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Metabolism > Amino acid metabolism > Flashcards

Amino acid metabolism Flashcards

1
Q

How are proteins digested?

A

Endopeptidase secreted from stomach and pancreas are cleave internal peptide bonds

Exopeptidases from pancreas remove amino acids at end of peptide chain

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2
Q

What endopeptidases are there and where are they secreted from?

A

Stomach: pepsin
Pancreas: trypsin, chymotrypsin and elastase

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3
Q

What are alternative names for endo and exo peptidases?

A

Olgiopeptidases and aminopeptidases

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4
Q

Why are proteins synthesised and degraded (4 reasons)?

A

Degrade damaged / incorrectly produced/folded proteins that need to be removed to maintain function

Signalling proteins need to be produced and removed as needed

Enzymes are often up/down regulated as part of regulatory mechanisms

In times of starvation protein stores in muscles are degraded in order to utilise amino acids as a fuel.

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5
Q

How are oligopeptides broken down?

A

Brush border membrane of enterocytes contains endopeptidases, aminopeptidases and dipeptidases that digest oligopeptides to dipeptides and tripeptides or AA which are taken into cell

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6
Q

How are amino acids and di/tri peptides absorbed?

A

By Na+ coupled transport system

Di and tri peptides taken up by proton coupled cotransporter and cleaved by intracellular peptidases into AA

AA leave enterocytes via basolateral membrane

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7
Q

What are essential AA, give examples

A

Can’t be made by body, must be received in the diet. (histidine, isoleucine, leucine, lysine methionine, phenylalanine, threonine, tryptophan and valine)

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8
Q

What are non-essential amino acids? What are they?

A

Made by the body (alanine, asparagine, aspartic acid and glutamic acid)

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9
Q

What are conditionally essential amino acids, give an example?

A

Usually not essential, except in times of illness and stress (arginine, cysteine, glutamine, glycine, proline, serine and tyrosine)

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10
Q

What are the metabolic fates of amino acids?

A

Synthesis of proteins/ protein derivatives e.g. hormones

Deaminated: remaining carbon skeleton can be: oxidised via TCA cycle, converted into glucose via gluconeogenesis (then stored as glycogen) or turned into fatty acid (stored as TAG)

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11
Q

What do glucogenic amino acids do?

A

Degraded to pyruvate or TCA intermediate

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12
Q

Give examples of glucogenic amino acids that enter at pyruvate (and mnemonic)

A 
Great Apple Crumble Takes Stress
Glycine
Alanine
Cysteine
Threonine
Serine
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13
Q

Examples of glucogenic amino acids that enter at TCA intermediates? (mnemonic)

A 
Always Give Holden Away
Aspartate
Glutamate
Hisidine
Asparagine
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14
Q

Give examples of both glucogenic and ketogenic amino acids, what is the mnemonic?

A 
Mnemonic = PITTT
Phenylalanine
Isoleucine
(Threonine)
Tryptophan
Tyrosine
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15
Q

What are ketogenic amino acids and what do they do?

A

Leucine
Lysine
Enter at Acetyl Coa

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16
Q

When is nitrogen balance negative?

A

After surgery

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17
Q

What happens when protein intake exceeds need?

A

Excess amino acids can’t be stored so carbon skeleton used as energy source and amino groups excreted
i.e. transport to liver and processed via urea

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18
Q

What happens when protein intake less than need?

A

Protein catabolism frees carbon skeleton for energy and excess amino acids secreted

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19
Q

How are proteins tagged for degradation?

A

With ubiquitin

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20
Q

How is ubiquitin attached to proteins, how is it activated and able to recognise?

A

Ubiquitin ligases: E1,E2,E3

E1 & E2 are responsible for activating the ubiquitin

E3 recognises the protein to be ubiquinated and is able to recognise damaged / misfolded proteins

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21
Q

What amino acid reactions occur in the skeletal muscle?

A

Transamination of pyruvate to alanine

Glutamate to glutamine (glutamate synthetase)

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22
Q

What is involved in the Cahill cycle?

A

Use of alanine in the liver and its conversion to pyruvate (transamination) which is then converted to glucose (gluconeogenesis).

The glucose returns the to muscles, is broken down to pyruvate (glycolysis) then pyruvate can be transaminated into alanine.
(In muscles (reverse to degradation) Glutamate transaminates pyruvate –> alanine and regenerate alpha keto glutarate)

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23
Q

What happens in transamination, what’s most commonly involved?

A

Responsible for deamination of AA.

NH3 of amino acid transferred to alpha keto acid.

Deaminated AA becomes another alpha keto acid.

Alpha ketoacid + NH3 becomes AA

(Most common acceptor is alpha ketoglutarate, forming glutamate)

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24
Q

Alanine + alpha ketoglutarate =

A

Pyruvate and glutamate

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25
Q

Aspartate + alpha keto glutarate =

A

Oxaloacetate and glutamate

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26
Q

Why is only one deamination pathway required?

A

Pooling of excess amino acids into glutamate as only AA that undergoes rapid oxidative deaminatoin

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27
Q

What happens in oxidative deamination (where does it happen and what’s produced)?

A

Glutamate dehydrogenase catalyses oxidative deamination of glutamate in mitochondrial matrix of cells

Glutamate + NAD+ + H2O = NADH+ alpha ketoglutarate + NH4+

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28
Q

What cofactor is required fro transamination, what does it do?

A

Pyridoxal phosphate.

Forms a Schiff base with amine component of lysine residues in the active site of the transaminase. This acts as an electron sink to stabilise the negatively charged intermediates.

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29
Q

What enzymes catalyse transamination?

A

Transaminases (aspartate transaminase / alanine transaminase)

30
Q

What can activate and inhibit glutamate dehydrogenase?

A

Activate: ADP and GDP. Signal AA needed as energy source

Inhibit: GTP (TCA cycle active)

31
Q

What happens to the ammonia generated by deamination?

A

Incorporated into urea for excretion

32
Q

How are serine and threonine deaminated and why?

A

Non-oxidative, they have an OH group. Catalysed by dehydratases because dehydration precedes deamination.

33
Q

Which AA undergo direct deamination?

A

Serine and threonine

34
Q

How are glutamine and asparagine deaminated, what enzyme?

A

Hydrolytic deamination, converted to another AA as NH4+ is removed.

Catalysed by -inases (glutaminase asparaginase)

35
Q

What AA undergo hydrolytic deamination?

A

Asparagine and glutamine

36
Q

What does transamination of serine and threonine produce?

A

Serine –> pyruvate

Threonine –> alpha ketobutyrate

37
Q

How do muscles produce NH4+?

A

Protein turnover and muscle catabolism in starvation and breakdown, transamination produces NH4+

NH4+ combines with alpha keto glutarate to form glutamate

Glutamate relases NH4+ in oxidative deamination

38
Q

What happens to alanine and glutamine produced in the muscles?

A

Alanine travels to liver for deamination and gluconeogenesis (alanine cycle).

Glutamine from muscle catabolism taken up by enterocytes.

39
Q

Describe relationship between glutamate and alpha keto glutarate?

A

Combine NH4+ and alpha ketoglutarate –> glutamate

40
Q

What can glutamate be used to generate in muscle?

A

Transaminate pyruvate to make alanine and regenerate alpha ketoglutarate

41
Q

What do enterocytes do?

A

Take up glutamine and release it as alanine

42
Q

What does renal cortex do to glutamine?

A

Deaminates and ammonium used to assist acidification of urine. Conserves HCO3-

43
Q

What is the role of brain in NH4+ production?

A

GABA broken down to succinate and NH4+ which combines with alpha ketoglutarate to form glutamate.

Another NH4+ incorporated to form glutamine which goes to liver for deamination + urea synthesis

44
Q

Why is AA release from muscle important?

A

Largest store of protein in body

Glucose still oxidised in starvation e.g. in brain so need glucose from non glucose stores (gluconeogenesis)

45
Q

What are the main AA in blood?

A

Alanine and glutamine

46
Q

Describe urea

A

Non toxic soluble way of eliminating excess ammonia

47
Q

Where are the two NH4 + in urea synthesis donated from?

A

One NH4+ from deamination and one donated form aspartate

48
Q

Where does the urea cycle occur?

A

Occurs in mitochondrial matrix and cytoplasm

49
Q

What is acute regulation of urea cycle?

A

High levels of arginine signal AA readily available so activates N acetyl glutamate (allosteric activator of CPS enzyme)

50
Q

What can activate CPS?

A

Ornithine, ammonia and N-acetyl Glutamate

51
Q

What is chronic regulation of urea cycle?

A

Urea enzymes induced 24-36 hours in response to increased ammonia levels in liver cells.

52
Q

Describe which cells generate urea?

A

Periportal liver cells

53
Q

Describe ammonia levels in liver disease

A

Rise

54
Q

Descibe what causes increased load on urea cycle?

A

Increased protein intake

Deficiency of essential amino acid

General starvation

Catabolic states, i.e. illness

55
Q

What two ways do the TCA cycle and urea cycle feed into one another?

A

The nitrogen provided by aspartate in the urea cycle can be obtained from the transamination of oxaloacetate to aspartate.

The fumarate that is produced in step three is also an intermediate in the citric acid cycle and is returned to that cycle.

56
Q

What is the committed step in the urea cycle?

A

Coupling of free ammonia with bicarbonate to from carbamoyl phosphate, catalysed by carbamoyl phosphate synthetase I (CPS1).

57
Q

What is the second and last reaction to happen inside the mitochondria ?

A

Carbamoyl group is transferred to ornithine to form citrulline, this is catalysed by ornithine transcarbamoylase.

58
Q

Is NH4+ a suitable substrate for CPS1?

A

No, only ammonia

59
Q

Which substance moves across the mitochondrial membrane?

A

Citrulline is transported from the mitochondrial matrix to the cytoplasm. .

60
Q

What is the final product formed in the cytosol (urea cycle) and what happens to it?

A

Finally, arginine is hydrolysed to generate urea and ornithine in a reaction catalysed by arginase.

Ornithine can then be transported back into the mitochondria to form citrulline, in the next cycle

61
Q

What form is waste nitrogen transported around body in fasting?

A

Glutamine

62
Q

What can be monitored in blood, and when found may suggest liver disease?

A

Aspartate aminotransferase/ aspartate transaminase, normally found in liver.

63
Q

What is the main AA used by liver in starvation for gluconeogenesis?

A

Alanine

64
Q

Amino acids which give rise directly to acetyl CoA are not glucogenic because…

A

For each acetyl CoA entering TCA cycle, an equivalent amount of co2 is evolved before formation of oxaloacetate

65
Q

% of fuel reserves of an adult represented by proteins?

A

25%

66
Q

The immediate source of the second nitrogen atom incorporated in the urea cycle

A

Aspartate

67
Q

What is the key enzyme for releasing amino groups as free ammonia for incorporation into carbamyl phosphate?

A

Glutamate dehydrogenase

68
Q

The form in which waste nitrogen is brought from peripheral tissue for excretion by the liver and kidney

A

Glutamine

69
Q

One way in which waste ammonia is removed from the body?

A

Ammonium ions (urine)

70
Q

Two ways in which nitrogen can be lost?

A

Liver - NH3 to urea - filtration - urine

Kidney - Glutamine to glutamate (and ammonium) - urine

71
Q

What amino acid is arguable whether it is both glucogenic and ketogenic?

A

Threonine (sometimes not recognised at ketogenic)

Metabolism (13 decks)

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