Utilization of Inorganic Nitrogen

Why is nitrogen availability limited?

Limited by the abilities of organisms to utilize different inorganic forms of nitrogen, not all organisms can synthesize ammonia (NH₃) from the far more abundant forms of inorganic nitrogen dinitrogen gas (N₂),

All organisms can convert ammonia (NH₃) to organic nitrogen compounds that is, substances containing C-N bonds. And they all can convert NH3 to N2 via Catabolism

diazotrophs

The reduction of N₂ to NH₃ carried out only by certain prokaryotes CALLED.

NITROGEN FIXATION

nitrogen economy

supply, demand, turnover, reuse, growth, and maintenance of a steady state.

Within the biosphere, a balance is maintained between total inorganic and total organic forms of nitrogen.

How is the nitrogen economy maintained in relation to biosphere

The conversion of inorganic to organic nitrogen, which starts with nitrogen fixation or nitrate reduction, is counterbalanced by processes that return inorganic nitrogen to the biosphere.

Vice verse, inorganic to organic nitrogen…

Denitrification

Denitrification

Catabolism of proteins and nucleic acids and other macromolecules yields ammonia and various organic nitrogenous end products, which can in turn be oxidized by various bacteria. This is done to generate energy from this proteins and acids. But in turn we convert the nitrogen all the way back to N2

Going from Ammonia to inorganic nitrogen, back to the biosphere

leguminous plants

Plants that form symbiotic root nodules infected with nitrogen-fixing bacteria (diazotrophs), allowing them to grow without an exogenous nitrogen source.

Rhizobium

A notable genus of diazotrophs (bacteria) that infects the root nodules of leguminous plants to perform biological nitrogen fixation.

Leghemoglobin

An abundant protein found in root nodules that maintains an anaerobic environment by binding to stray oxygen (O₂).

why is Anaerobic Environment required for root nodules?

Critical in root nodules because the nitrogen-fixing enzymes (like nitrogenase) are extremely sensitive to and damaged by oxygen.

Why rotate crops like clovers, soybeans, or alfalfa with non-nitrogen-fixing crops?

To replenish the soil with nitrogen.

Name three examples of nitrogen-fixing crops used in crop rotation.

Clovers, soybeans, and alfalfa.

Rotating crops like soybeans with non-nitrogen-fixing crops helps to __________ the soil with __________.

Replenish; nitrogen.

Why is N₂ difficult to reduce?

because of the triple bond

nitrogenase

Biological N₂ reduction is catalyzed by these enzymes

The most abundant and widely studied nitrogenase is the molybdenum (Mo)-dependent enzyme, such as that found in Azotobacter vinelandii.

Protein turnover

• What it is: Protein turnover is the ongoing cycle of tearing down existing proteins and building fresh replacements.

• Why the cell destroys them:

  1. Quality control: To clear out proteins damaged by cellular "rust" (reactive oxygen species) or misfolding, which cannot be repaired.

  2. Pathway regulation: To rapidly shut down metabolic pathways by intentionally shredding short-lived regulatory enzymes when they are no longer needed.

• How it happens: Internal cellular machines (like the ubiquitin-proteasome system) selectively target and shred those proteins into free amino acids.

• The recycling loop: Those reclaimed amino acids are dumped back into the cell's shared pool and reused by ribosomes to build brand-new, functional proteins from scratch, keeping metabolism running smoothly.

do cells store nitrogen?

No nitrogen is continually lost through catabolism and waste, animals must constantly replenish their nitrogen supply through their diet to maintain "nitrogen equilibrium".

nitrogen equilibrium

The Balance: A well-nourished adult maintains this equilibrium when the nitrogen they consume through their diet (primarily from protein) is equal to the nitrogen they excrete through urine, sweat, and other metabolic processes.

• Daily Target: To maintain nitrogen equilibrium, a healthy 70-kg adult needs to consume about 8 grams of nitrogen a day, which equates to roughly 52 grams of dietary protein.

Consequence of Nitrogen Imbalance

If dietary protein intake drops too low, the body still needs nitrogen. To survive, it begins breaking down existing functional proteins (mostly muscle tissue) to harvest the nitrogen, and these lost proteins are not replaced.

The Two Arms of Protein Turnover

The Ubiquitin-Proteasome System (UPS)
The Lysosomal / Autophagy System

Lysosomes

Contains enzymes.

Play various cellular roles: secretion of digestive enzymes, digestion of organelles destined for destruction, digestion of food particles or bacteria engulfed by phagocytosis, and the nonselective engulfment and degradation of bulk cellular constituents

proteasome

Location: The proteasome is distributed throughout the cytosol and the nucleus of eukaryotic cells.

Function: The proteasome selectively destroys individual intracellular proteins tagged with a polyubiquitin chain (proteins modified by ubiquitin)

Degradation: It cleaves the target protein into short peptide fragments (typically 3 to 15 amino acids long). Cytosolic peptidases then chop these peptides into free amino acids to refill the intracellular amino acid pool.

The Ubiquitin-Proteasome System (UPS)

Uses ubiquitination as a high-precision barcode tag.

Individual soluble proteins (misfolded proteins, regulatory enzymes, transcription factors).

What exactly do the lysosome and ubiquitin-proteasome systems target?

• Damaged or Misfolded Proteins: The systems act as quality control, tagging and destroying defective proteins before they cause cellular harm. (individual amino acid building blocks are usually perfectly fine and used for other functions)

• Healthy Regulatory Proteins: To immediately shut off a metabolic pathway, the cell will actively tag and destroy perfectly functional, newly synthesized enzymes simply because they are no longer needed.

• Normal Tissue Proteins: As part of standard continuous turnover (what Schoenheimer discovered), completely healthy structural proteins are routinely selected for breakdown and replacement to keep tissues fresh and adaptable.

What were Rudolf Schoenheimer's three major findings regarding amino acid metabolism and protein stability?

1. Dynamic Turnover: Body proteins are not static. Existing tissues are constantly broken down and rebuilt using a shared "free amino acid pool" that mixes newly digested amino acids with recycled ones from old tissues.

2. Amino Group Transfer: Ingested amino acids are frequently disassembled rather than used exactly as they are. Their nitrogens are stripped off and transferred to completely different carbon skeletons to serve new biological functions.

3. Nitrogen Equilibrium (No Storage): The body has no storage depot for intact proteins or excess amino acids. To maintain equilibrium, total nitrogen intake must exactly equal total excretion (e.g., eating 1 gram of protein means 1 gram of protein-equivalent nitrogen must be excreted as urea).