Why Do We Need Amino Acids?

• Proteins: Amino acids are the building blocks of proteins, essential for various biological functions.
• Synthesis of Nucleic Acids: Amino acids play a role in the biosynthesis of nucleic acids.
• Heme Group: Important for hemoglobin and myoglobin formation.
• Neurotransmitters: Serve as precursors for neurotransmitters, which transmit signals in the nervous system.

Atmospheric Nitrogen Problem

• Composition of Atmosphere:
  • 78% Nitrogen (N₂)
  • 21% Oxygen (O₂)
  • Trace gases (argon, carbon dioxide, helium, etc.)
• The Problem: How to utilize atmospheric nitrogen biologically?

Nitrogen Fixation

• Haber Bosch Process: Industrially converts atmospheric nitrogen (N₂) to ammonia (NH₃).

  • Reaction: $N<em>2 + 3H</em>2 <br /> ightarrow 2NH_3$
  • Uses high temperatures, pressures, and metal catalysts.

• Natural Fixation:

  • Bacteria, mainly Rhizobium, in legume root nodules fix nitrogen.
  • Almost 60% of Earth's nitrogen fixation is done by diazotrophic microorganisms.

Mechanism of Nitrogen Fixation

• Root Nodules:

  • Rhizobium bacteria colonize legume roots, forming nodules.
  • NodD, a transcriptional regulator, interacts with flavonoids to induce nod genes necessary for root nodule formation.

• Nitrogenase Complex:

  • Consists of two parts: Nitrogenase reductase (Fe-protein) and Nitrogenase (Fe-Mo protein).
  • Converts N₂ to NH₃ in the cytoplasm of nitrogen-fixing bacteria.
  • Requires 8 electrons and 16 ATP molecules for the reaction:
    $N<em>2 + 8e^- + 8H^+ + 16ATP + 16H</em>2O <br /> ightarrow 2NH<em>3 + H</em>2 + 16ADP + 16P_i$

• Protection Against Oxygen:

  • Nitrogenases are inactivated by oxygen.
  • Leghemoglobin: A hemoglobin-like protein that binds oxygen in nodules, protecting nitrogenases and allowing ammonia synthesis.

Sustainable Agriculture

• Fertilizers:
  • Increase soil nitrogen but lead to environmental impacts (e.g., excess algae growth in waterways due to nitrogen runoff).
  • Nitrous oxide acts as a greenhouse gas.
• Alternative Approaches:
  • Emphasis on practices that enhance natural nitrogen fixation, such as using indigenous plant varieties that effectively capture nitrogen from the air.

Amino Acid Synthesis

• Assimilation of Ammonium:
  • Ammonium ions are used to synthesize amino acids through intermediates in the citric acid cycle.
• Conversion Process:
  1. Reduction: Nitrate is reduced to nitrite and subsequently to ammonia via nitrate reductase and nitrite reductase.
  2. Nitrogen Donors: Glutamate and glutamine serve as nitrogen donors for amino acids.

Pathways to Amino Acids

• Biosynthetic Families:

  • Major metabolic precursors include glycolysis, citric acid cycle, and pentose phosphate pathway.

• Transamination Reactions:

  • Transfer of an amino group from one amino acid to an alpha-ketoacid to form new amino acids.
  • Example: Converting oxaloacetate to aspartate.

• Amidation Reactions:

  • Direct addition of an amide group to a molecule.

• One Carbon Transfer:

  • Conversion of serine to glycine requires tetrahydrofolate, a derivative of vitamin B9.
  • Example Pathway:
    $3 ext{-Phosphoglycerate} <br /> ightarrow ext{Serine} <br /> ightarrow ext{Cysteine} <br /> ightarrow ext{Glycine}$

Aromatic Amino Acid Biosynthesis

• Shikimate Pathway:
  • Converts phosphoenolpyruvate and erythrose-4-phosphate to aromatic amino acids (tyrosine, phenylalanine, tryptophan).

Essential vs Non-Essential Amino Acids

• Essential Amino Acids: Must be obtained from the diet (e.g., histidine, isoleucine, lysine, methionine, etc.).
• Non-essential Amino Acids: Can be synthesized by the body (e.g., alanine, glutamine, proline, etc.).

Summary

• Amino Acids are crucial for protein synthesis and various biological processes.
• Nitrogen Fixation is vital for converting atmospheric N₂ into a usable form for life, mainly facilitated by specific bacteria in legumes.
• Biosynthesis Pathways involve complex chemical transformations from simple precursors to produce the necessary amino acids for life.