Protein Breakdown and Urea Formation

Question 1

What are the sources of Amino Acids (AA)?

What happens to excess AA in the body?

What is the importance of the Urea cycle?

Answer 1

• From the breakdown of dietary and body proteins.
• Any excess aren’t stored, and are instead metabolised into ammonia + keto acids.
• Ammonia is toxic, so its converted into urea and excreted from the body to prevent accumulation.

Question 2

Conversion of ammonia to urea stages:

1. Transamination

Answer 2

• Controlled by enzymes – most important enzymes are Alanine (ALT) and Aspartate (AST) Transaminase – high levels of these enzymes in the blood indicates liver damage.
o They transfer the R group from an amino acid to an α-keto acid.

ALT: Alanine + α-ketoglutarate ⇄ Pyruvate + Glutamate
AST: Aspartate + α-ketoglutarate ⇄ Oxaloacetate + Glutamate

• α-ketoglutarate, pyruvate, and oxaloacetate are α-keto acids, which can be oxidised in the citric acid cycle or used in gluconeogenesis to form glucose.
• Requires Pyridoxal Phosphate (PLP)

Question 3

2. Ammonia Formation

Answer 3

• Ammonia can be made by oxidative deamination by the enzyme Glutamate dehydrogenase, using NADP – occurs in Mitochondrial Matrix:

Glutamate → α-ketoglutarate + NH3

• The ammonia formed is a substrate for the Urea Cycle

• Free ammonia is also eliminated
o In brain, it reacts with glutamate to form glutamine – using glutamine synthase: ATP + Glutamate + NH4+ → Glutamine + ADP
o Glutamine is the main transporter of Nitrogen

Question 4

3. Urea Cycle

LOOK AT PICTURE OF CYCLE!

Answer 4

• Enzymes for it are ONLY PRESENT IN LIVER, but not in muscle - takes place in the mitochondria and cytoplasm

• Substrates used are Bicarbonate, Aspartate, and Ammonium ions
           o Bicarbonate (from Carbon Skeleton) reacts with Ammonium ions (from Glutamine) to form Carbamoyl Phosphate in the mitochondria
           o Aspartate (from transamination of oxaloacetate and glutamate) reacts with Citrulline in the cytoplasm to form Argininosuccinate (ASS), which is then converted into Arginine
           o Finally, as the Arginine is converted into Ornithine by the Arginase enzyme, Urea is formed

Question 5

What is the Glucose-Alanine Cycle? How does it work?

LOOK AT PICTURE OF CYCLE!

Answer 5

• In prolonged exercise/starvation, the branched amino acids are used for energy
o Lots of ammonia produced, which needs to be eliminated, but the enzymes for the urea cycle aren’t present in the muscle - only in liver

• There are routes used to transport the nitrogen to the liver, which can be by
o Alanine via glutamate and pyruvate
o Glutamine made from glutamate as it’s a good N transporter

Question 6

What are the types of Carbon skeletons?

What can the energy be produced as?

Answer 6

• Skeletons can be from Ketogenic or Glucogenic AA:
  Ketogenic – converted into Acetyl CoA/AcetocetylCoA
  Glucogenic – converted into Pyruvate or other intermediates of the Krebs Cycle
• Can be produced as ATP or as Glucose/Ketone bodies

Question 7

What is Nitrogen Balance?

What are the types of Nitrogen Balance and what causes it?

Answer 7

• There is no storage of amino acids, any excess is secreted – maintains amino acid pool
o In normal metabolism (high insulin, low glucagon), amino acids are used for protein synthesis
o In normal starvation (low insulin, high glucagon), amino acids are moved to the liver to form glucose

• Positive Nitrogen Balance: more AA taken in than excreted - response to ↑Anabolic hormones e.g. Pregnancy

Negative Nitrogen Balance: more AA excreted than taken in - response to ↑Catabolic/↓Anabolic hormones e.g. Burns, Trauma