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Biochemistry > Chapter 17: Amino Acid Metabolism > Flashcards

Chapter 17: Amino Acid Metabolism Flashcards

1
Q

the amino acids that can be made given adequate dietary proteins

A

nonessential amino acids

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2
Q

amino acids that cannot be synthesized and must be obtained from dietary sources

A

essential amino acids

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3
Q

amino acids that are usually not essential, except in times of illness and stress

A

conditional amino acids

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4
Q

what is the correlation between the energetic expense of a certain pathway and whether an amino acid is essential?

A

the more ATP that is required, the more essential the amino acid is

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5
Q

which amino acids are essential?

A
  • histidine
  • isoleucine
  • leucine
  • methionine
  • phenylalanine
  • threonine
  • tryptophan
  • valine
  • lysine
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6
Q

which amino acids are non-essential?

A
  • alanine
  • aspartate
  • cysteine
  • glutamate
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7
Q

where does the nitrogen needed for amino acids come from?

A
  • nitrogen gas in the atmosphere

- nitrate (NO3-) from soil and water

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8
Q

what are the steps of the nitrogen cycle?

A
  • *One process:
    1. Atmospheric nitrogen (N2) is converted to nitrate (NO3-) by lightning
    2. Nitrate is converted to nitrite (NO2-) by nitrate reductase
    3. Nitrite converted to ammonia (NH3) by nitrite reductase
    4. NH3 used to make amino acids
  • *Second process:
    1. Atmospheric nitrogen (N2) converted to ammonia (NH3) by nitrogenase (nitrogen fixation)
    2. Ammonia used to make amino acids
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9
Q

how is ammonia integrated into a large number of low molecular weight metabolites?

A

through glutamate and glutamine

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10
Q

what is the predominate form of ammonia at physiological pH?

A

ammonium ion (NH4+) (pka = 9.2)

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11
Q

in most enzyme catalytic centers, what is the reactive species?

A

unprotonated ammonia (NH3) - acts as nucleophile

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12
Q

what is one way to achieve the assimilation of ammonia?

A
  • reductive ammination of alpha-ketoglutarate to glutamate
  • accompanied by the oxidation of NADPH to NADP+
  • carried out by glutamate dehydrogenase
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13
Q

how can glutamate be converted to glutamine after the conversion of alpha-ketoglutarate to glutamate?

A
  • glutamate can be converted to glutamine using a second molecule of ammonia
  • requires ATP hydrolysis to change the side chain
  • catalyzed by glutamine synthetase
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14
Q

what can the amino group of glutamate be used for?

A

-the amino group of glutamate can be transferred to many alpha-keto acids

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15
Q

what are the features of transamination reactions?

A
  • catalyzed by transaminases and amino transferases
  • enzymes require PLP as coenzyme
  • catalyze near-equilibrium reactions
  • the direction in which the reaction proceeds depends on the supply of substrates and removal of products
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16
Q

what is the PLP-enzyme Schiff base called sometimes?

A

an internal aldimine

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17
Q

what does PLP do in order for transamination to occur?

A
  • PLP links to the alpha-amino group of amino acids
  • once the alpha-amino group has transferred to PLP, an alpha-keto acid is released and the amino group remains bound to PLP, forming PMP
  • the next step in transamination is the reverse reaction using a different alpha-keto acid as a substrate
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18
Q

which amino acids are synthesized from 3-phosphoglycerate?

A
  • serine
  • cysteine
  • glycine
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19
Q

which amino acids are created from oxaloacetate?

A
  • aspartate
  • asparagine
  • lysine
  • methionine
  • threonine
  • cysteine
  • isoleucine
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20
Q

which amino acids are synthesized from pyruvate?

A
  • alanine
  • valine
  • leucine
  • isoleucine
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21
Q

which amino acids are synthesized from alpha-ketoglutarate?

A
  • glutamate
  • arginine
  • glutamine
  • proline
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22
Q

which amino acids are synthesized from ribose-5-phosphate?

A
  • histidine
  • phenylalanine
  • tryptophan
  • tyrosine
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23
Q

what can phenylalanine, tryptophan and tyrosine be made from in addition to ribose-5 phosphate/erythrose 4-phosphate?

A

phosphoenolpyruvate

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24
Q

how are aspartate and asparagine formed?

A
  • through transamination reactions starting with oxaloacetate
  • conversion of oxaloacetate into asparate requires input of glutamate and output of alpha-ketoglutarate and enzyme aspartate transaminase
  • conversion of aspartate into asparagine requires input of glutamine and output of glutamate, hydrolysis of ATP, and asparagine synthetase
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25
Q

what are lysine, methionine, and threonine made from?

A
  • aspartate
  • the first step is a phosphorylation followed by a reduction
  • ATP is required
  • NADPH is oxidized
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26
Q

what is the precursor for alanine, valine, leucine, and isoleucine?

A

pyruvate

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27
Q

what combines to form isoleucine?

A

alpha-ketobutyrate and pyruvate

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28
Q

what combines to form valine and leucine?

A

pyruvate and pyruvate

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29
Q

how is glutamate formed?

A

from oxaloacetate

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30
Q

what is the intermediate for proline and arginine?

A

glutamate and glutamine?

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31
Q

what combines with serine to form glycine?

A

tetrahydrofolate

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32
Q

what combines with serine to form cysteine?

A

acetyl CoA

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33
Q

in mammals, how is cysteine made?

A

-serine combines with homocysteine

from methionine biosynthetic pathway

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34
Q

what is the key intermediate in the formation of phenylalanine, tyrosine, and tryptophan?

A

chorismate

35
Q

what does tryptophan synthase do?

A
  • involved in the synthesis of tryptophan

- an alpha2beta2 tetramer that channels indole from one subunit to the other

36
Q

how does the synthesis of histidine begin?

A

the synthesis of histidine begins with the condensation of the ribose derivative PRPP and the purine ring of ATP

37
Q

what is the primary role for amino acids?

A

to serve as substrates for protein synthesis

38
Q

what are glutamate and glutamine important players in?

A
  • nitrogen assimilation
  • nitrogen donors in many transamination reactions
  • purine and pyrimidine biosynthesis
39
Q

what is arginine a substrate for?

A

arginine is a substrate in the synthesis of nitric oxide

40
Q

what is nitric oxide?

A
  • a short lived gas molecule
  • signaling molecule in the cardiovascular system
  • couples G protein-linked receptor stimulation in endothelial cells to relaxation of smooth muscle cells in blood vessels
41
Q

how does acetylcholine affect nitrous oxide?

A

causes the release of NO in vascular endothelial cells that causes the relaxation of the vascular smooth muscle (vasodilator)

42
Q

what is the process of vasodilation?

A
  • binding of acetylcholine to G protein receptors causes InsP3 production
  • Insp3 releases calcium ions from endoplasmic reticulum
  • calcium ions and calmodulin form complex which stimulates NO synthase to produce NO
  • NO diffuses from endothelial cell into adjacent smooth muscles
  • in smooth muscle cell, NO activates guanylyl cyclase to make cyclic GMP
  • cGMP activates protein kinase G which phosphorylates several muscle proteins to induce muscle relaxation
43
Q

what is ligin made from?

A

phenylalanine

44
Q

what is melanin made from?

A

tyrosine

45
Q

what does rapid protein turnover ensure?

A

ensures that regulatory proteins are degraded so the cell can respond to constantly changing conditions

46
Q

how are proteins degraded?

A

lysosomal hydrolysis or targeted degradation

47
Q

what is involved in lysosomal hydrolysis?

A
  • vesicles containing material to be destroyed fuse with lysosomes
  • lysosomal proteases hydrolyze the engulfed proteins
48
Q

what is involved in the targeted degradation of proteins?

A
  • lysine residues in the protein are covalently linked to ubiquitin
  • the proteasome then hydrolyzes the protein to peptides
49
Q

what is the fate of amino acids obtained from the degradation of proteins or diet?

A
  • can be used in the biosynthesis of new proteins

- catabolized in order to make use of their nitrogen and their carbon skeletons

50
Q

what are the steps of amino acid catabolism?

A
  • the first step in the process is the removal of the alpha-amino group
  • the carbon chain is altered to allow for entry into the central pathways of metabolism
51
Q

what happens to the carbon skeletons of amino acids?

A

converted to pyruvate, acetoacetate, acetyl CoA or citric acid cycle intermediates

52
Q

which amino acids can be used to make pyruvate?

A
  • threonine & serine –> glycine –> pyruvate

- tryptophan –> alanine & cysteine –> pyruvate

53
Q

which amino acids can be used to make oxaloacetate?

A

asparagine –> aspartate

54
Q

which amino acids can be used to make fumarate?

A

phenylalanine –> tyrosine & aspartate –> fumarate

55
Q

which amino acids can be used to make succinyl CoA?

A
  • isoleucine
  • methionine
  • valine
  • threonine
56
Q

which amino acids can be used to make alpha-ketoglutarate?

A

arginine, glutamine, histidine, & proline –> glutamate —-> alpha-ketoglutarate

57
Q

which amino acids can be used to make acetyl CoA?

A

-phenylalanine –> tyrosine, isoleucine, leucine, lysine, tryptophan, threonine –> acetyl CoA

58
Q

which amino acids can be used to make acetoacetate?

A

-phenylalanine –> tyrosine, leucine, lysine, tryptophan —> acetoacetate

59
Q

what is alanine converted to?

A

pyrvuate

60
Q

what is aspartate converted to?

A

oxaloacetate

61
Q

what is glutamate converted to?

A

alpha-ketoglutarate

62
Q

what is glutamine hydrolyzed to?

A

glutamate

63
Q

what is asparagine hydrolyzed to?

A

aspartate

64
Q

what are arginine, histidine, and proline degraded to?

A

glutamate

65
Q

what are the two pahways for breaking down serine?

A
  • conversion to pyruvate (minor)

- conversion to glycine (major)

66
Q

how is glycine degraded?

A
  • degraded by the glycine cleavage system to NH4+ and HCO3-

- lipoamide swinging arm

67
Q

what is the major route of cleavage of threonine?

A

-breaks down threonine into acetyl CoA and glycine

68
Q

what is the minor route of cleavage of threonine?

A

-break down threonine into acetylaldehyde and glycine

AND

-breaks down threonine into alpha-ketoglutarate and then propionyl CoA

69
Q

what are the steps of leucine, isoleucine, and valine catabolism?

A
  • transamination
  • oxidative decarboxylation
  • dehydrogenation
  • leucine –> acetyl CoA & acetoacetate –> acetyl CoA
  • valine –> propionyl CoA –> succinyl CoA
  • isoleucine –> acetyl CoA & propionyl CoA –> succinyl CoA
70
Q

what is one major role of methionine?

A

the conversion to the activated methyl donor: S-adenosylmethionine (SAM)

71
Q

how does methionine catabolism work?

A

-S-adenosylmethionine converted to
methyl group transfer leaves S-adenosylhomocysteine, which is degraded to adenosine and homocysteine
-homocysteine can either be methylated back to methionine or react with serine to eventually form cysteine and propionyl CoA

72
Q

how is cysteine degraded?

A

cysteine is degraded to pyrvuate by first being oxidized, followed by transamination and desulfurylation

73
Q

what is the common pattern of catabolism for phenylalanine, tryptophan, and tyrosine?

A
  • oxidation
  • transamination or hydrolysis
  • ring opening coupled with oxidation
74
Q

how is lysine degraded?

A

lysine is degraded by condensation with alpha-ketoglutarate, ultimately producing acetyl CoA

75
Q

where is urea produced?

A

in the liver through the urea cycle

76
Q

how is urea processed?

A

conversion of ammonia to urea, which is carried in the blood to the kidneys and excreted as the major solute in urine

77
Q

how is carbamoyl phosphate made?

A
  • made from bicarbonate through two phosphoryl group transfers
  • requires 2 ATP
78
Q

what are the steps of the urea cycle?

A
  1. carbamoyl phosphate reacts with ornithine, incorporating nitrogen from ammonia into citrulline
  2. the second nitrogen comes from aspartate, which condenses with citrulline to form argininosuccinate in the cytosol
  3. argininosuccinate is cleaved non-hydrolytically to form arginine and fumarate - arginine is the immediate precursor of urea
  4. the guanidinium group of arginine is cleaved to form ornithine and urea - the ornithine is transported into the mitochondria where it re-enters the cycle
79
Q

which step of the urea cycle is regulated?

A

carbamoyl phosphate synthetase 1

80
Q

do the reactions of the urea cycle convert equal amounts of nitrogen from ammonia and from aspartate into urea?

A

YES - this means the concentrations of ammonia and aspartate must be close to equal for efficient synthesis of urea

81
Q

what are the conversions of ammonia and aspartate to urea catalyzed by?

A
  • glutamate dehydrogenase

- aspartate transaminase

82
Q

where are some amino acids deaminated besides the liver?

A

muscle

83
Q

what is the glucose-alanine cycle?

A
  • describes the exchange of glucose and alanine between muscle and liver
  • provides an indirect means for muscle to elimate nitrogen and replenish its energy supply
  • pyruvate can be converted to alanine through the input of alpha-amino acid and output of alpha-keto acid
  • alanine is then shuttled back to liver and NH3 is kicked off to produce urea
84
Q

how is bicarbonate replenished?

A

by glutamate catabolism in the kidneys

Biochemistry (9 decks)

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