2024 Lecture 7 N2 fixation

Nitrogen Fixation & Nitrogen Anabolism Overview

  • Instructor: Professor Cornelia Welte

  • Institution: Radboud University, Department of Microbiology

  • Course: Microbial Metabolism

Learning Objectives

  • Understand microbial metabolism related to:

    • Biogeochemical nitrogen cycle

    • Biogeochemical carbon cycle

Key Concepts in Microbial Metabolism

Biosynthesis and Anabolism

  • Microorganisms synthesize cellular macromolecules de novo (e.g., amino acids, fatty acids, sugars). De novo refers to processes or syntheses that are initiated from scratch rather than being derived from existing components. In the context of microbial metabolism, it often refers to the synthesis of cellular macromolecules, like amino acids, from basic precursors rather than breaking down pre-existing molecules.

  • Anabolism involves assimilatory pathways where compounds like cysteine can be essential for some organisms (auxotrophs).

Metabolism Definition

  • Metabolism = Catabolism + Anabolism

    • Catabolism: Breakdown of molecules to generate energy.

    • Anabolism: Synthesis of cellular components from nutrients.

Amino Acid Biosynthesis

  • Amino acids are composed of a carboxyl and an amino group.

  • Nitrogen is obtained via ammonium for transaminase reactions.

  • Formation of polypeptides from amino acids through peptide bonds.

  • Secondary structure formation: alpha-helix and beta-sheets.

Nitrogen Fixation Process

  • Nitrogenase enzyme facilitates the conversion of N2 to NH3 under anoxic conditions.

  • Reaction: N2 + 8H+ + 8e- + 16ATP → 2NH3 + H2 + 16ADP.

  • Nitrogen fixation is energy-intensive due to the strong triple bond in N2.

  • Protection of nitrogenase from oxygen through several mechanisms:

    • Anoxic conditions

    • High respiratory rate

    • Slime production

    • Heterocysts in certain cyanobacteria

Nitrogen Cycle

  • Nitrogen fixation makes nitrogen biologically available and counteracts its loss through processes like denitrification.

  • Symbiotic nitrogen fixation in legumes enhances soil fertility.

Root Nodule Symbiosis

  • Legume plants, like soybeans, form symbiotic relationships with nitrogen-fixing bacteria.

  • Bacteria fix N2 into ammonium, and plants provide sugars and shelter to bacteria.

  • ¼ of the planet's fixed nitrogen derives from root nodules.

  • Steps in the formation of root nodules include recognition, signaling, and formation of modified bacterial cells (bacteroids).

Conclusion: Importance of Nitrogen Fixation

  • Both symbiotic and free-living bacteria contribute to N2 fixation.

  • Essential to prevent depletion of nitrogen in ecosystems.

Lecture Questions

  • Key questions addressed include:

    • How is nitrogen incorporated into amino acids?

    • How is N2 incorporated into biomolecules?

    • Mechanism and role of nitrogenase?

    • Overall significance of nitrogen fixation in the nitrogen cycle?

    • Benefits of root nodule symbiosis?