31. Protein Breakdown and Urea Formation

Question 1

What happens to proteins in the diet? (3pts)

Answer 1

1. Proteins obtained in the diet are broken down into constituent amino acids.
2. In normal circumstances the amino acid pool and the body protein pool are in equilibrium so newly synthesised protein equals protein turnover.
3. Amino acids are used for biosythentic products and are broken down into nitrogen free intermediates which are used to generate glucose, energy source and ketone bodies.

Question 2

What happens to Ammonia released from amino acids?

Answer 2

It is metabolised by the liver to form Urea which is excreted by the kidney.

Question 3

What happens in a normal healthy indivdual?

Answer 3

The synthesis of new proteins is balanced by the breakdown of old proteins.

Question 4

Describe a positive nitrogen balance?

Answer 4

Build up of body proteins so the amino acid pool is directed towards the synthesis of new proteins

Question 5

Describe a negative nitrogen balance? (3pts)

Answer 5

1. Loose body protein
2. Increase production of urea
3. Takes place during:
4. Wasting disease, during burns/disease
5. Response to an increase catabolic or a reduction of anabolic hormones.

Question 6

Describe what happens to amino acids in the body?

Answer 6

Amino acids are used or broken down but not stored.

Question 7

What are amino acids made up of? (2pts)

Answer 7

1. Carbon skeleton

2. Nitrogen

Question 8

What happens during the metabolism of an amino acid?

Answer 8

an a-keto acid and Ammonia is formed via the enzyme transaminase or aminotransferase

Question 9

What is the carbon skeleton used for? (2pts)

Answer 9

1. Energy metabolism

2. Biosynthetic products

Question 10

What is nitrogen used for? (2pts)

Answer 10

1. Nitrogenous compounds

2. Urea

Question 11

Describe Transamination?

Answer 11

1. Amino acid A reacts with a-keto acid b.

2. Under the influence of transaminase a-ketone acid A and amino acid B is formed.

Question 12

What are A-keto acids? (3pts)

Answer 12

1. A-ketoglutarate
2. Pyruvate
3. Oxalocetate

Question 13

Describe a Transaminase reaction?

Answer 13

Alanine + a-ketoglutrate= Pyruvate+ glutamate

Question 14

Describe a Transaminase reaction?

Answer 14

Aspartate+ a-ketoglutrate= glutamate+oxalocetate

Question 15

What happens in the liver?

Answer 15

OAA and a-ketoglutarate is used to make glucose

Question 16

What happens in the muscle?

Answer 16

Pryuvate is used in the TCA cycle and electron transport chain to make ATP.

Question 17

What happens if there are high levels of AST and ALT in the blood?

Answer 17

High levels of AST and ALT in the blood are indicative of tissue damage particularly liver and cardiac muscle

Question 18

How is Ammonia formed?

Answer 18

Glutamate undergoes oxidative deamination to form Ammonia

Question 19

How is Ammonia eliminated?

Answer 19

Glutamine+ NH4+ + ATP= glutamine+ ADP

• occurs via glutamine synthase
• Glutamine is soluble and readily transported in the blood

Question 20

Describe the removal of nitrogen? (4pts)

Answer 20

1. Ammonia is toxic so has to be removed safely
2. Levels of Ammonia in the blood are low
3. Nitrogen is transported by the amino acids alanine and glutamine.
4. In mammals the ammonia is converted to urea and excreted in the urine.

Question 21

How is nitrogen transferred to urea? (3pts)

Answer 21

1. Transamination
2. Formation of ammonia
3. Synthesis of urea

Question 22

Describe the Urea cycle? (4pts)

Answer 22

1. Urea cycle excretes nitrogen
2. Enzymes are present in the liver but not muscle
3. Takes place in the mitochondria and the cytoplasm.
4. Substrates are bicarbonate, aspartate and ammonium ions.

Question 23

Describe the steps of the UREA cycle? (7pts)

Answer 23

1. CO2 and NH4+ react to form Carbamoyl Phosphate.
2. Carbonyl Phosphate reacts with orthinine to form Citruline which moves out of the mitochondria into the cytosol where it reacts with Aspartate.
3. The Aspartate is formed from the transamination of alpha-amino acids.
4. The citruline and aspartate react to form Argino-succinate which then forms arginine.
5. Arginine then forms urea and ornithine via the enzyme arginase.
6. Fumarate is converted to malate and oxaloacetate which alllows the cycle to continue.
7. Cycle continues.

Question 24

Describe what happens in the muscle? (3pts)

Answer 24

1. The enzymes of the urea cycle are not present
2. In prolonged exercise or starvation branched amino acids are used for energy.
3. 2 routes are used to transport nitrogen to the liver:
4. Alanine
5. Glutamine

Question 25

Describe the glucose Alanine cycle? (5pts)

Answer 25

1. Glucose is metabolised in muscles to pyruvate
2. Pryuvate will form alanine via transamination reaction.
3. Alanine is transported in the blood to the liver where it undergoes formation of glutamate and pyruvate
4. Pyruvate is used to make glucose which is transported across the body.
5. The glutamate forms the ammonium ion which is required for the formation of urea.

Question 26

What is the fate of the carbon skeleton? (2pts)

Answer 26

1. Ketogenic amino acids will be broken down to form acetly coA which will then be used to form ketone bodies
2. Glucogenic Amino acids will be broken down and fed into the TCA cycle.

• some amino acids are in both categories