Protein Metabolism

Question 1

What is meant by nitrogen balance?

Answer 1

It is the equilibrium produced in the body when nitrogen intake is equal to nitrogen output.

Question 2

When might you have a positive nitrogen balance?

Answer 2

When growing
Pregnant
Recovering from malnutrition.

Question 3

When might you have a negative nitrogen balance?

Answer 3

Trauma
Infection
Malnutrition

Question 4

What is protein turnover?

Answer 4

It is the replacement of old proteins.
Proteins are broken down to amino acids by proteolysis.
They are then synthesised again from the amino acids.

Question 5

How can proteins be used for energy?

Answer 5

Proteolysis of proteins to produce amino acids.

The amino acids are broken down further into the amino group and the carbon skeleton.

The amino group is excreted.

The carbon skeleton is then used to produce energy by being fed into certain metabolic pathways.

Question 6

What is a glucogenic amino acid?

Answer 6

An amino acid which can be converted into glucose via gluconeogenesis to be used as energy.

Question 7

What is a ketogenic amino acid?

Answer 7

Amino acids that can be converted into ketone bodies to be used for energy.

Question 8

Why is it important that the amino group is excreted?

Answer 8

It could be converted to ammonia which is very toxic to the body.

Question 9

How is the amino group excreted?

Answer 9

It is first converted to urea which dissolves in urine to be excreted.

Question 10

Which hormones inhibit protein metabolism?

Answer 10

Insulin & Growth Hormones

Question 11

Which hormones stimulate protein metabolism?

Answer 11

Glucocorticoids e.g Cortisol

Question 12

What are the 9 essential amino acids?

Answer 12

Isoleucine (IF)
Lysine  (LEARNED)
Threonine (THIS)
Histidine (HUGE)
Leucine (LIST)
Methionine (MAY)
Phenylalanine (PROVE)
Tryptophan (TRULY) 
Valine (VALUABLE)

Question 13

What is creatinine and why is it measured?

Answer 13

Creatinine is a breakdown product of creatine and creatine phosphate.
It is excreted via urine.

The amount excreted is proportional to, and can therefore be used to estimate muscle mass.

It can also be used as a indicator of renal function. (Levels are raised when nephrons are damaged)

Question 14

Name two methods for the removal of the amino group from an amino acid:

Answer 14

Transamination

Deamination

Question 15

Describe the basis for transamination:

Answer 15

The amino group of one amino acid is removed and added to a keto acid to form a new amino acid.
The new amino acid is more readily inserted into the urea cycle.

These amino acids are:
Glutamate
Aspartate

Question 16

Which substrates (other than the amino acid to be converted) are used in transamination and what amino acids do they produce?

Answer 16

a-ketoglutarate (produces glutamate)

Oxaloacetate (produces aspartate)

Question 17

Name two key aminotransferases and explain their clinical relevance:

Answer 17

Alanine aminotransferase
Aspartate aminotransferase

Plasma levels of these enzymes are monitored to test liver function. They are usually only found in the liver so if they’re found in blood then the liver has been damaged.

Elevated levels of these enzymes may indicate:
Viral hepatitis
Autoimmune liver diseases
Toxic Injury

Question 18

Describe the principle of deamination:

Answer 18

Occurs mainly in the liver but also the kidneys.

Removes the amino group from an amino acid in the form of ammonia.

A keto acid is also produced which can be used for energy.

The toxic ammonia must be removed.

Question 19

What is the urea cycle?

Answer 19

The cycle which is used to dispose of ammonia created from deamination and the amino groups transferred in transamination.

Occurs in the liver partially within mitochondria and the cytoplasm.

Aspartate, Glutamate, Ammonia and CO2 are all fed in and urea is produced.

Question 20

What makes urea suitable for disposing of nitrogen?

Answer 20

It has a high nitrogen content.
It is extremely water soluble.
Chemically inert.

Question 21

Name two inborn errors of amino acid metabolism:

Answer 21

Phenylketonuria

Homocystinuruas

Question 22

Describe phenylketonuria:

Answer 22

Usually phenylalanine is converted to tyrosine by phenylalanine hydroxylase.

In phenylketonuria there’s a deficiency in that enzyme. Caused by an autosomal recessive disease.

This means that there’s a tyrosine deficiency so any pathways which require that amino acid are dysfunctional.

This includes the production of noradrenaline, adrenaline, dopamine and thyroid hormone.

Can be treated by a diet which supplements tyrosine and avoids phenylalanine.

Question 23

What are the symptoms of phenylketonuria?

Answer 23

Severe intellectual disability
Developmental delay
Microcephaly (Small head)
Seizures
Hypopigmentation.

Question 24

Describe homocystinurias:

Answer 24

Autosomal recessive disorder which leads to defect in cystathionine B-synthase.

This means that methionine can’t be broken down.

Affects connective tissue, muscles, CNS, CVS.