Chapter 22: Biosynthesis of Amino Acids, Nucleotides, and Related Molecules

Learning Objectives

• Nitrogen Metabolism
• Biosynthesis of Amino Acids
• Molecules Derived From Amino Acids
• Biosynthesis and Degradation of Nucleotides

Nitrogen Metabolism

The Nitrogen Cycle (Refer to Fig. 22-1)

• Nitrogen fixation process: Conversion of atmospheric nitrogen (N₂) to ammonia (NH₄⁺) by bacteria and archaea.
• Most organisms assimilate nitrogen effectively from ammonia (NH₃).
• Soil bacteria oxidize ammonia into nitrite (NO₂⁻) and nitrate (NO₃⁻) through nitrifying reactions.
• Both plants and microorganisms assimilate nitrogen from nitrite and nitrate.
• Some bacteria utilize nitrate as the electron acceptor to establish a proton gradient for ATP synthesis through denitrifying reactions.
• A few bacteria can convert ammonia and nitrite back into nitrogen gas (N₂) via the anammox reaction.
• Summary Points:
  • Synthesis occurs in plants and microorganisms.
  • Degradation occurs in animals and microorganisms.

Nitrogen Fixation: The Nitrogenase Complex

• Components:
  • Fe-Mo cofactor, ADP, and Fe-S clusters are involved in nitrogen fixation reactions.
  • The dinitrogenase reductase, a Fe protein, is crucial in the nitrogenase complex.

Nitrogen Fixation Process

1. Electron Transfer: Pyruvate donates electrons to either ferredoxin or flavodoxin.
2. Reducing Agent: Ferredoxin/flavodoxin transfer electrons to dinitrogenase reductase.
3. ATP Interaction: Dinitrogenase reductase interacts with ATP to enhance reduction potential.
4. Final Electron Transfer: Dinitrogenase receives electrons from dinitrogenase reductase.
5. Nitrogen Conversion: Dinitrogenase converts nitrogen gas (N₂) to ammonia (NH₃).

Reaction Summary

• The net reaction for nitrogen fixation can be summarized as:
  $ext{N}<em>2 + 10 ext{H}^+ + 8 ext{e}^- + 16 ext{ATP} + 16 ext{H}</em>2 ext{O} = 2 ext{NH}<em>4^+ + ext{H}</em>2 + 16 ext{ADP} + 16 ext{P}_i$
• Catalysis by Dinitrogenase (FeMo protein):
  • Transfers 6 electrons to nitrogen (producing NH₃).
  • Transfers 2 electrons to protons (generating H₂).
• Biological synthesis of ammonia (NH₃) requires substantial ATP hydrolysis.

Incorporation of Ammonium Ion (NH₄⁺) into Biomolecules

Through Glutamine and Glutamate

• Glutamine Synthesis:
  • Reaction: Glutamate + NH₄⁺ + ATP
  • Catalyzed by Glutamine Synthetase.
• Net Reaction: Glutamine produced along with ADP, Pi, and H⁺.
• Glutamate Synthesis in Mammals:
  • Reaction involves $ ext{a-Ketoglutarate}$ + NH₄⁺ using Glutamate Dehydrogenase.
  • The process may also involve NAD(P)H equipment.

Glutamate Synthesis in Bacteria and Plants

• In bacteria and plants, glutamate is produced primarily from glutamine via Glutamate Synthase.
• Net Reaction:
  $ext{α-ketoglutarate} + ext{NH}_4^+ + ext{NADPH} + ext{ATP} \ <br /> ightarrow ext{Glutamate} + ext{ADP} + ext{Pi} + ext{H}^+$

Glutamine as a Nitrogen Donor: Amidotransferases

• Amidotransferase moves nitrogen from an amide group in glutamine.
• Aminotransferases move nitrogen from amine groups.
• Mechanism: Involves two structural domains of the enzyme with conserved cysteine acting as a nucleophile.

Biosynthesis of Amino Acids

Overview

• Amino acids (AAs) are synthesized from metabolic intermediates of glycosylation, citric acid cycle, and pentose phosphate pathway.
• Essential vs. Nonessential Amino Acids:
  • Essential Amino Acids (9): Cannot be synthesized by the body, must be sourced from diet (e.g., Leucine, Methionine, etc.).
  • Conditionally Nonessential: Required in specific situations (e.g., during growth).
  • Nonessential Amino Acids (11): Synthesized by the body from essential AAs or through normal protein breakdown.

Amino Acid Biosynthetic Families (6 Groups)

• α-Ketoglutarate Family: Glutamate, Glutamine, Proline, Arginine
• Oxaloacetate Family: Aspartate, Asparagine, Methionine, Lysine, Threonine
• Pyruvate Family: Alanine, Valine, Leucine, Isoleucine
• Phosphoenolpyruvate + Erythrose 4-Phosphate Family: Phenylalanine, Tyrosine, Tryptophan
• 3-Phosphoglycerate Family: Serine, Cysteine, Glycine

Detailed Pathway Descriptions

α-Ketoglutarate to Glutamate/Glutamine

• Enzyme: Glutamate Dehydrogenase facilitates conversion of α-ketoglutarate to glutamate using NAD(P)H.
• Reaction:
  $ext{α-Ketoglutarate} + ext{NH}<em>4^+ + ext{NAD(P)H} \rightarrow ext{Glutamate} + ext{NAD(P)}^+ + ext{H}</em>2 ext{O}$

Conversion Steps to Other Amino Acids

Glutamate to Proline (Reduction)

• Glutamate is reduced to Glutamate y-semialdehyde and further converted to Proline.

Glutamate to Arginine (Reduction)

• Arginine later synthesized in the urea cycle, sometimes considered essential due to its loss to urea and ornithine.

3-Phosphoglycerate Family

• Pathway from 3-Phosphoglycerate to Serine: Involves several enzymatic reactions, with glutamate serving as an amino donor.
• Reaction Detailed in steps:
  1. Conversion of 3-Phosphoglycerate to 3-Phosphohydroxypyruvate.
  2. Aminotransferase mediates conversion to phosphoserine.
  3. Phosphoserine transformed to Serine.

Pyruvate Family Pathways

• Pathway to Alanine, Valine, Isoleucine, and Leucine: Involves multiple enzymatic steps with aminotransferases as key components.
• Branched-chain AA Synthesis: Valine and isoleucine share pathways after an initial step with branched synthesis pathways.

Summary of Essential Amino Acid Synthesis

• Branched chain AAs: Valine, leucine, and isoleucine directly derive from pyruvate.
• Through transamination, non-essential amino acids are synthesized including alanine and aspartate.
• Aromatic AAs: Synthesized from phosphoenolpyruvate and erythrose 4-phosphate.
• Histidine synthesis: Relies on ribose-5-phosphate.

Ethical, Philosophical, or Practical Implications

• Understanding amino acid biosynthesis is crucial for fields like nutrition, medicine, and biochemistry, emphasizing the importance of dietary sources.
• Conditions affecting amino acid availability can influence health, requiring insights into metabolism and synthesis for better clinical outcomes.