S3) Protein & Amino Acid Metabolism

Question 1

Identify three major nitrogen containing compounds

Answer 1

• Amino acids
• Proteins
• Purines + Pyrimidines (DNA / RNA)

Question 2

Identify three minor nitrogen containing compounds

Answer 2

• Creatine
• Neurotransmitters e.g. dopamine
• Some hormones e.g. adrenaline

Question 3

What is creatinine?

Answer 3

Creatinine is a breakdown product of creatine & creatine phosphate in muscle

Question 4

Explain how creatinine can be used a clinical marker for renal function

Answer 4

• Produced at constant rate and filtered via kidneys into urine
• Creatinine urine excretion over 24h is proportional to muscle mass
• Provides estimate of muscle mass

Question 5

What is nitrogen balance?

Answer 5

Nitrogen balance is the measure of nitrogen input minus nitrogen output i.e. nitrogen input — nitrogen loss

Question 6

Which three concepts revolve around nitrogen balance?

Answer 6

• Nitrogen equilibrium
• Positive nitrogen balance
• Negative nitrogen balance

Question 7

What are the clinical features of nitrogen equilibrium?

Answer 7

• Intake = output
• No change in total body protein
• Normal state in adult

Question 8

What are the clinical features of a positive nitrogen balance?

Answer 8

• Intake > output
• Increase in total body protein
• Normal state in growth, pregnancy or adult recovering from malnutrition

Question 9

What are the clinical features of a negative nitrogen balance?

Answer 9

• Intake < output
• Net loss of body protein
• Never normal (trauma, infection, malnutrition)

Question 10

What is protein turnover?

Answer 10

Protein turnover is the balance between protein synthesis and protein degradation

Question 11

Illustrate the pathways involved in protein turnover

Answer 11

Question 12

Provide an example of the following:

• Glucogenic amino acid
• Ketogenic amino acid
• Both ketogenic and glucogenic amino acid

Answer 12

• Glucogenic amino acid: alanine
• Ketogenic amino acid: leucine
• Both ketogenic and glucogenic amino acid: isoleucine

Question 13

When are protein stores mobilised?

Answer 13

Occurs under extreme stress (starvation)

Question 14

Describe the hormonal control over the mobilisation of protein reserves

Answer 14

Question 15

In de novo amino acid synthesis, where do the carbon atoms come from?

Answer 15

• Intermediates of glycolysis (C3)
• Pentose phosphate pathway (C4 & C5)
• Krebs cycle (C4 & C5)

Question 16

In the de novo amino acid synthesis, where does the amino group come from?

Answer 16

Amino group provided by other amino acids by the process of transamination or from ammonia

Question 17

Which compounds are synthesised from tyrosine?

Answer 17

• Catecholamines
• Melanin
• Thyroid hormones

Question 18

Which compound is synthesised from histidine?

Answer 18

Histamine

Question 19

Which compound is synthesised from arginine?

Answer 19

Nitric oxide

Question 20

Which compound is synthesised from cysteine?

Answer 20

Glutathione

Question 21

Which compounds are synthesised from tryptophan?

Answer 21

• Serotonin (5HT)
• Melatonin

Question 22

Which molecules are synthesised from glycine?

Answer 22

• Purines
• Glutathione
• Haem
• Creatine

Question 23

Why does nitrogen have to be removed from amino acids in protein metabolism?

Answer 23

• Essential to allow carbon skeleton of amino acids to be utilised in oxidative metabolism
• Once removed nitrogen can be incorporated into other compounds or excreted from body as urea

Question 24

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

Answer 24

• Transamination
• Deamination

Question 25

Explain the process of transamination

Answer 25

- Most **aminotransferase** enzymes use **a-ketoglutarate** to funnel the amino group to **glutamate** - Exception to rule is **aspartate aminotransferase** which uses **oxaloacetate** to funnel amino group to **aspartate**

Question 26

Which aminotransferase enzymes are measured routinely as part of liver function test?

Answer 26

- **Alanine aminotransferase** (ALT) which converts alanine to glutamate - **Aspartate aminotransferase** (AST) which converts glutamate to aspartate

Question 27

High plasma AST and ALT levels are associated with which conditions?

Answer 27

- Viral hepatitis - Autoimmune liver diseases - Toxic injury

Question 28

Explain the process of deamination

Answer 28

- Liberates amino group as **free ammonia** - Mainly occurs in **liver & kidney** - Keto acids can be utilised for **energy**

Question 29

Several enzymes can deaminate amino acids. Identify three

Answer 29

- Amino acid oxidases - Glutaminase - Glutamate dehydrogenase

Question 30

What happens to ammonia at physiological pH?

Answer 30

At physiological pH, ammonia (NH₃) is rapidly converted to ammonium ion (NH₄⁺)

Question 31

Ammonia (and ammonium ions) are very toxic and must be removed. How does this occur?

Answer 31

Ultimately converted to **urea** or excreted directly in **urine**

Question 32

Describe four properties of urea

Answer 32

- High nitrogen content - Non-toxic - Extremely water soluble - Chemically inert in humans

Question 33

What is the urea cycle?

Answer 33

- The **urea cycle** is a process occurring in liver and involves 5 enzymes - It is used to dispose of ammonia

Question 34

Explain how the amount of urea cycle enzymes normally related to need to dispose of ammonia

Answer 34

- High protein diet **induces enzyme levels** (up-regulation) - Low protein diet or starvation **represses enzyme levels** (down-regulation)

Question 35

What is refeeding syndrome?

Answer 35

- **Refeeding syndrome** is a condition which can occur when nutritional support given to severely malnourished patients - Ammonia toxicity significant factor (urea cycle down regulated)

Question 36

What are the risk factors for refeeding syndrome?

Answer 36

- BMI \< 16 - Unintentional weight loss \> 15% in 3-6 months - 10/more days with little or no nutritional intake

Question 37

What are the effects of autosomal recessive genetic disorders caused by deficiency of one of enzymes in the urea cycle?

Answer 37

- Hyperammonaemia - Accumulation/excretion of urea cycle intermediates

Question 38

What does the severity of defects in the urea cycle depend on?

Answer 38

- Nature of defect - Amount of protein eaten

Question 39

What are the symptoms of genetic disorders due to defects in the urea cycle?

Answer 39

- Vomiting - Lethargy - Irritability - Mental retardation - Seizures

Question 40

What is the management for autosomal recessive disorders due to defects in the urea cycle?

Answer 40

- Low protein diet - Replace amino acids in diet with keto acids

Question 41

What is the biochemical basis for ammonia toxicity?

Answer 41

- Ammonia is readily **diffusible** and extremely **toxic to brain** - Blood level needs to be kept low (25-40 µmol/L)

Question 42

Identify 5 toxic effects of ammonia

Answer 42

- Interference with **amino acid transport** and **protein synthesis** - Disruption of **cerebral blood flow** - **pH** effects (alkaline) - Interference with metabolism of **excitatory amino acid neurotransmitters** *e.g. glutamate and aspartate* - Alteration of the **blood–brain barrier**

Question 43

Two mechanisms are utilised for the safe removal of ammonia from tissues for disposal. In 4 steps, outline the use of glutamine

Answer 43

⇒ Ammonia combines with glutamate to **form glutamine** ⇒ Glutamine transported in blood to **liver /kidneys** ⇒ **Cleavage by glutaminase** to reform glutamate and ammonia ⇒ Ammonia fed into **urea cycle** (liver) / excreted directly in **urine** (kidneys)

Question 44

Two mechanisms are utilised for the safe removal of ammonia from tissues for disposal. In 5 steps, outline the use of alanine

Answer 44

⇒ Ammonia combines with pyruvate to **form alanine** ⇒ Alanine transported in blood to **liver** ⇒ Conversion to pyruvate by **transamination** ⇒ Amino group **fed via glutamate into urea cycle** for disposal as urea ⇒ Pyruvate is used to **synthesise glucose**

Question 45

Identify 5 clinical conditions which can utilise the heel prick test

Answer 45

- Sickle cell disease - Cystic fibrosis - Congenital hypothyroidism - Phenylketonuria (PKU) - Homocystinuria

Question 46

What is phenylketonuria?

Answer 46

- **PKU** is the common inborn error of amino acid metabolism due to an autosomal recessive deficiency in phenylalanine hydroxylase - Phenylalanine accumulates in the tissue, plasma & urine, hence presenting with phenylketones in urine (musty smell)

Question 47

Outline the treatment of phenylketonuria

Answer 47

- Strictly controlled **low phenylalanine diet** - Avoid **artificial sweeteners** (contain phenylalanine) - Avoid **high protein foods** such as meat, milk, and eggs

Question 48

Identify 5 symptoms of PKU

Answer 48

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

Question 49

Illustrate the affected pathways in PKU

Answer 49

Question 50

What is homocystinuria?

Answer 50

- **Homocystinuria** is an autosomal recessive disorder, commonly due to a defect in cystathionine β-synthase, leading to an inability in breaking down methionine - Excess homocystine (oxidised form of homocysteine) is excreted in urine

Question 51

Which tissues/systems are affected in homocystinuria?

Answer 51

- Connective tissue - Muscles - CNS - CVS

Question 52

Outline the treatment of homocystinuria

Answer 52

- **Low-methionine diet** - **Avoid**: milk, meat, fish, cheese, eggs and nuts - **Supplements**: cysteine, Vit B₆, Betaine, B₁₂ & Folate

Question 53

Illustrate the affected pathways in homocystinuria

Answer 53