Urea Cycle and Nitrogen Metabolism: Key Concepts for Biochemistry

What are the two main sources of amino acids in the body?

Dietary protein and normal body-protein degradation/turnover.

What is an amino-acid pool?

A collection of free amino acids in blood and cells available for protein synthesis and catabolism.

What is nitrogen balance?

The relationship between nitrogen intake and nitrogen excretion.

What defines a balanced nitrogen state?

Nitrogen intake equals nitrogen excretion.

What is a negative nitrogen balance?

Nitrogen excretion is greater than nitrogen intake, often due to burns, malnutrition, or tissue injury.

What is a positive nitrogen balance?

Nitrogen intake is greater than nitrogen excretion, typically during growth or tissue repair.

How does nitrogen balance correlate with BUN/urea levels?

Greater protein breakdown increases nitrogen disposal as urea, leading to higher BUN levels.

What is the relationship between ammonia (NH₃) and ammonium (NH₄⁺)?

NH₄⁺ is the predominant form at physiological pH, while NH₃ is the membrane-crossing form.

Why do humans need a nitrogen-disposal system?

To eliminate excess nitrogen generated from protein turnover and amino-acid catabolism.

What toxic molecule can build up due to nitrogen imbalance?

Ammonia, which can cause hyperammonemia and is neurotoxic.

What are AST and ALT?

Aminotransferases involved in transamination reactions.

What does elevated plasma ALT indicate?

Liver-cell injury.

What does elevated plasma AST indicate?

Liver-cell injury.

What is the function of PLP in transamination?

PLP (pyridoxal phosphate) is required for aminotransferases to function.

What is the muscle-liver glutamine cycle?

A cycle that transports nitrogen safely from peripheral tissues to the liver using glutamine.

What is the glucose-alanine cycle?

A cycle that carries nitrogen from muscle to liver while recycling carbon back to muscle as glucose.

What is the pKa of the NH₄⁺/NH₃ equilibrium?

The pKa is 9.3.

At physiological pH, which form of nitrogen predominates?

NH₄⁺ predominates at physiological pH.

What happens to nitrogen during positive nitrogen balance?

Nitrogen is retained for protein synthesis, reducing the amount sent for disposal.

What are possible manifestations of hyperammonemia?

Tremor, slurred speech, somnolence, vomiting, cerebral edema, blurred vision, coma, and death.

What role does the liver play in nitrogen metabolism?

The liver converts excess nitrogen to urea for safe elimination.

What is the primary function of the urea cycle?

To convert free ammonia into urea in the liver.

What are the two nitrogen sources incorporated into urea?

Free ammonia (NH₃) and aspartate.

What is the main product of the urea cycle?

Urea.

What role does glutamate play in nitrogen metabolism?

It is the immediate precursor of both nitrogen sources for the urea cycle.

Which enzyme captures free ammonia in the urea cycle?

Carbamoyl phosphate synthetase I (CPS-I).

What is the function of ornithine transcarbamylase (OTC) in the urea cycle?

Transfers the carbamoyl group to ornithine to form citrulline.

What does arginase do in the urea cycle?

Hydrolyzes arginine to produce ornithine and urea.

Where does the urea cycle primarily take place?

In the liver, with steps occurring in both the mitochondrial matrix and cytosol.

What happens to urea excretion during early fasting?

Urea excretion increases due to protein breakdown supporting gluconeogenesis.

What is the expected urea excretion during a high-glucose fed state?

Low urea excretion due to less need to catabolize amino acids for glucose.

What laboratory value is most indicative of urea cycle deficiency?

Elevated blood ammonia (hyperammonemia).

What is the relationship between blood urea nitrogen (BUN) and urea cycle defects?

BUN is low because urea synthesis is impaired.

What differentiates CPS-I deficiency from OTC deficiency in terms of orotic acid levels?

CPS-I deficiency has low orotic acid, while OTC deficiency has high orotic acid.

What is the main nitrogen collector in amino acid metabolism?

Glutamate.

What is the significance of the glucose-alanine cycle?

It transports nitrogen and recycles glucose between muscle and liver.

What metabolic state is characterized by a shift toward ketone use and sparing of muscle protein?

Long fasting (5-6 weeks).

What is the pH ratio of NH₄⁺ to NH₃ at physiological pH (7.4)?

~100:1.

What is the pKa of ammonia?

9.3.

What occurs during prolonged fasting in terms of nitrogen metabolism?

Decreased muscle breakdown and urea excretion as the brain uses ketones.

What is the toxic nitrogen product produced in the body?

Ammonia.

What is the role of vitamin B6 in amino acid metabolism?

It acts as a cofactor for transamination reactions.

What happens to ammonia levels in the case of any urea cycle defect?

Ammonia levels increase and BUN decreases.

What is the classic differentiator for CPS-I deficiency?

Hyperammonemia with low orotic acid.

What is the classic differentiator for OTC deficiency?

Hyperammonemia with high orotic acid.

What is the overall pathway for nitrogen disposal in the body?

Amino-acid nitrogen → Glutamate → NH₃/Aspartate → Urea cycle → Urea.