Nitrogen Fixation and Assimilation

Study Notes

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.

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.

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.

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

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).

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.

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.

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

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).