Session 3-Protein And AA Metabolism

Question 1

What is creatinine the breakdown product of?

Answer 1

Creatine

Creatine phosphate

Question 2

True or false: creatinine urine excretion over 24 hours is proportional to muscle mass

Answer 2

TRUE

Question 3

What is creatinine commonly used as an indicator of?

Answer 3

Renal function

Question 4

What is the normal state of nitrogen balance in an adult?

Answer 4

N equilibrium, intake=output

Question 5

What is the normal state of nitrogen balance in growth and pregnancy or in an adult recovering from malnutrition?

Answer 5

Positive N balance, intake>output

Question 6

Which state of nitrogen balance is never normal and what are its causes?

Answer 6

Negative N balance, intake < output

Trauma, infection and malnutrition

Question 7

What is protein turnover?

Answer 7

Proteins undergo continuous breakdown and resynthesis

Question 8

What does the rate of protein turnover depend on?

Answer 8

Depends on the protein and varies during growth (fast) and ageing (slows)

Question 9

True or false: normally the rate of protein breakdown equals the rate of resynthesis

Answer 9

TRUE

Question 10

What effect do insulin and growth hormone have on protein synthesis?

Answer 10

Increases protein synthesis

Question 11

What effect does cortisol have on skeletal muscle?

Answer 11

Promotes breakdown of muscle protein (proteolysis) and release of AA

Question 12

When an AA is catabolised and an amino group is removed, what is it converted to?

Answer 12

Urea, excreted from the body in urine

Question 13

After an amino group has been removed from an AA, what can the remaining C-skeleton be converted to? (6)

Answer 13

Pyruvate
Oxaloacetate 
Fumarate 
Alpha-ketoglutarate
Succinyl-CoA
Acetyl-CoA

Question 14

What are ketogenic amino acids?

Answer 14

AA that produce acetyl-CoA because acetyl-CoA can be used to produce ketone bodies

Question 15

Name two ketogenic AA

Answer 15

Lysine

Leucine

Question 16

What are glucogenic AAs?

Answer 16

AAs that give rise to the other products of C-skeleton conversion as they can be used for glucose synthesis by gluconeogenesis

Question 17

Name an AA which is both glucogenic and ketogenic

Answer 17

Isoleucine 
Threonine 
Phenylalanine 
Tyrosine 
Tryptophan

Question 18

What effect do insulin and growth hormone have on protein degradation?

Answer 18

Decreases

Question 19

What effect do glucocorticoids (cortisol) have on protein synthesis?

Answer 19

Decreases

Question 20

What effect do glucocorticoids (cortisol) have on protein degradation?

Answer 20

Increases

Question 21

In which condition can excessive protein breakdown occur and this can lead to the formation of what?

Answer 21

Cushing’s syndrome (excess cortisol)

Weakens skin structure, leading to the formation of striae

Question 22

Where do the carbon atoms for non-essential AA synthesis come from?

Answer 22

Intermediates of glycolysis
Pentose phosphate pathway
Krebs cycle

Question 23

Where does the amino group for AA synthesis come from?

Answer 23

Form other AA by transamination or from ammonia

Question 24

Why is the removal of an amino group from an AA essential?

Answer 24

To allow carbon skeleton to be utilised in oxidative metabolism

Question 25

What are the two main pathways that facilitate the removal of nitrogen from AA?

Answer 25

1) transamination | 2) deamination

Question 26

Which enzymes are involved in transamination?

Answer 26

Aminotransferases (transaminases)

Question 27

What do aminotransferase enzymes use to funnel the amino group to glutamate?

Answer 27

Alpha-ketoglutarate

Question 28

What does aspartate aminotransferase (AST) convert?

Answer 28

Glutamate to aspartate

Question 29

Which coenzyme do all aminotransferases require?

Answer 29

Pyridoxal phosphate (derivative of vitamin B6)

Question 30

What does alanine aminotransferase (ALT) convert?

Answer 30

Alanine to glutamate

Question 31

When are plasma ALT and AST levels measured?

Answer 31

Part of liver function test

Question 32

When are levels of plasma ALT and AST high?

Answer 32

In conditions that cause extensive cellular necrosis such as: Viral hepatitis Autoimmune liver disease Toxic injury

Question 33

What is deamination?

Answer 33

Liberation of an amino group as free ammonia, mainly in liver and kidney

Question 34

What are L and D-amino acid oxidases?

Answer 34

Low specificity enzymes that convert AA to keto acids and ammonia

Question 35

Which cells have a high activity of D-amino acid oxidase?

Answer 35

Liver cells

Question 36

How do D-amino acids enter through the diet?

Answer 36

Found in plant and bacterial cell

Question 37

Why must D-amino acids not be used for protein synthesis?

Answer 37

Proteins would be structurally abnormal and non-functional as the enzyme converts them to keto acids that aren't optically active

Question 38

Which enzymes can deaminate AA? (3)

Answer 38

Amino acid oxidases (L and D) Glutaminase Glutamate dehydrogenase

Question 39

What is ammonia rapidly converted to at physiological pH?

Answer 39

Ammonium ion (NH4+)

Question 40

True or false: urea is not soluble in water

Answer 40

FALSE - extremely water soluble

Question 41

True or false: urea is chemically inert in humans

Answer 41

TRUE - bacteria can break it down to release ammonia

Question 42

Where do the amino groups of urea come from?

Answer 42

NH4+ and aspartate

Question 43

Where does the urea cycle occur and how many enzymes are involved in it?

Answer 43

In liver | 5 enzymes

Question 44

True or false: low protein diet induces enzyme levels in the urea cycle

Answer 44

FALSE - high protein diet induces enzyme levels

Question 45

True or false: the urea cycle is inducible but not regulated

Answer 45

TRUE

Question 46

What is refeeding syndrome?

Answer 46

When nutritional support is given to severely malnourished patients - too much protein leads to hyperammonaemia

Question 47

What do defects in the urea cycle lead to?

Answer 47

Autosomal recessive genetic disorders caused by deficiency of one of enzymes in the cycle, leading to hyperammonaemia and the accumulation/excretion of urea cycle intermediates

Question 48

What are the symptoms of urea cycle disorders?

Answer 48

``` Vomiting Lethargy Irritability Mental retardation Seizures Coma ```

Question 49

What is the management methods for urea cycle disorders?

Answer 49

Low protein diet | Replace AA in diet with keto acids

Question 50

True or false: ammonia is readily diffusible and very toxic to the brain

Answer 50

TRUE

Question 51

How is glutamine used in the disposal of ammonia?

Answer 51

ammonia combines with glutamate to form glutamine, which is transported to liver or kidneys where it is cleaved by glutaminase to reform glutamate and ammonia. In liver, ammonia fed into urea cycle and in kidney is excreted directly in urine

Question 52

How is alanine used to dispose of ammonia?

Answer 52

Ammonia combines with pyruvate to form alanine. Alanine is transported to liver where it is converted back to pyruvate by transamination. Amino group fed via glutamate into urea cycle -> urea.

Question 53

When is a heel prick test used?

Answer 53

- Sickle cell disease - Cystic fibrosis - Congenital hyperthyroidism

Question 54

What is phenylketonuria (PKU)?

Answer 54

Autosomal recessive condition - deficiency in phenylalanine hydroxylase

Question 55

What does PKU lead to an accumulation of and what does this lead to?

Answer 55

Phenylalanine in tissue, plasma and urine -> phenylketones in urine

Question 56

What are the symptoms of PKU?

Answer 56

``` Severe intellectual disability Developmental delay Microcephaly (small head) Seizures Hypopigmentation ```

Question 57

How can PKU be treated?

Answer 57

1) controlled low phenylalanine diet 2) avoid artificial sweeteners 3) avoid high protein foods such as meat, milk and eggs

Question 58

What is homocystinuria?

Answer 58

Autosomal recessive - problem with breaking down methionine so excess homocystine (oxidised form of homocysteine) excreted in urine

Question 59

What is the most common deficiency in homocystinuria?

Answer 59

Cystathionine beta-synthase

Question 60

What homocystinuria affect?

Answer 60

Connective tissue Muscles CNS CVS

Question 61

How can homocystinuria be treated?

Answer 61

1) low methionine diet 2) avoid milk, meat, fish, cheese, eggs, nuts and peanut butter 3) cysteine, vitamin B6, Betaine, B12 and folate supplement

Question 62

Name an AA that is glucogenic

Answer 62

``` Any one of the following: Alanine Glycine Threonine Cysteine Serine Arginine Proline Histidine Glutamine Methionine Valine Aspartate Asparagine ```