L2: Protein and Amino Acid Metabolism

Question 1

What are the major nitrogen containing compounds?

Answer 1

Amino acids, proteins and purines and pyrimidines (DNA/RNA).

Smaller amount in creatine kinase, neurotransmitters, some hormones, prophyrins etc…

Question 2

How much nitrogen is there in the body (70kg male)?

Answer 2

2.0kg in body protiens = 3% of body weight

Question 3

What is creatinine?

Answer 3

Breakdown product of creatine and creatine phosphate in muscle. Good estimate of muscle mass.

Question 4

How is creatinine eliminated from the body? Normal range? Why is it a useful clinical marker?

Answer 4

It is excreted by the kidney’s into urine.
Men 14-26 mg/kg, women 11-20 mg/kg
Usually produced at a constant rate. Elevated levels indicate muscle wasting or renal failure.

Question 5

What is meant by nitrogen balance? What do we say when they are unbalanced?

Answer 5

The steady rate at which nitrogen is taken into the body and excreted from the body.
N equilibrium - intake = output
Positive N balance - intake > output
- growth, pregnancy, recovery from malnutrition
Negative N balance - intake < output
- never normal, trauma, infection or malnutrition

Question 6

What happens during stage 1 of protein catabolism?

Answer 6

Broken down in GI tract by proteases and peptidases into amino acids. Amino acids used for synthesis of new proteins.

Question 7

What happens during stage 2 metabolism?

Answer 7

Removal of NH2 group –> converted to urea (CO(NH2)2) and excreted.
Carbon skeleton–> ketogenic or glucogenic amino acids –> energy

Question 8

What are ketogenic and glucogenic amino acids?

Answer 8

Ketogenic amino acids produce acetyl CoA which can be used in the TCA cycle or produce ketone bodies.
- e.g. leucine and lysine
Glucogenic amino acids are used to produce glucose by gluconeogenesis
- e.g. alanine, glycine, cysteine and serine

Question 9

What is meant by protein turnover?

Answer 9

All proteins undergo continuous breakdown and synthesis, normally equal.
Rate varies on growth and ageing and type of protein.
Normal T1/2= 80 days.

Question 10

What hormones have an effect on protein synthesis and breakdown?

Answer 10

Insulin and growth hormone increase the rate of synthesis and decrease rate of breakdown.
Glucocorticoids (cortisol) decrease rate of synthesis and increase the rate of breakdown.

Question 11

What is the structure of an amino acid?

Answer 11

Diagram on slide 9
Amino group
R group
Carboxyl group

Question 12

What type of bond forms when two amino acids join together? What is the name of the reaction? What is released?

Answer 12

Peptide bond
Condensation reaction
Water

Question 13

What are essential amino acids? What are the 9 essential amino acids?

Answer 13

Essesntial amino acids are amino acids that cannot be synthesised by the body they must be obtained from the diet.
If - Isoleucine 
Learned- Lysine 
This - Threonine
Huge - Histidine
List - Leucine
May - Methionine
Prove - Phenylalaine 
Truly - Tryptophan 
Valuable - Valine

Question 14

What are conditionally essential amino acids? Name them?

Answer 14

Become essential amino acids during certain periods e.g. growth periods
Arginine, Tyroisine and Cysteine

Question 15

What are high quality proteins? What are low quality proteins?

Answer 15

High quality proteins are from animal origin, contain all essential amino acids. Low quality proteins are form plant origin, don’t contain all essential amino acids.

Question 16

How are amino acids synthesised?

Answer 16

Non essential amino acids.
Carbon group comes from intermediate of glycolysis (C3), pentose phosphate pathway (C4 + C5) or krebs cycle (C4 + C5).
Amino group from transamination or ammonia

Question 17

What are amino acids required for?

Answer 17

They are needed for protein synthesis or N-containing compounds
Nitric oxide –> formed from arginine
hydrogen sulphide –> formed from cysteine
GABA –> formed from glutamate
Histamine –> formed from Histidine etc…

Question 18

What are the two main pathways for removing nitrogen from amino acids?

Answer 18

Nitrogen needs to be removed so the carbon skeleton can be utilised in other compounds.
Removal of nitrogen by
-Transamination
-Deamination

Question 19

What is transamination? What ketoacids are usually used?

Answer 19

Transfer of the amino group to ketoacid to form a new amino acid.
alpha-ketoglutarate –> glutamate
oxaloacetate –> aspartate

Question 20

What enzyme is required for transamination?

Answer 20

Aminotransferase enzyme and co enzyme pyridoxal phosphate derived from vitamin B6

Question 21

What are two important aminotransferase enzymes? Why are the important?

Answer 21

Alanine aminotransferase (ALT) --> alanine to glutamate
Aspartate aminotransferase (AST) --> Glutamate to aspartate 
Measured in a liver function test
High levels indicate extensive cellular necrosis e.g. viral hepatitis, autoimmune liver disease, toxic injury

Question 22

What is deamination?

Answer 22

Removal of amino group to form free ammonia. It occurs mainly in the liver and kidneys.
Ammonia is very toxic and must be removed.
Converted to urea and excreted from the body.
Important for deamination of dietary D-amino acids (not good isomer)

Question 23

What are the different steroisomers of amino acids?

Answer 23

L isomers and D isomers.
L isomers are used to make proteins
D isomers are not as they would make structurally abnormal proteins.
Converted to keto acids that are not optically active.

Question 24

What enzymes can deaminate amino acids?

Answer 24

Amino acid oxidase
Glutaminase
Glutamate dehydrogenase

Question 25

What is urea? How is it produced?

Answer 25

Urea is a substance that is water soluble, chemically inert, non toxic, and has high nitrogen content.
It is produced by the urea cycle and is excreted in urine.

Question 26

What is the urea cycle?

Answer 26

Cycle that produced urea from ammonia or transminated amino acids glutamate and asparatate. Half the cycle takes place in the mitochondria, half takes place in the cytoplasm.
Takes place in the liver.

Question 27

How is the urea cycle regulated?

Answer 27

5 enzymes involved in the urea cycle. Produced on an ‘as needed’ basis.

Question 28

What is refeeding syndrome? How is it treated?

Answer 28

Malnourished patients given nutritional support, can often lead to hypophosphataemia (electrolyte abnormality) and ammonia toxicity, this is because the enzymes used in the urea cycle are down regulated so causes a sudden increase in ammonia.
Re-feed at 5-10 kcal/kg/day, raise gradually to full needs within a week.

Question 29

What causes defects in the urea cycle? What does it lead to?

Answer 29

Autosomal recessive genetic disorder leads to deficiency in one enzymes in the urea cycle.
Results in hyperammonaemia and accumulation/ excretion of urea cycle intermediates.
Complete loss of enzyme is fatal.

Question 30

What are the clinical symptoms of urea cycle defects? How is it managed?

Answer 30

Severity depends on nature of defect and amount of protein eaten. 
Severe defects show symptoms within one day, if untreated patient will die. 
Mild defects may not show symptoms until much later in childhood.
Symptoms
- vomiting
- lethargy
- irritability 
- mental retardation
- seizures 
- coma
Management--> low protein diet
--> Replace AA with ketoacids

Question 31

What is ammonia toxicity?

Answer 31

Build up of ammonia which is toxic to cells. Normally kept at 25-40 micromol/ litre.
At physiological pH (7.4) it is usually in the form of ammonium.

Question 32

How is ammonia transported from the tissue to the liver/kidney? Explain each mechanism?

Answer 32

There are two mechanisms of transport.
Glutamine
- glutamate and ammonia (glutamine synthetase needed)–> glutamine
- taken to kidney or liver
- Cleaved by glutaminase to reform glutamate and ammonia
- In liver enters urea cycle
-In kidney directly excreted
Alanine
- Amine group transfer to glutamate by transamination, then transferred to pyruvate to form alanine
- Transported to liver
- Turned back into pyruvate (transamination - alanine aminotransferase)–> glutamate –> enter urea cycle
- pyruvate used to synthesis glucose

In both glutamate dehydrogenase converts glutamate to NH3 → Urea

Question 33

What causes amino acid metabolism problems? What can it lead to? How is it detected?

Answer 33

Over 50 inherited disease in amino acid metabolism. It is rare but collectively as a group make a significant proportion of paediatric genetic diseases.
Clinical consequence is usually seen when it effects breakdown.
Usually only partial loss of function.
Can lead to intellectual impairment.
Detected by heel prick test?

Question 34

How are clinical conditions caused by amino acid metabolism problems treated?

Answer 34

Remove specific amino acids from the diet.

Question 35

What are some of the common disorders?

Answer 35

Phenylketonuria (PKU), Homocystinuria, Maple syrup urine disease, isovaleric acidaemia, glutaric aciduria.

Question 36

What is phenylketonuria? How is it treated?

Answer 36

Autosomal recessive chromosome 12
Deficiency in Phenylalanine hydroxylase enzyme.
Inhibits conversion to tyrosine
Accumulation of phenylalanine.
Converted to phenylketones by transamination, excreted in urine, musty smell.
Treatment:
–> Low controlled phenylalanine diet
–> Tyrosine
–> Avoid protein rich food (eggs, milk, meat) and artificial sweeteners

Question 37

What causes PKU? What are the symptoms?

Answer 37

Autosomal recessive disorder on chromosome 12.
Most common.
Severe intellectual disability, developmental delay, microcephaly, seizures and hypopigmentation

Question 38

What is homocystinuria? What causes it? What are the symptoms?

Answer 38

Autosomal recessive
Excess Homocysteine or methionine
HomocystINE two homocysteine together–> oxidised form in the urINE
Deficiency in cystathionine beta-synthase–> unable to convert homocysteine to cysteine
Causes disorders of connective tissue, muscles, CNS and cardiovascular system.

Question 39

What is homocystinuria often mistaken for and why?

Answer 39

Often mistaken as Marfans syndrome (defect in fibrillin 1 gene) because symptoms are similar.
Marfans genetic disorder of connective tissue
Neurological dysfunction seen in homcystinuria not in Marfans

Question 40

How is homocystinuria treated?

Answer 40

Low methionine diet
Avoid milk, fish, meat, cheese, eggs, nuts and peanut butter
Cysteine, Vit B6, Betaine, B12 and folate supplement (to make remaining cystathionine B-synthase most effective)