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Metabolism, Endocrinology and Haematology > Protein and Amino Acid Metabolism > Flashcards

Protein and Amino Acid Metabolism Flashcards

1
Q

What are the main nitrogen-containing compounds in our body?

A
  • Amino acids
  • Proteins
  • Purines + Pyramidines
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2
Q

In which compounds is nitrogen found in small amounts?

A
  • Porphyrins (haem)
  • Creatine phosphate
  • Neurotransmitters (dopamine)
  • Some hormones (adrenaline)
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3
Q

What percentage of our body’s mass is nitrogen?

A

~3%

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4
Q

What is creatinine?

A
  • Nitrogen-containing breakdown product of creatine and creatine phosphate
    • creatine phosphate is an immediate source of ATP in skeletal muscles (used at the start of a sprint)
  • Produced at a constant rate depending on muscle mass
    • amount of creatinine present in pts urine (over 24hrs) can give an indication of muscle mass
    • not a constant rate when muscle is wasting
  • Filtered by the kidneys into the urine
  • Can be used to determine renal function
    • high creatinine = damage to nephrons (particularly in elderly)
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5
Q

What are the reference ranges of creatinine excreted in the urine per day?

A

Men = 14-26 mg/kg

Women = 11-20 mg/kg

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6
Q

What is nitrogen balance?

A

Nitrogen input - nitrogen output = nitrogen balance

N equilibrium:

  • intake = output
  • no change to normal body protein
  • normal state in an adult

Positive N balance:

  • intake > output
  • increase in total body protein
  • normal state of growth (children and adolescents) and pregnancy
  • normal in an adult recovering from malnutrition

Negative N balance:

  • intake < output
  • net loss of protein
  • never normal
  • caused by; trauma, infection or malnutrition
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7
Q

Describe the nitrogen balance in a 70kg male

A
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8
Q

What is protein turnover?

A

The continuous breakdown and resynthesis of protein.

  • rate of turnover depends on:
    • growth (increased rate)
    • ageing (decreased rate)
  • half-life of body protein in ~80 days
  • total protein turnover = 300-400g/day
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9
Q

What are glucogenic amino acids and give two examples?

A

Amino acids that can be used to synthesise glucose or glycogen.

Examples:

  • Aspartate
  • Asparagine
  • Alanine
  • Glycine
  • Cysteine
  • Serine
  • Arginine
  • Proline
  • Histidine
  • Glutamine
  • Glutatmate
  • Methionine
  • Valine
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10
Q

What are ketogenic amino acids and give two examples

A

Amino acids that can be used to synthesise fatty acids or ketone bodies.

Examples:

  • Lysine
  • Leucine
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11
Q

Which amino acids are both glucogenic and ketogenic? (2 examples)

A
  • Threonine
  • Tryptophan
  • Tyrosine
  • Phenylalanine
  • Threonine
  • Isoleucine
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12
Q

What are the 9 essential amino acids?

A

Body cannot produce ‘essential’ amino acids - they need to be sourced from our diet

If - Isoleucine

Learned - Lysine

This - Threonine

Huge -Histidine

List -Leucine

May - Methionine

Prove - Phenylalanine

Truly - Tryptophan

Valuable -Valine

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13
Q

What extra amino acids do children and pregnant women require?

A

They have a high rate of protein synthesis so also require arginine, tyrosine ad cysteine from the diet.

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14
Q

How are amino acids in the body synthesised?

A
  • Carbohydrates from:
    • glycolysis
    • pentose phosphate pathway
    • krebs cycle
  • Amino group from other amino acids (transamination) or from ammonia
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15
Q

Which important compounds are synthesised from amino acids?

A
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16
Q

Why are amino groups removed from amino acids?

A
  • removal of an amino group:
    1. exposes the carbon skeleton to be utilised in oxidative metabolism
    2. allows nitrogen to be used in other compounds
    3. allows nitrogen to be excreted in urea
17
Q

How is nitrogen removed from amino acids?

A

Transamination

  • aminotransferases use alpha ketoglutarate to funnel amino acid to glutamate
  • exception = aspartate aminotranferases use oxaloacetate to funnel amino group to aspartate
  • All aminotrasferases require coenzyme pyridoxal phosphate which is a derivative of vitamin B6

Deamination

  • removes amino groups as free ammonia
  • takes place in liver and kidneys
    • ammonia needs to quickly be converted into urea
  • enzymes that deaminate amino acids:
    • amino acid oxidases
    • glutaminase
    • glutamate dehydrogenase
18
Q

Which aminotransferases are key biomarkers?

A

Alanine aminotransferase (ALT)

  • converts alanine to glutamate

Aspartate aminotransferase (AST)

  • converts glutamate to aspartate

Plasma levels of AST and ALT are measured routinely as part of a liver function test:

  • high levels when hepatocyte necrosis occurs
    • viral hepatitis
    • Autoimmune liver diseases
    • Toxic injury
19
Q

What is urea?

A

Non-toxic high nitrogen content molecule that is extremely water soluble. Mostly excreted in urine through the kidney where it also performs an osmotic role in the kidney tubules.

20
Q

What is the urea cycle?

A

An inducible unregulated cycle that converts excess ammonia into urea using the mitochondria of liver cells.

  • occurs in the liver and involves 5 enzymes
    • enzymes are expressed in relation to the amount of ammonia needed tobe exposed
      • high protein diet = induces enzyme levels
      • low protein diet = represses levels
21
Q

What is refeeding syndrome?

A

An increase in the ammonia toxicity due to refeeding severely malnourished pts as their urea cycle enzymes have been downregulated.

Risk factors:

  • BMI <16kg/m2
  • unintentional weight loss >15% in 3-6 months
  • 10 days or more with little or no nutritional intake

How to refeed:

  • 5 - 10 kcal/kg/day
  • raise gradually to fulfil needs within a week
22
Q

What are the defects of the urea cycle?

A

Autosomal recessive genetic disorders caused by deficiency of one of the enzymes in the urea cycle

  • ~1 in 30,000 live births
  • mutations can cause a partial loss of enzyme function causing:
    • hyperammonaemia
    • accumulation/excretion of urea cycle intermediates

Clinical picture:

  • the severity of NH3 nature of defect and amount of protein eaten
  • severe disorders are apparent within 1 day; child will die if untreated
  • mild disorders may not be apparent until early childhood

Treatment:

  • Low protein diet
  • Replace amino acids with keto acids
23
Q

What are the symptoms of urea cycle defects?

A

Symptoms:

  • vomiting
  • lethargy
  • irritability
  • mental retardation
  • seizures
  • coma
24
Q

What is the biochemical basis of ammonia toxicity?

A
  • readily diffusible and extremely toxic to brain
  • blood level should be kept low (25-40umol/L)
  • potential toxic effects
    • disrupts cerebral blood flow
    • interfere with a.a. transport, protein synthesis and TCA cycle
    • increases pH
    • disrupts the metabolism of glutamate and aspartate neurotransmitters
    • alters blood-brain barrier
25
Q

How is amino acid nitrogen safely transported?

A

Glutamine

  • ammonia is combined with glutamate = glutamine
  • cleaved by glutaminase in liver and kidneys
    • liver: ammonia -> urea cycle
    • kidney: ammonia -> urine

Alanine

  • 1st transamination: to glutamate
  • 2nd transamination: pyruvate by glutamate to form alanine
  • transported to liver by blood
  • transaminated back to pyruvate, the amino group fed via glutamate into the urea cycle
26
Q

What does the ‘heel prick test’ test for?

A

Also known as newborn blood spot test. Carried out when the newborn is 5 days old.

Tests for:

  • Sickle cell disease
  • Cystic fibrosis
  • Congenital hypothyroidism
  • Phenylketonuria (PKU)
  • Maple syrup urine disease
  • Isovaleric acidaemia (IVA)
  • Glutaric aciduria type 1
  • Homocystinuria
  • Medium-chain acyl-CoA dehydrogenase deficiency

(Metabolic disorders)

27
Q

What is phenylketonuria?

A

Autosomal recessive deficiency in phenylalanine hydroxylase

  • converts phenylalanine to tyrosine
  • most common inborn error of a.a. metabolism
  • affects gene on chromosome 12
  • causes an accumulation of phenylalanine in tissue, plasma and urine
  • phenyl ketones in urine (musty smell)

Treatment:

  • low phenylalanine diet enriched with tyrosine
  • avoid artificial sweeteners
  • avoid high protein foods
28
Q

What are the symptoms of PKU?

A

All symptoms can be avoided with early treatment

  • severe intellectual disability
  • developmental delay
  • microcephaly (small head)
  • seizures
  • hypopigmentation
29
Q

What are the affected pathways in PKU due to lower levels of tyrosine?

A

Production of:

  • noradrenaline
  • adrenaline
  • dopamine
  • melanin
  • thyroid hormone
  • protein synthesis
30
Q

What is homocystinuria?

A

Autosomal recessive disorder in breaking down methionine

  • excess homocystine is excreted in urine
  • normally due to a defect in cystathionine ß-synthase
  • affects connective tissue, muscles, CNS and CVS
    • elevated plasma homocysteine associated with CVD

Treatment:

  • low methionine diet
  • avoid milk, meat, fish, cheese, eggs and nuts
  • supplement with cysteine, Vit B6, betaine, B12 and folate
31
Q
A

Metabolism, Endocrinology and Haematology (5 decks)

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