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Biochemistry 2 > AA Catabolism > Flashcards

AA Catabolism Flashcards

1
Q

Under what circumstances do AAs need to be broken down in mammals?

A
  1. Normal protein synthesis and degradation
  2. Surplus AA ingested
  3. Starvation
  4. Uncontrolled diabetes, tumor, etc – body starts eating its own protein and body weight drops drastically
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2
Q

What is the % of proteins eaten are degraded for energy in carnivores vs herbivores?

A

Carnivores: 9%

Herbivores: close to none

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3
Q

Where are most AAs metabolized?

A

Liver

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4
Q

What two paths can the AAs that turn into ammonia (NH4+) take?

A
  1. recycled and used in the biosynthesis of AAs. nucleotides, and biological amines.
  2. Converted to urea for excretion
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5
Q

What 4 AAs play a central role in nitrogen metabolism? What is special about these 4 AAs?

A
  1. glutamate
  2. glutamine
  3. alanine
  4. aspartate

They are the ones most easily converted into TCA cycle intermediates

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6
Q

Which TCA cycle intermediate do glutamate, glutamine, alanine, and pyruvate most easily turn into?

A

Glutamate & glutamine –> alpha-ketoglutarate

Alanine –> pyruvate

Aspartate –> oxaloacetate

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7
Q

What two products do AAs break down into?

A

NH4+ + Carbon skeletons

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8
Q

What is the path of the carbon skeletons?

A

TCA cycle, then gluconeogenesis

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9
Q

Where are gastric glands located?

A

Stomach lining

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10
Q

What 3 types of cells can be found in the gastric glands of the stomach lining? What do each secrete?

A
  1. Parietal cells (secrete HCl)
  2. Chief cells (secrete pepsinogen)
  3. Gastric mucosa (secretes gastrin)
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11
Q

Entry of dietary protein into the stomach stimulates ______ to secrete ______?

A
  1. the gastric mucosa

2. the hormone: gastrin

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12
Q

What is the role of gastrin in the stomach?

A

The presence of gastrin causes partial cells to secrete HCl and chief cells to secrete pepsinogen.

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13
Q

What is the acidity of gastric juice (pepsinogen + HCl + gastrin)?

A

1.0-2.5

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14
Q

What are 2 characteristics of gastric juice?

A

Antiseptic: kills bacteria & foreign cells

Denaturing agent: unfolds globular proteins, causing their internal peptide bonds to be more accessible to enzymatic hydrolysis

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15
Q

What does pepsinogen (an inactive precursor) turn into in the stomach?

A

Pepsin

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16
Q

What is the role of pepsin in the stomach?

A

Hydrolyzes ingested proteins. Cleaves long peptide chains into smaller peptides

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17
Q

What is secreted as the contents from the stomach pass into the small intestine?

A

Low pH of the contents triggers secretion of the hormone secretin into blood

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18
Q

What does secretin stimulate?

A

The pancreas to secrete bicarbonate into the small intestine to neutralize the pH to 7

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19
Q

What is a zymogen?

A

a precursor

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20
Q

Arrival of AAs into the duodenum (upper intestine) causes release of what?

A

Cholecytokinin hormone into the blood.

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21
Q

What does the release of cholecytokinin into the blood stimulate?

A

Secretion of the pancreatic precursors trypsinogen, chymotryprinogen, and carboxypeptidase A & B by the exocrine cells of the pancreas.

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22
Q

What is trypsinogen (a precursor) converted into? Via what enzyme?

A

Trypsin via enteropeptidase

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23
Q

What does free trypsin then further activate? (3 things)

A

Converts additional trypsingogen to trypsin.

Trypsin also activates chymotrypsinogen and the carboxypeptidases A & B.

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24
Q

What is the role of trypsin and chymotrypsin once they have been activated?

A

Further hydrolyze the peptides that were produced by pepsin in the stomach.

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25
Q

Why is the stage of protein digestion accomplished very efficiently by trypsin, chymotrypsin and pepsin?

A

Because they each target different AAs

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26
Q

How does the final mixture of free amino acids enter the blood capillaries? Where do they travel next?

A

They are imported into the epithelial cells that line the small intestine through which the AAs enter the blood capillaries in the villi, then travel to the liver

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27
Q

What is the 1st step in the catabolism of L-amino acids once they have reached the liver? What enzyme mediates this? What are these reactions called?

A

Removal of the alpha-amino grou (NH3+).

Amino trasferase (aka transaminases)

Transamination reactions

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28
Q

What is the net reaction of a transamination reactio?

A

L-amino acid + alpha-ketogluterate –> L-glutamate + alpha-keto acid

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29
Q

Alpha-ketogluterate is known as what in transamination reactions?

A

General amino acceptor. It accepts the amino from the L-amino acid and becomes L-glutamate. The L-amino acid minus it’s amino group now becomes alpha-keto acid.

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30
Q

What is a prosthetic group?

A

A non-protein group forming part of or combined with a protein

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31
Q

What enzyme cataylzes transamination reactions? What is this enzyme’s prosthetic group?
What is the primary function of this prosthetic group in cells?

A

Amino transferases/transaminases

Pyridoxal Phosphate (PLP).

Primary function: metabolism of molecules with amino groups.

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32
Q

What is the function of pyridoxal phosphate in transamination reactions?

A

Function: intermediate carrier of amino groups at the active site of animotransferases

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33
Q

Why is there no net loss in amino groups in transamination reactions?

A

The amino group from the AA is donated to alpha-ketogluterate

34
Q

What other enzyme is PLP a cofactor for?

A

Glycogen phosphorylase

35
Q

PLP’s major role is in what?

A

AA metabolism

36
Q

What is the overall purpose of transamination reactions?

A

Collect the amino groups from many different AAs in the form of L-glutamate. Glutamate then acts as an amino group donor for biosynthetic pathways/excretion pathways that serve to eliminate nitrogenous waste products.

37
Q

What is the purpose of deamination reactions? Where do they occur?

A

To remove Amino groups from glutamate to prepare them for excretion

Mitochondria

38
Q

What is one function of glutamate?

A

Amino group donor for excretion and biosynthetic pathways

39
Q

What does glutamate become after undergoing a deamination reaction? Enzyme used?

A

Alpha-ketogluterate via glutamate dehydrogenase

40
Q

What are two biproducts of the deamination reaction?

A

NADPH, NH+4 (use H2O to remove)

41
Q

Which of 3 pathways can the alpha-ketogluterate go to after undergoing the deamination reaction?

A

Transamination, TCA cycle or gluconeogenesis

42
Q

What is glutamate converted to once free ammonia has been added to it? Via what enzyme? How many steps does this reaction occur in?

A

Glutamine.

Glutamine synthetase.

2 steps.

43
Q

What are the 2 steps in the reaction from glutamate to glutamine? Where is glutamine exported to once it is greated?

A
  1. Glutamate + ATP –> ADP + y-glutamyl phosphate. Via: glutamine synthetase
  2. y-glutamyl phosphate + NH4+ –> L-glutamine + Pi. Via: glutamine synthetase

Liver

44
Q

Why is glutamine ideal to transport ammonia?

A

Ammonia = toxic

Glutamine = a non-toxic transport form of ammonia

45
Q

What is the excess glutamine (that is not needed for biosynthesis) transported to? What is glutamine converted to here?

A

Intestine, liver (mitochondria), kidneys for processing.

Glutamate + NH4+ via glutaminase

46
Q

Where is the final NH4+ from the kidney and intestine transported to?

A

Liver for excretion via urea synthesis

47
Q

What is the cycle that involves the transport of ammonia from the muscle to the liver via alanine?

A

Glucose-alanine cycle

48
Q

Draw out the transport of Ammonia from muscle!

A

Nice work Soph!

49
Q

Where does the urea cycle begin?

A

Mitochondrea

50
Q

What are the origional AAs that are used for the urea cycle?

A

Glutamine, Alanine (from muscle), other AAs

51
Q

What AA is alanine & the other AAs turned into before it enters the urea cycle? Via what reaction?

A

Glutamate via transamination reaction. (This happens outside of mitochondria)

52
Q

What two amino acids actually enter the mitochondria for the urea cycle?

A

Glutamine, glutamate

53
Q

What is glutamine converted to once it enters the mitochondria? Via what enzyme? What is released?

A

Glutamate via enzyme glutaminase. This releases an NH4+

54
Q

What enzyme allows glutamate to give up its NH4+ in the mitochondria during the urea cycle? What molecule does glutamate turn into once it releases its NH4+?

A

glutamate dehydrogenase.

alpha-ketoglutarate

55
Q

Oxaloacetate then reacts with alpha-ketoglutarate to form what AA? Enzyme?

A

Oxaloacetate reacts with alpha-ketoglutarate to form aspartate via aspartate aminotransferase

56
Q

How does ammonia from the intestine enter the mitochondria and contribute to the ammonia pool ?

A

Bacteria in GI tract use protein for energy…ammonia is a metabollic byproduct –> ammonia enters liver via portal vein

57
Q

What does a bicarbonate molecule look like?

A

HCO3

58
Q

What accumulates along with the NH4+ pool in the mitochondria for the urea cycle?

A

Bicarbonate (HCO3) + ATP

59
Q

What is the 1st intermediate of the urea cycle that is produced from the NH4+ pool in the mitochondria? Enzyme?

A

NH4(+) + HCO3 + 2 ATP –> Carbamoyl phosphate

Via Carbamoyl phosphate synthetase 1

60
Q

What is the 1st step of the urea cycle? Where does it occur? Enzyme?

A

Carbamoyl phosphate binds with ornithine to form citrulline

Enzyme: Ornithine transcarbamoylase

61
Q

What happens to citrulline once it is produced in the mitochondria?

A

Kicked out into the cytosol

62
Q

What is the 2nd step of the urea cycle? Dependent on? Enzyme?

A

Citrulline –> Agrininosuccinate.

ATP dependent.

Enzyme: Arginosuccinase synthase

63
Q

What step in the urea cycle is aspartate that was produced in the mitochondria used? What does aspartate donate to Argininosuccinate?

A

Step 2. Aspartate gives its amino group to Agrininosuccinate

64
Q

What is the 3rd step in the urea cycle? Enzyme?

A

Argininosuccinate –> Fumarate + Arginine

Agrinosuccinase (cleaves Argininosuccinate into fumerate and arginine)

65
Q

What is the 4th step in the urea cycle?

A

Arginine is cleaved (hydrolysis) to yield urea and ornithine via arginase

66
Q

Which two enzymes of the TCA cycle are also present as isozymes in the cytosol?

A

Fumerase and malate dehydrogenase

67
Q

How does the fumerate that leaves the urea cycle end up in the TCA cycle? What does it transform into?

A

Frumerate is converted to malate in the cytosol, then transported into the mitochondria to enter the TCA cycle

68
Q

What 4 molecules closely link the TCA and Urea cycle?

A

Aspartate, oxaloacetate, malate, fumerate

69
Q

What does ketogenic mean?

A

can be degraded directly into acetyl-CoA, which is the precursor of ketone bodies.

Notes: forms acetyl-CoA and Fumarate

70
Q

What does glucogenic mean?

A

Capable of being converted into glucose or glycogen by the process of gluconeogenesis

Notes: forms pyruvate

71
Q

Which 2 AAs are exclusively ketogenic?

A

Only leucine and lysine

72
Q

Which 5 amino acids are both ketogenic and glucogenic?

A

Isoleusine, phenylalanine, threonine, tryptophan, tyrosine

73
Q

When is the ability of ketogenic AAs to form ketone bodies particularly evident?

A

During starvation, diabetes, cancer: here the liver produces large amounts of ketone bodies from FAs & ketogenic AAs

74
Q

Which AAs lead to the formation of pyruvate of the TCA cycle?

Are these ketogenic or glucogenic?

A 
Alanine
Tryptophan
Cysteine 
Serine 
Glycine
Threonine

Both

75
Q

Which AAs lead to the formation of pyruvate of the TCA cycle?

Are these ketogenic or glucogenic?

A 
Tryptophan
Lysine
Phenylalanine
Tyrosine
Leucine
Isoleucine
Threonine

Lysine and Leucine are strictly ketogenic. The rest are both

76
Q

Which AAs lead to the formation of alpha-ketogluterate of the TCA cycle?

Are these ketogenic or glucogenic?

A 
Proline
Glutamate 
Glutamine 
Arginine
Histidine

Both

77
Q

Which AAs lead to the formation of succinyl-CoA of the TCA cycle?

Are these ketogenic or glucogenic?

A

Methionine
Isoleucine
Threonine
Valine

Both

78
Q

Which AAs lead to the formation of Oxaloacetate of the TCA cycle?

Are these ketogenic or glucogenic?

A

Asparagine

Aspartate

79
Q

Which AAs lead to the formation of Fumarate of the TCA cycle?

Are these ketogenic or glucogenic?

A

Phenylalanine

Tyrosine

80
Q

What activates glutamate dehydrogenase? What inactivates gluatmate dehydrogenase?

A

ADP positively regulates it

GTP negatively regulates it

81
Q

What are all 4 enzymes of the urea cycle and carbamoyl phosphate synthase I regualted by?

A

Nutritional status (diet and starvation)

Starvation & protein rich diet: enzymes are elevated because ammonia needs to be excreted

82
Q

The high energy (3 ATP) used by urea cycle is offset by what?

A

NADH (2.5 ATP) malate-oxaloacetate conversion reaction

Biochemistry 2 (8 decks)

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