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MEH > Session 1: Protein and Amino Acid Metabolism > Flashcards

Session 1: Protein and Amino Acid Metabolism Flashcards

Explain nitrogen balance and protein turnover Describe how amino acids are catabolised Explain defects in amino acid metabolism Define the terms glycogenic and ketogenic amino acids Describe how ammonia is metabolised Explain why creatinine is measured

1
Q

What are some major nitrogen containing compounds?

A 
Amino acids
Proteins
Purines and pyrimidines in DNA and RNA.
Porphyrins
Creatine phosphate
Neurotransmitters
Hormones
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2
Q

What is the precursor to creatinine?

A

Creatine and creatine phosphate. Produced at a constant rate unless muscle wasting.

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3
Q

How is creatinine filtered?

A

Via the kidneys into the urine.

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4
Q

Why is creatinine important?

A

It is used as a marker to estimate muscle mass and can also be an indicator of renal function. A lot of drugs are standardised to creatinine as well.

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5
Q

How much nitrogen is there in a human?

A

Around 2 kg.

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6
Q

Where do we get nitrogen from?

A

From dietary proteins. Around 16 g N.

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7
Q

How is nitrogen excreted?

A

By loss of skin, hair and nails. However most is excreted via faeces and urine.

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8
Q

What does a positive N balance mean? When can this happen? Is it dangerous?

A

That more nitrogen is going into the body than going out.

Its common in growth of a human, pregnancy and in adult covering from malnutrition. It is not a problem.

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9
Q

What does a negative N balance mean? When can this happen? Is it dangerous?

A

When more nitrogen is going out than in. This means a net loss of body protein. This is never normal. It can be caused by trauma, infection or malnutrition.

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10
Q

Outline the protein turnover.

A

Dietary protein is digested into free amino acids. As amino acids they can go into the muscles to synthesise more muscle or they can go to the liver. In the liver they can be metabolised into the carbon skeleton or the amino group (-NH2). The carbon skeleton goes onto form glucogenic amino acids and ketogenic acids. Glucogenic amino acids undergo gluconeogenesis to form energy. Ketogenic amino acids turn into ketone bodies which can then be used for energy.

The amino group will turn into urea and be excreted in the urine.

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11
Q

Which amino acids are glucogenic?

A 
Arginine
Histidine
Proline
Glutamine
Glutamate
Methionine
Valine
Glycine
Serine
Cysteine
Alanine
Aspartate
Asparagine
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12
Q

Which amino acids are ketogenic?

A

Lysine

Leucine

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13
Q

What happens to the effect of protein synthesis and protein degradation in the presence of insulin and growth hormone?

A

Protein synthesis increases

Protein degradation decreases

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14
Q

What happens to the effect of protein synthesis and protein degradation in the presence of glucocorticoids such as cortisol?

A

Protein synthesis decreases

Protein degradation increases

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15
Q

What is Cushing’s syndrome?

A

When there is an excessive breakdown of protein as there is an excess of cortisol.

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16
Q

What is a consequence of Cushing’s syndrome?

A

Weakened skin structure leading to striae formation.

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17
Q

What is the structure of an amino acid?

A

Amino group, a carbon, a hydrogen, a R-group and a carboxyl group.

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18
Q

How do amino acids bind to each other?

A

They bind via a peptide bond between the carboxyl group and the amino group. Water is formed.

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19
Q

What 9 essential amino acids are there?

A 
If learned this huge list may prove truly valuable.
Isoleucine
Lysine
Threonine
Histidine
Leucine
Methionine
Phenylalanine
Tryptophan
Valine
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20
Q

What are conditionally essential amino acids?

A

Amino acids that are only essential in some cases. For example arginine, tyrosine and cysteine is required in children and pregnant women due to high rate of protein synthesis.

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21
Q

What are high quality proteins?

A

Proteins of animal origin.

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22
Q

Why are they considered high quality?

A

Because they have all essential amino acids.

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23
Q

What are low quality proteins? Why?

A

Proteins of plant origin. Because they can lack essential amino acids.

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24
Q

Where do the carbon atoms for non-essential amino acid synthesis come from?

A

Intermediates of glycolysis
Pentose phosphate pathway
Krebs cycle.

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25
Q

In addition to protein synthesis amino acids are also required for synthesis of other important compounds. Give examples.

A 
Tyrosine - catecholamines
Cysteine - glutathione
Tryptophan - Serotonin
Histidine - Histamine
Glutamate - GABA
Glycine - Purines
Serine - sphingosine
Arginine - Nitric oxide
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26
Q

Why is the removal of amino groups essential?

A

To allow carbon skeleton of amino acids to be utilised in oxidative metabolism.

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27
Q

How can nitrogen be excreted?

A

As urea.

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28
Q

What are the two main pathways of how removal of nitrogen can happen?

A

Transamination

Deamination

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29
Q

Explain transamination.

A

Amino acid 1 + Keto acid 2 -> Amino acid 2 + Keto acid 1

This means that the first amino acid turns into a keto acid and the keto acid in the left row turns into an amino acid.

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30
Q

Why is transamination important?

A

Because some amino acids can more easily feed into the urea cycle.

31
Q

Give examples of transamination.

A

alpha-ketoglutarate turns into glutamate. The other keto acid and amino acid are not important.
Oxaloacetate to aspartate.

32
Q

What enzymes are used in each case?

A

Most aminotransferase enzymes use alpha-ketoglutarate to turn into glutamate.
Aspartate aminotransferase is used to turn oxaloacetate into aspartate.

33
Q

What coenzyme is required in all aminotransferases?

A

Pyridoxal phosphate, which is a derivative of vitamin B6.

34
Q

Which are the key aminotransferases?

A

Alanine aminotransferase

Aspartate aminotransferase

35
Q

Why are they important?

A

Because plasma levels of ALT and AST are measured as part of liver function test.

36
Q

What conditions could be a consequence of an altered level of ALT and AST? Elevated or lowered?

A

A high level of ALT and AST can cause extensive cellular necrosis:
Viral hepatitis
Autoimmune liver disase
Toxic injury

37
Q

Explain deamination.

A

The removal of an amino group to create free ammonia. The keto acids that are formed can be utilised for energy.

38
Q

Where does deamination mainly occur?

A

In the liver and kidneys.

39
Q

What is also important in deamination?

A

Dietary D-amino acids that are found in plants and micro-organisms.

40
Q

What is ammonia and ammonium?

A

Ammonia NH3 and ammonium NH4+. They are very toxic and must be removed by the body. They are converted to urea or excreted directly in urine. At physiological pH ammonia is rapidly converted into ammonium.

41
Q

Give examples of enzymes that can deaminate.

A

Amino acid oxidases
Glutaminsases
Glutamate dehydrogenase

42
Q

What is urea?

A

Urea is a waste product of high nitrogen content. It is extremely water soluble and chemically inert in humans.

43
Q

Is urea toxic?

A

No.

44
Q

Bacteria can break down urea into what?

A

Into NH3.

45
Q

How does urea leave the body?

A

It is usually excreted in urine via the kidneys.

46
Q

They have an important role in the kidney tubules, which?

A

Osmotic role.

47
Q

Where does the urea cycle take place?

A

In the liver. Half in the mitochondria and half in the cytoplasm.

48
Q

Outline the urea cycle.

A

In the mitochondria:
CO2 and NH4+ reacts to form Carbamoyl phosphate. This requires 2 ATP.
Carbamoyl phosphate then reacts with ornithine to form Citrulline.
Citrulline then exits the mitrochondria.
Cytoplasm:
Citrulline reacts with Aspartate to form Argininosuccinate.
Arginino succinate then lyses to form arginine and fumarate. Fumarate is then recycled back into aspartate to be used again. Arginine then reacts with water to form urea, a part of arginine also breaks off to produce ornithine.

49
Q

How does the enzyme amount relate to ammonia?

A

The amount of enzymes from the urea cycle is normally related to the disposal of ammonia. This means that if there is more ammonia more enzymes are needed.

50
Q

What regulates the enzyme levels of the urea cycle?

A

High protein diet increases enzyme levels
Low protein diet decreases enzyme levels
Cycle is inedible but not regulated.

51
Q

What is refeeding syndrome?

A

This occur when you give nutritional support to a severely malnourished patient. Since the enzymes in the urea cycle are already down-regulated ingesting high protein food will increase the ammonia levels severely. The urea cycle can then not take care of the excess ammonia and it raises to toxic levels. This can have detrimental damage, especially to the liver.

52
Q

What are defects in the urea cycle?

A

Automsal recessive disorders caused by a deficiency in one of the enzymes.

53
Q

What will enzyme deficiency lead to?

A

Hyperammonaemia

Accumulation/excretion of urea cycle intermediates

54
Q

What happens if a child has severe urea cycle disorder?

A

It will show within a day of birth. If it is left untreated the child will die.

55
Q

What happens if a child has a mild urea cycle disorder?

A

It might now show up in a while until early childhood.

56
Q

How do you manage urea cycle disorders?

A

By a low protein diet or replace amino acids altogether with keto acids instead. Keto acids do not have an amine group and therefore no nitrogen.

57
Q

How do keto acids and amino acids differ?

A

Keto acids do not have an amino group and therefore no nitrogen.

58
Q

Why is ammonia toxic?

A

It is readily diffusible and extremely toxic to the brain.

59
Q

How can ammonia have toxic effects?

A

Interference with amino acid transport and protein synthesis.
Disruption of cerebral blood flow.
pH effects making it more alkaline.
Interference with metabolism of excitatory amino acid neurotransmitters.
Alteration of the blood-brain barrier.
Interference with the TCA cycle.

60
Q

How can amino acid nitrogens (ammonia) be safely transport from the tissues to the liver for disposal?

A

It is usually transported by either glutamine or alanine.

61
Q

Explain glutamine transportation of amino acids nitrogen.

A

Ammonia reacts with glutamate to form glutamine. (NH3 + ATP -> H3PO4 + ADP).
This is done by an enzyme called glutamine synthetase.
Glutamine can the be transported via the blood to the liver where it is then cleaved by glutaminase to form glutamate and ammonia. The ammonia can now be safely fed into the urea cycle.

62
Q

Explain alanine transportation of amino acids nitrogen.

A

Amine groups are transferred to glutamate by transamination. Pyruvate then transaminates by glutamate to form alanine.
The alanine can now be safely transported to the liver via blood. Alanine is then converted back into glutamate.
Pyruvate is used to synthesis glucose which can be fed back into tissue and glutamate goes into urea cycle.

63
Q

What happens if you leave an amino acid metabolism disorder untreated?

A

Usually leads to intellectual impairment.

64
Q

Give examples of inborn errors of metabolism.

A 
Phenylketonuria (PKU)
Maple syrup urine disease
Isovaleric acadaemia (IVA)
Glutaric aciduria
Homocystinuria
65
Q

Explain phenylketonuria.

A

It is the most common inborn error of amino acid metabolism.
There is a deficiency of the enzyme phenylalanine hydroxylase.
This is an autosomal recessive disorder affecting a gene on chromosome 12.
This causes accumulation of phenylalanine in tissue, plasma and urine as phenylalanine transaminases into phenylpyruvate and phenylacetate and phenyl lactate is then formed.
There is a presence of phenylketones in urine.
Urine has a musty smell.

66
Q

How do you treat PKU?

A

Strictly controlled low phenylalanine diet enriched with tyrosine.
Avoid artificial sweeteners because they contain phenylalanine.
Avoid high protein foods such as meat, milk and eggs.

67
Q

Why is tyrosine enriched food important?

A

Because phenylalanine hydroxylase turns phenylalanine into tyrosine.

68
Q

Why is tyrosine important?

A

It is used in noradrenaline, adrenaline, dopamine, melanin, thyroid hormone, protein synthesis.

69
Q

What are symptoms of PKU if left untreated?

A 
Severe intellectual disability
Delay in development
Microcephaly
Seizures
Hypopigmentation
70
Q

Explain homocystinurias.

A

Problem metabolising methionine.
There is an excess amount of homocystine (the oxidised form of homocysteine) in the urine.
Autosomal recessive.
Defect in cystathionine beta-synthase usually.
It affects connective tissue, muscles, CNS and CVS.

71
Q

What are treatments for homocystinuria?

A

Low-methionine diet
Avoid milk, meat, fish, cheese and eggs.
Nuts and peanut butter also.
Cysteine, vit b6, betaine, b12 and folate supplement.

72
Q

What is elevated plasma homocysteine a sign of?

A

Potential cardiovascular disease.

73
Q

Why would you give cysteine to a patient with homocystinuria?

A

Because methionine is eventually turned into cysteine.

74
Q

Why would you give betaine, b1 and folate to a patient with homocystinuria?

A

Because methionine is turned into homocysteine. However these supplements promote the methionine instead so there is a lower accumulation of homocysteine.

MEH (22 decks)

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