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Metabolism > Lecture 2- Protein and amino acid metabolism > Flashcards

Lecture 2- Protein and amino acid metabolism Flashcards

1
Q

Major nitrogen containing compounds:

A
  • Amino acids
  • Proteins
  • Purines and pyrimidines (DNA/RNA)
  • Creatine phosphate
  • Neurotransmitters (dopamine)
  • Some hormones (adrenaline)
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2
Q

Nitrogen balance

A

Nitrogen balance is a measure of nitrogen input minus nitrogen output. Nitrogen Balance = Nitrogen intake - Nitrogen loss

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3
Q

Nitrogen equilibrium

A

Intake = outake

  • No change in total body protein
  • Normal state in adult
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4
Q

Positive N balance

A

Intake >output

  • Increase in total body protein
  • Normal state in growth and pregnancy or in adult recovering from malnutrition
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5
Q

Negative N balance

A

Intake < output

  • Net loss of body protein
  • Never normal
  • Trauma
  • Infection
  • malnutrition
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6
Q

creatinine can be used to estimate

A

muscle mass

Creatinine urine excretion over 24h proportional to muscle mass

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7
Q

creatinine is also a useful indicator of

A

renal function

  • raised levels when nephron are dmaage
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8
Q

creatinine is the breakdown product of

A

creatine & creatine phosphate in muscle

  • Usually produced at constant rate depending on muscle mass (unless muscle is wasting)
  • Filtered via kidneys into urine
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9
Q

reference range of creatinine

A
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10
Q

amino acid structure

A
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11
Q

how many different types of amino acid

A

20

  • 20 different side chains
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12
Q

essential amino acids

A

Isoleucine

Leucine

Threonine

Histidine

Lycine

Methionine

Phenylalanine

Tryptophan

Valine

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13
Q

Conditionally essential aa =

A

arginine, tyrosine and cysteine (children and pregnant women- high rate of protein synthesis)

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14
Q

Animal origin protein=

A

high qual

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15
Q

Plant origin =

A

low qual

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16
Q

What contributes to free amino acids

A
  • Dietary protein gets broken down into free amino acids
  • Some amino acids produced de novo
  • Some muscle protein broken down into amino acids via proteolysis
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17
Q

Free amino acids are transporter to the liver and :

A
  • Either lose their amino group (-NH2) à urea à urine
  • Uses carbon skeleton (carboxyl group (COOH) and R groups)
    • R groups determine if the amino acid is:
      • Glucogenic
      • Ketogenic amino acids
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18
Q

glucogenic

A

glucoseneogenesis

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19
Q

ketogenic

A

ketone bodies

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20
Q

glucoseneogensiss and ketone bodies both used tod erive nergy

A
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21
Q

Glucogenic amino acids used in gluconeogenesis

A
  • Glutamine
  • Arginine
  • Proline
  • Histidine
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22
Q

Ketogenic amino acids used to form ketone bodies

A

• lysine and leucine (provide acetyl CoA)

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23
Q

Both glucogenic and ketogenic

A
  • Threonine
  • Tryptophan
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24
Q

mobilisation of protein reserve

A

during extreme stress (starvation)

  • under hromonal control
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25
Q

insuilin and growth hormone

A

both increase the rate of protein synthesis and decrease the rate of protein degradation

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26
Q

Glucocorticoids (e.g. cortisol)

A

decrease protein synthesis and increase protein degradation

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27
Q

Cushing’s syndrome

A
  • Excessive breakdown of proteins can occur (excess cortisol)
  • Weakens skin structure leading to striae formation
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28
Q

what sort of amino acids can the body synthesie

A

non-essential AA

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29
Q

Carbon for non-essential amino acid synthesis come from

A
  • Intermediates of glycolysis (C3)
  • Pentose phosphate pathway (C4 &C5)
  • Kreb cycle (C4 7C5)
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30
Q

Amino group provided by

A

other amino acids by process transamination or from ammonia

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31
Q

In addition to protein synthesis (requires all 20 amino acids) amino acids also required for synthesis of other important compounds (requires specific amino acids).

A
32
Q

Removal of nitrogen from amino acids

A

Allow carbon skeleton of amino acids to be utilised in oxidative metabolism

33
Q

Once removed N can be

A

incorporated into other compounds or excreted from body as urea

34
Q

Two main pathways facilitate removal of nitrogen from amino acids:

A
  1. transamination
  2. deamination
35
Q

transamtionation

A
  • Most aminotransferase enzyme use alpha-ketoglutarate to funnel the amino group to glutamate
  • Exception to rule is aspartate aminotransferase which uses oxaloacetate to funnel amino groups to aspartate
36
Q

All aminotransferase requires is

A

the coenzyme pyridoxal phosphate which is a derivative of vitamin B6

37
Q

Key aminotransferase enzymes

A

1) Alanine aminotransferase (ALT)

Converts alanine to glutamate

2) Asparate aminotransferase (AST)

Converts glutamate to aspartate

38
Q

Plasma ALT and AST

A

Measured routinely as part of liver function test. Levels high in conditions that cause extensive cellular necrosis:

  • Viral hepatitis
  • Autoimmune liver damage
  • Toxic injury
39
Q

amanita phalloides

A

death cap mushrooms which cause acute liver failure if ingested (plasma ALT levels up 20x normal)

40
Q

Deamination

A
  • Liberate amino group as free ammonia
  • Mainly occurs in liver and kidney
41
Q

Ammonia very toxic and must be removed…

A

Ultimately converted to urea or excreted directly in urine

42
Q

important deamination enzymes

A
  • Amino acids oxidases
  • Glutaminase
  • Glutamate dehydrogenase
43
Q

At physiological pH, NH3 is rapidly converted to

A

ammonium (NH4+)

44
Q

urea

A
  • High nitrogen content
  • Non-toxic
  • Extremely water soluble
  • Chemically inert in humans
  • Most urea is excreted in the urine via the kidneys
  • Also performs useful osmotic role in kidney tubules
45
Q

urea cycle

A
  • Occurs in the liver and involves 5 enzymes
  • Amount of urea cycle enzymes normally related to need to dispose of ammonia
  • High protein diet induces enzyme levels
  • Low protein diet or starvation represses levels
  • Cycle is inducible but not regulated
46
Q

refeeding syndrome

A
  • can occur when nutiritonal support is given to severly malnourished patient
  • ammonia toxicity significant factor (urea cycle down regulated)
  • re-feed @ 5 to 10 kcal/kg/day
  • raid gradually to full needs withing a week
47
Q

Defects in the urea cycle

A
  • Autosomal recessive genetic disorder caused by a deficiency of one of the 5 enzymes in the cycle
  • Occurs in 1 in 30,000 live births
  • Mutation cause partial loss of enzyme function
48
Q

Defects in the urea cycle can lead to

A
  • Hyperammonaemia
  • Accumulation/excretion of urea cycle intermediates
49
Q

clinical severity resulting from defects in the urea cycle depend on

A
  • Nature of defect
  • Amount of protein eaten
50
Q

Severe urea cycle disorders show

A
  • within 1 day of birth- if untreated child will die
51
Q

Mild urea cycle enzyme deficiencies may

A

not show symptoms until early childhood

52
Q

symptoms of urea cycle enzyme deficiency

A
  • Vomiting
  • Lethargy
  • Irritability
  • Mental retardation
  • Seizures
  • Coma
53
Q
A
53
Q

Management of urea cycle enzyme deficiency

A
  • Low protein diet
  • Replace amino acids in diet with keto acids
54
Q

biochemical basis of ammonia toxicity

A
  • Readily diffusible and extremely toxic to brain
  • Blood level needs to be kept low (25-40 umol/l)
  • Several potential toxic effects
55
Q

Several potential toxic effects of ammonia toxicity

A
  • Interference with amino acid transport and protein synthesis
  • Disruption of cerebral blood flow
  • pH effects (alkaline)
  • interference with metabolism of excitatory amino acid NT (glutamate and aspartate)
  • alteration of BBB
  • interference with TCA cycle
56
Q

transport of amino acid nitrogen from tissues to liver for safe disposal

A

two transport mechanisms utilised

1) Glutamine
2) Alanine

57
Q

Transport mechanisms utilised

1) Glutamine

A
  • Ammonia combined with glutamate to form glutamine
  • Glutamine transported in blood to liver or kidneys where it is cleaved by glutaminase to reform glutamate and ammonia
  • In liver ammonia fed into urea cycle
  • In kidneys excreted directly into urine
58
Q

Transport mechanisms utilised

2) Alanine

A
  • Amine groups transferred to glutamate by transamination
  • Pyruvate then transaminase by glutamate to form alanine
  • Alanine transported in blood to liver where it is converted back to pyruvate by transamination
  • Amino group fed via glutamate into urea cycle for disposal as urea whereas pyruvate is used to synthesise glucose which can be fed back to tissues
59
Q
A
60
Q

Clinical problems of amino acid metabolism

A
  • Over 50 inherited diseases involving defects in amino acid metabolism
  • Either total, or more commonly partial loss of enzyme activity
  • Rare (many diseases <1:250,000) although collectively as a group constitute a significant portion of paediatric genetic disease
61
Q

if untreated those with probelsm of amino acid emtbaolism

A

are intellectually impaired

62
Q

treatment of those with problems of aminoa acid metabolism

A

restricting amino acids in diet

63
Q

how are problems of aminoa acid metabolism diagnosed

A

heel prick test

64
Q

what does the heel prick test test for

A
  • Sickle cell
  • CF
  • Congenital hypothyroidism
  • Inborn errors of metabolism:
    • Phenylketonuria (PKU)
    • Homocystinuria
65
Q

Inborn errors of metabolism:

A
  • Phenylketonuria (PKU)
  • Homocystinuria
66
Q

Phenylketonuria (PKU)

A
  • Moss common inborn error of amino acid metabolism (1 in 15,000 births)
  • Deficiency in phenylalanine hydroxylase
  • Autosomal recessive (chromosome 12)
  • Accumulation of phenylalanine in tissue, plasma and urine
  • Phenylketones in urine
  • Musty smell
67
Q

treatment of phenylketonuria (PKU)

A
  • Low phenylalanine diet enriched with tyrosine
  • Avoid artificial sweeteners (contain phenylalanine)
  • Avoid high protein foods such as meat, milk, and eggs
68
Q

homocystinuria

A
  • Problem breaking down methionine
  • Excess homocysteine (oxidised form of homocysteine) excreted in urine
  • Autosomal recessive disorder
  • 1 in 344,000 incidence
  • Defect in cystathionine B-synthase (CBS)
  • Affects connective tissue, muscle, CNS and CVS
69
Q
A
70
Q

treatment of homocysti uria

A

Treatment

  • Low methionine diet
  • Avoid milk, meat, fish, cheese, eggs, nuts
  • Cysteine, Vit B6, Betaine, B12 and folate supplement
71
Q

homocystinuria is often confused with

A

Marfans

  • Some of the clinical features of homocystinuria such as – lens dislocation, skeletal deformities are similar to Mar fans
72
Q

High levels of homocysteine affect connective tissue and bone

A
  • Excess homocysteine causes damage to collagen and elastin fibres in connective tissue by binding to lysine residues in proteins
73
Q
A
74
Q

Metabolites of methionine are toxic to

A

neurones and cause neurological dysfunction

Metabolism (11 decks)

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