Protein breakdown and urea formation Flashcards
What can the carbon skeleton of a polypeptide be used for? [2 marks]
- Biosynthesis pathways
- Energy metabolism
Enzymes involved in transamination [2 marks]
- Alanine transaminase (ALT)
- Aspartate transaminase (AST)
What does ALT do to alaninie? [2 marks]
alanine + α-ketoglutarate ↔ pyruvate + glutamate
(equilibrium to the right)
α-ketoglutarate and pyruvate can be oxidised or converted to glucose
What does AST do to aspartate? [2 marks]
aspartate + α-ketoglutarate ↔ oxaloacetate + glutamate
(equilibrium to the left)
α-ketoglutarate and oxaloacetate can be oxidised or converted to glucose
Which amino acids don’t go under transamination? [2 marks]
- Threonine
- Lysine
Where does oxidative deamination take place? [1 mark]
The mitochondrial matrix
What happens in oxidative deamination? [2 marks]
- Glutamate is converted to α-ketoglutarate.
- NADPH and NH3 is released.
How is ammonia removed? [1 mark]
It is combined with glutamate to give glutamine.
Where does the urea cycle take place? [2 mark]
- In the liver
- In the mitochondria and cytoplasm
What are the substrates in the urea cycle (from glutamine or glutamate)? [3 marks]
- Bicarbonate
- Aspartate
- Ammonium ions
(from breakdown of the carbon skeleton)
What does the muscle use in prolonged exercise or starvation? [3 marks]
Branched amino acids such as
- Leucine
- Isoleucine
- Valine
How is nitrogen transported to the liver? [2 marks]
- Alanine and glutamine
- Nitrogen transferred to alanine via glutamate and pyruvate.
Why doesn’t the urea cycle happen in muscle? [2 marks]
- It lacks enzymes for the formation of urea
- Transaminase
What does glutamine synthase do? [1 mark]
Converts glutamate to glutamine with ATP and ammonia.
What can glutamate form? [2 mark]
Urea
OR
Pyruvate
What happens in muscle (glucose-alanine cycle)? [3 marks]
- Glucose undergoes glycolysis to form pyruvate.
- Pyruvate is converted to alanine.
- Alanine reaches the liver via the bloodstream.
What happens in liver (glucose-alanine cycle)? [3 marks]
- Alanine undergoes transamination to form urea.
- Pyruvate is converted to glucose.
- Glucose goes to muscle via bloodstream.
What does protein/amino acid metabolism look like after a meal? [4 marks]
- Positive protein balance
- Most amino acids from a meal will be used for protein synthesis in peripheral tissues.
- Excess amino acids will be used as sources of energy.
Nitrogen will be incorportated into urea in the liver and excreted.
What does protein/amino acid metabolism look like during starvation? [3 marks]
- Negative protein balance
- Net flow of amino acid from muscle to liver.
- Increased production of glucose and urea.
What can a negative protein balance cause and how? [3 marks]
Muscle wasting due to:
- Decreased protein synthesis
- Increased protein breakdown
Where can muscle wasting occur? [4 marks]
- Untreated diabetes
- Chronic infections
- Late stage cancer
- Trauma e.g. burns, surgery
