Nitrogen Fixation and Assimilation

Nitrogen Fixation

• Nitrogen from the atmosphere is reduced to ammonia (NH3) by nitrogen-fixing bacteria.
• Plants and animals are unable to carry out nitrogen fixation.
• Nitrate assimilation is the primary mechanism by which green plants, algae, and other microorganisms acquire nitrogen.
• Denitrification: nitrate is converted back into atmospheric nitrogen, making it non-reactive.
• Ammonia is converted into nitrate before being used by plants.

Nitrate Assimilation

• Two-step process:
  • Nitrate ion is reduced to nitrite (cytosol).
  • Nitrite is reduced to ammonium ion (chloroplast) catalyzed by nitrite reductase, involving a transfer of 8 electrons.

Dynamics of Protein Metabolism

• Primary source of protein is dietary proteins, which are converted into amino acids through digestion.
• Amino acids are transported in blood to cells, where they will undergo protein synthesis to produce functional proteins.
• Functional proteins undergo protein degradation in proteasomes, followed by ubiquitin, and are converted back into amino acids, which can then be reused for protein synthesis.

Dietary Proteins

• Digestion of dietary proteins provides:
  • Amino acids for protein biosynthesis.
  • Nitrogen for nitrogen-containing compounds.
  • Energy when carbohydrate and lipid resources are depleted.

Digestion of Dietary Proteins

• Mouth: no digestion, only mechanical breaking of food into smaller particles.
• Stomach:
  • Gastric mucosa secretes gastrin, stimulating the secretion of HCl and pepsinogen.
  • HCl denatures proteins, and pepsinogen is converted into pepsin.
• Small intestine:
  • Secretin stimulates the release of HCO3-, making the pH neutral.
  • Cholecystokinin stimulates the pancreas to secrete proteases (trypsin, chymotrypsin, thermolysin, carboxypeptidase, and aminopeptidase).

Protein Turnover

• Process of degradation and re-synthesis of proteins in the body.
• Constantly occurring in cells due to different protein half-lives.
• Non-functional or misfolded proteins are targeted for degradation to reuse amino acids for synthesizing other proteins.

Ubiquitin

• A small protein (8.5 kDa) present in all eukaryotic organisms.
• Tags proteins for degradation by forming an isopeptide bond between a Lys residue of the protein and ubiquitin's C-terminal glycine residue.

Protein Denaturation

• Targeted proteins are degraded in proteasomes.
• Pathway: ubiquitin is added to the protein substrate, then transported and released.

Free Amino Acids

• Channeled to:
  • Protein synthesis.
  • Synthesis of nitrogen-containing compounds (purines, pyrimidines, heme of hemoglobins, neurotransmitters, hormones).
  • Energy production (excess amino acids are degraded for energy).