasm
  • Catalyzed by nitrate reductase enzyme
  • Requires NADH or NADPH as electron donor
• Nitrite further reduced to ammonium in chloroplasts
  • Catalyzed by nitrite reductase enzyme
  • Uses reduced ferredoxin as electron donor
• Microorganisms have similar pathways with variations
  • Some can use nitrate as terminal electron acceptor in anaerobic respiration

Incorporation into organic compounds

• Ammonium incorporated through glutamine synthetase-glutamate synthase (GS-GOGAT) cycle
• Two key steps in GS-GOGAT cycle:
  1. Glutamine synthetase catalyzes glutamine formation
     • Combines ammonium with glutamate
     • Requires ATP for energy
  2. Glutamate synthase (GOGAT) produces two glutamate molecules
     • Transfers amide group from glutamine to α-ketoglutarate
     • Uses NADH or NADPH as reducing agent
• Assimilated nitrogen distributed to other compounds
  • Transamination reactions transfer amino groups
  • Biosynthetic pathways produce various amino acids
  • Nucleotides and other nitrogenous compounds synthesized