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[AMS] Metabolism, Endocrinology & Haematology > S3) Protein & Amino Acid Metabolism > Flashcards

S3) Protein & Amino Acid Metabolism Flashcards

1
Q

Identify three major nitrogen containing compounds

A
  • Amino acids
  • Proteins
  • Purines + Pyrimidines (DNA / RNA)
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2
Q

Identify three minor nitrogen containing compounds

A
  • Creatine
  • Neurotransmitters e.g. dopamine
  • Some hormones e.g. adrenaline
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3
Q

What is creatinine?

A

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

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4
Q

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

A
  • 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
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5
Q

What is nitrogen balance?

A

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

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6
Q

Which three concepts revolve around nitrogen balance?

A
  • Nitrogen equilibrium
  • Positive nitrogen balance
  • Negative nitrogen balance
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7
Q

What are the clinical features of nitrogen equilibrium?

A
  • Intake = output
  • No change in total body protein
  • Normal state in adult
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8
Q

What are the clinical features of a positive nitrogen balance?

A
  • Intake > output
  • Increase in total body protein
  • Normal state in growth, pregnancy or adult recovering from malnutrition
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9
Q

What are the clinical features of a negative nitrogen balance?

A
  • Intake < output
  • Net loss of body protein
  • Never normal (trauma, infection, malnutrition)
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10
Q

What is protein turnover?

A

Protein turnover is the balance between protein synthesis and protein degradation

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11
Q

Illustrate the pathways involved in protein turnover

A
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12
Q

Provide an example of the following:

  • Glucogenic amino acid
  • Ketogenic amino acid
  • Both ketogenic and glucogenic amino acid
A
  • Glucogenic amino acid: alanine
  • Ketogenic amino acid: leucine
  • Both ketogenic and glucogenic amino acid: isoleucine
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13
Q

When are protein stores mobilised?

A

Occurs under extreme stress (starvation)

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14
Q

Describe the hormonal control over the mobilisation of protein reserves

A
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15
Q

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

A
  • Intermediates of glycolysis (C3)
  • Pentose phosphate pathway (C4 & C5)
  • Krebs cycle (C4 & C5)
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16
Q

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

A

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

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17
Q

Which compounds are synthesised from tyrosine?

A
  • Catecholamines
  • Melanin
  • Thyroid hormones
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18
Q

Which compound is synthesised from histidine?

A

Histamine

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19
Q

Which compound is synthesised from arginine?

A

Nitric oxide

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20
Q

Which compound is synthesised from cysteine?

A

Glutathione

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21
Q

Which compounds are synthesised from tryptophan?

A
  • Serotonin (5HT)
  • Melatonin
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22
Q

Which molecules are synthesised from glycine?

A
  • Purines
  • Glutathione
  • Haem
  • Creatine
23
Q

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

A
  • 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
24
Q

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

A
  • Transamination
  • Deamination
25
Q

Explain the process of transamination

A
  • 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
26
Q

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

A
  • Alanine aminotransferase (ALT) which converts alanine to glutamate
  • Aspartate aminotransferase (AST) which converts glutamate to aspartate
27
Q

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

A
  • Viral hepatitis
  • Autoimmune liver diseases
  • Toxic injury
28
Q

Explain the process of deamination

A
  • Liberates amino group as free ammonia
  • Mainly occurs in liver & kidney
  • Keto acids can be utilised for energy
29
Q

Several enzymes can deaminate amino acids.

Identify three

A
  • Amino acid oxidases
  • Glutaminase
  • Glutamate dehydrogenase
30
Q

What happens to ammonia at physiological pH?

A

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

31
Q

Ammonia (and ammonium ions) are very toxic and must be removed.

How does this occur?

A

Ultimately converted to urea or excreted directly in urine

32
Q

Describe four properties of urea

A
  • High nitrogen content
  • Non-toxic
  • Extremely water soluble
  • Chemically inert in humans
33
Q

What is the urea cycle?

A
  • The urea cycle is a process occurring in liver and involves 5 enzymes
  • It is used to dispose of ammonia
34
Q

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

A
  • High protein diet induces enzyme levels (up-regulation)
  • Low protein diet or starvation represses enzyme levels (down-regulation)
35
Q

What is refeeding syndrome?

A
  • Refeeding syndrome is a condition which can occur when nutritional support given to severely malnourished patients
  • Ammonia toxicity significant factor (urea cycle down regulated)
36
Q

What are the risk factors for refeeding syndrome?

A
  • BMI < 16
  • Unintentional weight loss > 15% in 3-6 months
  • 10/more days with little or no nutritional intake
37
Q

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

A
  • Hyperammonaemia
  • Accumulation/excretion of urea cycle intermediates
38
Q

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

A
  • Nature of defect
  • Amount of protein eaten
39
Q

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

A
  • Vomiting
  • Lethargy
  • Irritability
  • Mental retardation
  • Seizures
40
Q

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

A
  • Low protein diet
  • Replace amino acids in diet with keto acids
41
Q

What is the biochemical basis for ammonia toxicity?

A
  • Ammonia is readily diffusible and extremely toxic to brain
  • Blood level needs to be kept low (25-40 µmol/L)
42
Q

Identify 5 toxic effects of ammonia

A
  • 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
43
Q

Two mechanisms are utilised for the safe removal of ammonia from tissues for disposal.

In 4 steps, outline the use of glutamine

A

⇒ 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)

44
Q

Two mechanisms are utilised for the safe removal of ammonia from tissues for disposal.

In 5 steps, outline the use of alanine

A

⇒ 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

45
Q

Identify 5 clinical conditions which can utilise the heel prick test

A
  • Sickle cell disease
  • Cystic fibrosis
  • Congenital hypothyroidism
  • Phenylketonuria (PKU)
  • Homocystinuria
46
Q

What is phenylketonuria?

A
  • 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)
47
Q

Outline the treatment of phenylketonuria

A
  • Strictly controlled low phenylalanine diet
  • Avoid artificial sweeteners (contain phenylalanine)
  • Avoid high protein foods such as meat, milk, and eggs
48
Q

Identify 5 symptoms of PKU

A
  • Severe intellectual disability
  • Developmental delay
  • Microcephaly (small head)
  • Seizures
  • Hypopigmentation
49
Q

Illustrate the affected pathways in PKU

A
50
Q

What is homocystinuria?

A
  • 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
51
Q

Which tissues/systems are affected in homocystinuria?

A
  • Connective tissue
  • Muscles
  • CNS
  • CVS
52
Q

Outline the treatment of homocystinuria

A
  • Low-methionine diet
  • Avoid: milk, meat, fish, cheese, eggs and nuts
  • Supplements: cysteine, Vit B6, Betaine, B12 & Folate
53
Q

Illustrate the affected pathways in homocystinuria

A

[AMS] Metabolism, Endocrinology & Haematology (20 decks)

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