S1 - Protein + Amino Acid Metabolism

Question 1

What is the difference between nitrogen balance and protein turnover?

Answer 1

Nitrogen balance = the balance between protein intake and outake.
Protein turnover = the balance between protein synthesis and protein degradation.

Question 2

Name some states when someone might have a positive nitrogen balance.

Answer 2

Pregnancy
Growth
Recovering from malnutrition

Question 3

Name some states when someone might have a negative nitrogen balance.

Answer 3

Malnutrition

After trauma or infection

Question 4

What is the difference between glucogenic and ketogenic amino acids?

Answer 4

In terms of proteolysis, glucogenic amino acids are converted into glucose.
Ketogenic amino acids are converted into acetyl CoA (which is a precursor of ketone bodies).

Question 5

Give an example of a glucogenic and ketogenic amino acid, and one which is both.

Answer 5

Glucogenic = valine
Ketogenic = lysine
Both = tyrosine

Question 6

What are essential amino acids? Name a few examples.

Answer 6

Amino acids that can't be synthesised by the body, therefore must be present in the diet.
Isoleucine
Lysine
Theonine
Histidine
Leucine
Methionine
Phenylalanine
Tryptophan
Valine
If Learned This Huge List May Prove Truly Valuable

Question 7

What are conditionally essential amino acids? Name a few.

Answer 7

Amino acids that are essential during periods of rapid growth and pregnancy (where there is an increased rate of protein synthesis).
Tyrosine, cysteine, arginine

Question 8

What can be used as a clinical marker for muscle injury and death?

Answer 8

Creatinine (breakdown product of creatine and creatine kinase) -> it is released when muscle is damaged.

Question 9

How do creatinine levels related to muscle mass?

Answer 9

Creatinine is released from injured muscle and is filtered by the kidneys and into the urine. The urine excretion of creatinine over 24 hours is proportional to muscle mass.

Question 10

How would creatinine levels vary with kidney damage?

Answer 10

Creatinine levels are an indicator of kidney function. The levels would increase with kidney damage.

Question 11

When metabolising an amino acid, what are the 2 methods of nitrogen removal?

Answer 11

Deamination - removal of nitrogen group using enzymes.
Transamination - transfers the amino group to a keto acid to produce a new amino acid and keto acid (uses aminotransferase enzymes).

Question 12

What 3 markers are there that when levels are raised, suggest inflammation?

Answer 12

C-reactive protein
ALT - alanine aminotransferase
AST - aspartate aminotransferase
(ALT and AST are also markers of liver function tests as they are released when the liver is damaged).

Question 13

Describe transamination - what are the 2 enzymes used, what keto acids can be used, and what are the products produced?

Answer 13

ALT - alanine aminotransferase -> uses alpha-ketoglutarate as the keto acid (most common keto acid used) and converts amino acid alanine into glutamate.
AST - aspartate aminotransferase -> uses oxaloacetate as the keto acid and converts amino acid glutamate into aspartate.

Question 14

Describe deamination - what enzymes can be used to remove the nitrogen group and what is produced?

Answer 14

3 enzymes can be used:
Amino acid oxidases
Glutaminase
Glutamate dehydrogenase

Ammonia is produced, which is very toxic so must be removed and converted to urea.

Question 15

How is toxic ammonia removed from the body?

Answer 15

Ammonia is converted into urea in the urea cycle. Urea is highly soluble, so is excreted directly into urine.

Question 16

What are some of the toxic effects of ammonia?

Answer 16

Alters blood-brain barrier
Disrupts cerebral blood flow
Interferes with amino acid transport and protein synthesis

Question 17

What induces the urea cycle?

Answer 17

A high protein diet - makes the 5 enzymes in the urea cycle much more active.

Question 18

What is refeeding syndrome?

Answer 18

A malnourished patient with little protein in their diet will have reduced activity of the enzymes in the urea cycle. If protein is reintroduced into their diet too quickly, the ammonia won’t be converted to urea and they will get hyperammonaemia.
The protein needs to be reintroduced gradually in malnourished patients.

Question 19

What are the symptoms of hyperammonaemia due to urea cycle defects?

What is the management of urea cycle defects?

Answer 19

Vomiting, mental retardation.

Can reduce these symptoms by having a low protein diet, and replacing amino acids in the diet with keto acids.

Question 20

What are the 2 safe methods of moving ammonia to the liver, in order for it to enter the urea cycle?

Answer 20

1. Glutamine - carries ammonia to the liver (ammonia + glutamate = glutamine), ammonia is then fed into urea cycle.
2. Alanine - also takes ammonia to the liver for urea cycle via a series of side reactions.

Question 21

What is phenylketonuria? What are the symptoms and how can it be managed?

Answer 21

Deficiency in phenylalanine hydroxylase - leads to a build of phenylalanine.
Symptoms: developmental delay.
Treatment: diet low in phenylalanine and high in tyrosine.

Question 22

What is homocysteinuria? How can it be managed?

Answer 22

Excess homocysteine due to a problem breaking down methionine.
Treatment: low methionine diet, supplemented with cysteine.