Overview of the flow of nitrogen in the biosphere. Nitrogen, nitrites and nitrates are acted upon by bacteria (nitrogen fixation) and plants and we assimilate these compounds as protein in our diets. Ammonia incorporation in animals occurs through the actions of glutamate dehydrogenase and glutamine synthase. Glutamate plays the central role in mammalian nitrogen flow, serving as both a nitrogen donor and nitrogen acceptor.

Schematic of AA Catabolism(Breakdown)

1. Aminotransferase transfers NH3+ group

2. Oxidative deamination yields ammonia

3. Ammonia (toxic!) disposed of with the urea cycle

Urea Cycle

1 2

3

• Catalyzed by aminotransferases (aka transaminases), ex:

– Aspartate aminotransferase

– Aspartate + α-ketoglutarate ↔ Oxaloacetate + Glutamate

– Alanine aminotransferase

– Alanine + α-ketoglutarate ↔ Pyruvate + Glutamate

- Glucose-Alanine Cycle

1

• Catalyzed by glutamate dehydrogenase

• Located inside the mitochondria

• Compartmentalize release of toxic NH4+

• Allosterically regulated, responsive to energy charge

• Inhibited by GTP/ATP; activated by GDP/ADP

2

• Overview

• Step 3 requires the urea cycle (NH4+ → Urea)

3

Simplified Urea Cycle

Urea Cycle Step 1

• In the mitochondria

• Carbamoyl phosphate synthetase (CPS I)

• Irreversible reaction due to breakdown of 2 ATP

• Enzyme requires N-acetylglutamate– Signals free AA’s present; regulation tied to presence of AA’s

Urea Cycle Step 2

• In the mitochondria

• In the cytoplasm; Asp donates 2nd amino group

• Aspartate can be generated by aspartate aminotransferases.

• Oxaloacetate + Glutamate ↔ Aspartate + α-ketoglutarate

Urea Cycle Step 3

Urea Cycle Step 4

• Carbon skeleton of aspartate leaves as fumarate

Urea Cycle

2

1

3

45

Connections b/w Urea Cycle & CAC

Fatty Liver Cirrhosis of the Liver Healthy

• Defects in the urea cycle cause hyperammonemia (↑ NH4+)

• Ammonia is toxic to the nervous system

Neurotoxicity Associated with Ammonia

1. Ammonia (NH4+) buildup is neurotoxic, caused by failure to make urea via the urea cycle or to eliminate urea through kidneys.

2. It will affect the blood’s pH, increase pH due to Ammonia buildup3. In the brain it traverses the brain blood barrier and in the brain is converted to

glutamate via glutamate dehydrogenase, depleting amount of alpha keto glut.

4. As KetoGlut ↓, Oxaloacetate ↓ , CAC halts Absence of aerobic oxidative phosphorylation and TCA cycle activity Irreparable cell damage and neural cell death ensue.

Electron Transport Chain

Oxidative Phosphorylation

5. Also, ↑ glutamate leads to glutamine formation by glutamine synthetase. depleting levels of glutamate ↓, which are needed in neural tissue since

glutamate is both a neurotransmitter and a precursor for the synthesis of GABA.

GABA is chief inhibitory neurotransmitter in the mammalian CNS, it is directly responsible for the regulations of muscle tone in human.

Therefore, ↓ glutamate affect energy production + neurotransmission.

Neurotoxicity Associated with Ammonia