Protein breakdown and urea formation

Question 1

What can the carbon skeleton of a polypeptide be used for? [2 marks]

Answer 1

• Biosynthesis pathways

- Energy metabolism

Question 2

Enzymes involved in transamination [2 marks]

Answer 2

• Alanine transaminase (ALT)

- Aspartate transaminase (AST)

Question 3

What does ALT do to alaninie? [2 marks]

Answer 3

alanine + α-ketoglutarate ↔ pyruvate + glutamate
(equilibrium to the right)
α-ketoglutarate and pyruvate can be oxidised or converted to glucose

Question 4

What does AST do to aspartate? [2 marks]

Answer 4

aspartate + α-ketoglutarate ↔ oxaloacetate + glutamate
(equilibrium to the left)
α-ketoglutarate and oxaloacetate can be oxidised or converted to glucose

Question 5

Which amino acids don’t go under transamination? [2 marks]

Answer 5

• Threonine

- Lysine

Question 6

Where does oxidative deamination take place? [1 mark]

Answer 6

The mitochondrial matrix

Question 7

What happens in oxidative deamination? [2 marks]

Answer 7

• Glutamate is converted to α-ketoglutarate.

- NADPH and NH3 is released.

Question 8

How is ammonia removed? [1 mark]

Answer 8

It is combined with glutamate to give glutamine.

Question 9

Where does the urea cycle take place? [2 mark]

Answer 9

• In the liver

- In the mitochondria and cytoplasm

Question 10

What are the substrates in the urea cycle (from glutamine or glutamate)? [3 marks]

Answer 10

• Bicarbonate
• Aspartate
• Ammonium ions

(from breakdown of the carbon skeleton)

Question 11

What does the muscle use in prolonged exercise or starvation? [3 marks]

Answer 11

Branched amino acids such as

• Leucine
• Isoleucine
• Valine

Question 12

How is nitrogen transported to the liver? [2 marks]

Answer 12

• Alanine and glutamine

- Nitrogen transferred to alanine via glutamate and pyruvate.

Question 13

Why doesn’t the urea cycle happen in muscle? [2 marks]

Answer 13

• It lacks enzymes for the formation of urea

- Transaminase

Question 14

What does glutamine synthase do? [1 mark]

Answer 14

Converts glutamate to glutamine with ATP and ammonia.

Question 15

What can glutamate form? [2 mark]

Answer 15

Urea
OR
Pyruvate

Question 16

What happens in muscle (glucose-alanine cycle)? [3 marks]

Answer 16

• Glucose undergoes glycolysis to form pyruvate.
• Pyruvate is converted to alanine.
• Alanine reaches the liver via the bloodstream.

Question 17

What happens in liver (glucose-alanine cycle)? [3 marks]

Answer 17

• Alanine undergoes transamination to form urea.
• Pyruvate is converted to glucose.
• Glucose goes to muscle via bloodstream.

Question 18

What does protein/amino acid metabolism look like after a meal? [4 marks]

Answer 18

• 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.

Question 19

What does protein/amino acid metabolism look like during starvation? [3 marks]

Answer 19

• Negative protein balance
• Net flow of amino acid from muscle to liver.
• Increased production of glucose and urea.

Question 20

What can a negative protein balance cause and how? [3 marks]

Answer 20

Muscle wasting due to:

• Decreased protein synthesis
• Increased protein breakdown

Question 21

Where can muscle wasting occur? [4 marks]

Answer 21

• Untreated diabetes
• Chronic infections
• Late stage cancer
• Trauma e.g. burns, surgery