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Metabolism > Protein and Nitrogen Metabolism > Flashcards

Protein and Nitrogen Metabolism Flashcards

0
Q

What happens to the a’a released from protein breakdown in the GI lumen?

A

-Absorbed into the bloodstream and enter the circulation to be used in protein synthesis and synthesis of N-containing compounds

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1
Q

What is the first stage in protein metabolism and where does it occur?

A

-Lumen of GI tract
-Break down proteins into a’a by peptidases and proteases hydrolysing peptide bonds
-

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2
Q

What effect do insulin and GH have a’a?

A

-Stimulates there uptake into skeletal muscle, adipose tissue and liver

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3
Q

What effect does cortisol have on protein metabolism?

A

-Stimulates proteolysis

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4
Q

What are the essential dietary a’a?

A

-lysine, isoleucine, leucine, methionine, threonine, valine , tryptophan, phenylalanine

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5
Q

Define a’a pool

A

-Total amount of free a’a in the body

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6
Q

Define nitrogen metabolism

A

-Metabolic processes which relate to N-containing compounds

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7
Q

What is a’a reutilisation?

A

-approx.75% of a’a released during breakdown are used in synthesis

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8
Q

What happens to the a’a that are not reutilised?

A
  • Oxidised to release energy

- Synthesis of other N-containing compounds

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9
Q

What is stage 2 of protein catabolism?

A
  • NH2 removed from a’a

- Converted to intermediates of TCA cycle

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10
Q

What is nitrogen balance?

A

-Nin=Nout

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11
Q

What is the source of Nin?

A

-Proteins

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12
Q

What is the source of Nout?

A
  • Urea
  • Urine
  • Faeces
  • Sweat
  • Hair, nails skin
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13
Q

When would there be a positive nitrogen balance

A

-During periods of growth

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14
Q

When would there be a negative nitrogen balance?

A

-During periods of starvation, malnutrition and trauma

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15
Q

What is protein turnover and what does its rate depend on?

A
  • Breakdown and synthesis

- Depends on protein type, protein demand and varies with growth and ageing

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16
Q

What is the approximate half-life of a protein?

A

-80 days

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17
Q

Which a’a can be synthesised in the body?

A

Non-essential a’a

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18
Q

What are the substrates for a’a synthesis?

A
  • C atoms from pentose phoshate pathway and TCA cycle

- N from other a’a or transamination

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19
Q

Where does a’a breakdown occur?

A

-Liver

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20
Q

What happens to the C-containing compounds in a’a breakdown?

A
  • Enter carb/lipid metaboism

- Converted to intermediates

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21
Q

What is the first step in a’a breakdown?

A
  • Transamination

- Deamination

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22
Q

What happens to the majority of N in the body?

A

-Excreted as urea

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23
Q

What is transamination?

A

-The removal of NH2 from an a’a by combination with a-ketoglutarate or oxaloacetate and transaminase enzymes which is transfer the NH2 to synthesise glutamate or asparatate and a keto-acid (depending on the starting a’a)

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24
Q

What are the two possible equations for transamination?

A
  • a’a+a-ketoglutarate->glutamate + keto acid

- a’a+oxaloacetate->aspartate +keto acid

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25
Q

Why is transamination useful if it just produces other a’a?

A

-Aspartate and glutamate can enter the urea cycle for excretion

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26
Q

What are two clinical important transaminase enzymes and why?

A
  • alanine aminotransaminase
  • aspartate aminotransaminase
  • Can be used as a measure of liver function -> High levels indicates liver damage
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27
Q

What effect does cortisol have on transaminases?

A

-Stimulates their production in the liver

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28
Q

What is deamination?

A

-The removal of NH2 from a’a to free NH3

29
Q

What is NH3?

A

-Ammonia

30
Q

What happens to NH3?

A

-Spontaneously ionised to NH4+

31
Q

Why is NH3 a problem?

A
  • Reduces the TCA cycle

- Effects neurotransmitter synthesis

32
Q

What are the toxicity problems of NH4+?

A
  • Blurred vision
  • Tremors
  • Coma
  • Death
33
Q

How is NH3 removed from the body?

A

-Converted to urea and excreted

34
Q

What enzymes are responsible for deamination? (umberella term)

A

-L-/D-a’a oxidases

35
Q

Where does deamination occur?

A

-Liver/Kidney

36
Q

Which a’a does ALT convert?

A

-Converts Alanine to glutamate + ketoacid

37
Q

Which a’a does AST convert?

A

-Glutamate ->aspartate+ketoacid

38
Q

Why is urea better than NH3?

A
  • High N content for excretion
  • Non-toxic
  • Water-soluble so can travel in blood
  • Chemically inert in humans
  • Useful osmotic effects in kidney tubules
39
Q

Where does the urea cycle occur, specifically?

A

-Partly in the cytoplasm and partly in the mitochondrial matrix of hepatocytes

40
Q

How many enzymes are involved in the urea cycle?

A

5

41
Q

What is the purpose of the urea cycle?

A

-To remove NH3 and NH4+ which are toxic to the body

42
Q

From where is urea excreted?

A

-In urine via the kidneys (urea diffuses from the liver into the blood)

43
Q

What happens to the urea which is reabsorbed from the kidney tubules?

A

-Enters small intestine and is processed by bacteria which produces ammonia

44
Q

What are the substrates for the urea cycle?

A
  • NH4+ from deamination
  • NH3+ from the GI lumen and from the breakdown of excess glutamine
  • Aspartate and Glutamate from transamination
45
Q

What are the two mechanisms of NH3 excretion?

A
  • Converted to glutamine via Glutamate+NH3->glutamine

- Converted to urea

46
Q

How is glutamine excreted?

A

-Directly in the urine via kidneys

47
Q

What happens to excess glutamine which is not excreted via kidneys?

A

-NH3 is converted to urea

48
Q

What enzyme catalyses NH3 incorporation with glutamate?

A

-Glutamine synthase

49
Q

Describe the regulation of the urea cycle, and state in what situation this can be a problem

A
  • No negative feedback from product as aim is to excrete urea
  • Inducible enzymes -> high protein diet -> high urea enzymes
  • Can cause refeeding syndrome; in people with low protein metabolism such as marasmus, giving protein rich food can lead to death as the ammonia in the body will rise, and conversion to urea will not be adequate due to lack of enzymes, leading to toxicity and death
50
Q

What is the clinical picture of NH3 toxicity?

A
  • Vomiting
  • Lethargy
  • Irritability
  • Mental retardation in children
  • Seizures
  • Coma
51
Q

What happens if there is a complete loss of a urea enzymes?

A
  • Fatal

- Death

52
Q

What is the treatment of urea enzyme defects?

A

-Reduction in the intake of a’a

53
Q

What type of inheritance in phenylketouria?

A

-Autosomal recessive

54
Q

What is the enzyme deficiency in phenylketouria?

A

-Phenylalanine hydroxylase

55
Q

Where is the gene located for phenylalanine hydroxylase?

A

-Chromosome 12

56
Q

What is the incidence of phenylketouria?

A

-1 in 15,000

57
Q

Is phenylketouria used in a standard screening test in neonates?

A

-Yes, heel prick test

58
Q

What is the normal metabolism of phenylalanine?

A

-Phenylalanine->tyrosine via phenylalanine hydroxylase

59
Q

What happens in phenylketouria?

A
  • Phenylalanine hydroxylase absent
  • Phenylalanine accumulates in blood and tissues
  • Converted to Phenylpyruvate (phenylketone)
60
Q

How are phenylketones excreted and how can this be used in diagnosis?

A
  • Excreted in the urine

- Test for phenylketones in urine

61
Q

Why is phenylpyruvate problematic?

A

-Inhibits pyruvate uptake into the mitochondria

62
Q

Why is phenylketouria a problem in relation to tyrosine?

A
  • Tyrosine used to synthesis adrenaline,noradrenanline and dopamine
  • Deficiency in dopamine results in disruption of CNS development in children if untreated
63
Q

What inheritance pattern is homocysteinuria?

A

-Rare autosomal recessive

64
Q

What is the enzyme deficiency in homocystinuria?

A

-Cystathione-b-synthase

65
Q

What is the normal metabolism of homocystiene?

A
  • Homocystiene->Cystathione via cystathione-b-synthase

- Cystathione->Cysteine

66
Q

What cofactor does cystatione-b-synthase need?

A

-Vitamin B6

67
Q

What happens in homocystinuria?

A
  • No cystathione-b-synthase
  • Homocysteine begins to accumulate in blood->oxidised to homocystine
  • Homocysteine also converted to methionine via vitamin B12
68
Q

Why is conversion to methionine better than oxidation to homocystine?

A

-Although methionine is damaging, homocystine accumulation in the blood can lead to chronic disorders of connective tissue, muscles, CNS and CVS

69
Q

How does chronic homocystine lead to connective tissue damage?

A

-Disrupts the bonds in the fibrillin-1-protein which is an important connective tissue protein

70
Q

What CVS problems can homocystinuria cause?

A

-Angina by the age of 30 via an unknown mechanism

71
Q

What are two important signalling molecules resulting from a’a metabolism?

A
  • Nitric oxide (vasodilator and neurotransmitter)

- Hydrogen sulphide (Vasodilator)

Metabolism (14 decks)

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