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GI > Nitrogen Elimination and Carbon Chain Metabolism I > Flashcards

Nitrogen Elimination and Carbon Chain Metabolism I Flashcards

1
Q

The free amino acids that are generated in the gut are mobilized by peptide transporters in intestinal epithelial cells, and delivered via the portal system to the

A

Liver

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2
Q

In the liver, a family of related enzymes removes α-NH3 groups from amino acids and transfers them to an acceptor molecule, typically

A

α-ketoglutarate

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3
Q

Transfer of α-NH3 to α-ketoglutarate results in the formation of

A

Glutamate

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4
Q

While this first step does not alter the overall amount of nitrogen, it does reposition the nitrogen for more efficient elimination during the

A

Second stage

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5
Q

In this second step, glutamate dehydrogenase can release nitrogen (as ammonia), which is then assimilated into

A

Urea

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6
Q

After the removal of the alpha-amino group, carbon chains can be modified and channeled into the

-generates energy

A

TCA cycle

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7
Q

The remnant carbon skeleton can also be stored, following their assimilation into

A

Fat and carbohydrates

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8
Q

The carbon chain of amino acids can be used for generating energy, or stored as fat and carbohydrate after removal of the

A

Alpha-amino group

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9
Q

Rarely coded in gene sequences, and this amino acid is underrepresented in proteins

A

Methionine

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10
Q

Methionine is encoded by the single codon

A

ATG

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11
Q

Degraded to yield amino acids, primarily in the gut, and absorbed by epithelial cells that line the intestine

A

Dietary Proteins

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12
Q

Because the stability of cellular proteins can vary from seconds to years, their degradation is

A

Highly regulated

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13
Q

Required for the uptake of amino acids in the gut, and their transfer to the portal system

-In the intestinal wall

A

Trans-membrane protein channels

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14
Q

Help recover amino acids which are inadvertently released into the urine.

-Mutations in these pumps result in loss of amino acids which can cause amino acid deficiencies

A

Membrane channels in the kidney

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15
Q

There are three primary sites where proteins are synthesized. These locations are termed

A

Gastric, pancreatic, and intestinal

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16
Q

The function of the mouth/saliva is to masticate food so that it is easily transferred to the stomach. This initiates the first of three stages of

A

Proteolysis

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17
Q

Generally named after critical residues in their active site

A

Proteases

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18
Q

Proteases can also be named after a

A

Cofactor, substrate, or location

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19
Q

The extreme acidity of the stomach begins protein unfolding and

A

Denaturation

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20
Q

Digestive proteases are synthesized as inactive precursors, called

A

Zymogens

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21
Q

The zymogen is catalytically inactivated by an inhibitory

-when removed, enzyme is activated

A

Prodomain

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22
Q

Synthesized in intestinal mucosal cells, and its key function is to convert trypsinogen to active trypsin

A

Enteropeptidase

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23
Q

Trypsin then activates the rest of the enzymes. This mechanism of autocatalytic protease activation is activated by hormonal signaling, and occurs primarily in the

A

Intestinal tract

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24
Q

Secrete dipeptidases that can generate free amino acids

A

Intestinal cells

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25
Q

Amino acid and dipeptide transporters take up amino acids/peptides into the intestinal cells, where we see complete

A

Peptide hydrolysis

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26
Q

Free amino acids are transferred to the liver by the

A

Portal vein

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27
Q

Proteases in the gut. These non-specific and potent enzymes are classified as

A

Endo- and exopeptidases

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28
Q

These proteases are indiscriminate, because they degrade all, and not specific proteins

A

Endo- and exopeptidases

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29
Q

May be either amino- or carboxypeptidases

A

Exopeptidases

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30
Q

A serineprotease that cleaves after basic residues (lysine and arginine).

-The hydroxyl group in serine contributes to the catalytic properties of this endopeptidase.

A

Trypsin

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31
Q

The first step in amino acid breakdown involves the removal of the

A

Alpha amino group

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32
Q

A family of aminotransferase enzymes transfers α-NH3 from most amino acids to

A

α-ketoglutarate

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33
Q

Although this mechanism does not alter the balance in nitrogen (because net nitrogen level is unchanged), it provides an efficient way to concentrate all the

A

α-NH3 molecules on one or a few molecules

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34
Q

The primary acceptor for NH3 in the liver, and most other tissues

A

α-ketoglutarate

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35
Q

However, is generated and is to the liver in muscle cells (which produce abundant pyruvate)

A

Alanine (alanine = pyruvate + NH3)

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36
Q

Are detected in the blood following a high protein meal because they provide a safe way to transfer excess NH3 to the liver for the synthesis of urea, and excretion in the urine

A

Alanine, Glutamine, and Asparagine

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37
Q

Not transported in its free form because it is highly toxic to the central nervous system

A

Ammonia

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38
Q

Can be released from glutamate via oxidative deamination to produce ammonia

A

NH3

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39
Q

Oxidative deamination is mediated by

-present in all tissues

A

Glutamate dehydrogenase

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40
Q

Alternatively, α-ketoglutarate can enter the

A

TCA cycle

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41
Q

The amino group can be transferred from glutamate to oxaloacetate by

A

Aspartate aminotransferase

42
Q

Both ammonia and aspartate provide a nitrogen in the assimilation of

A

Urea in the liver

43
Q

Can add NH3 to glutamate to increase the capture and transport of nitrogen (ammonia) from peripheral tissues to the liver

A

Glutamine Synthase

44
Q

Can add NH3 to aspartate (asparagine)

A

Asparagine synthesis

45
Q

Under conditions of ammonia toxicity, these reactions can rapidly sequester excess

A

NH3

46
Q

The transamination of pyruvate yields

-can be safely transported to the liver

A

Alanine

47
Q

Alanine is converted back to pyruvate by transamination, and can then enter the pathway of

A

Gluconeogenesis

48
Q

The critical enzyme that initiates the net removal of nitrogen from amino acids (specifically glutamate)

A

Glutamate dehydrogenase

49
Q

Glutamate dehydrogenase is regulated by allosteric mechanisms. It is inhibited by

A

ATP/GTP

50
Q

Glutamate dehydrogenase is regulated by allosteric mechanisms. It is activated by

A

ADP/GDP

51
Q

Stimulation of glutamate dehydrogenase when energy levels are low increases the deamination of glutamate, to yield higher amounts of

A

α-ketoglutarate

52
Q

This reaction occurs primarily in the mitochondrion, and requires the cofactor

A

NAD

53
Q

NH3 that is released from glutamate is condensed with carbon dioxide (in the mitochondrion) to form

A

Citrulline

54
Q

Glutamate transfers NH3 (transamination) to

-forms aspartate

A

Oxaloacetate

55
Q

Glutamate transfers NH3 (transamination) to oxaloacetate (forming aspartate), which donates the nitrogen to citrulline, yielding the

A

2nd nitrogen in urea

56
Q

This reaction occurs in the

A

Cytosol of liver cells

57
Q

In addition to α-ketoglutarate, there are other intermediates in the formation of urea that are linked to the TCA cycle. one example is

A

Oxaloacetate

58
Q

Transaminated to aspartate by aspartate aminotransferase

A

Oxaloacetate

59
Q

Arginino-succinate is cleaved to yield

A

Arginine and fumarate

60
Q

The conversion of arginine to ornithine requires the enzyme

-expressed only in the liver

A

Arginase

61
Q

Transamination and oxidative deamination reactions are

A

Reversible

62
Q

The incorporation of ammonia into CO2 by carbamoyl phosphate synthase I requires

A

2 ATPs

63
Q

The conversion of aspartate to argininosuccinate requires

A

1 ATP

64
Q

The addition of NH3 to glutamate/aspartate by glutamine/asparagine synthase requires

A

1 ATP

65
Q

Digestive proteases are synthesized in the stomach, pancreas and intestine, but are active only in an

A

Extracellular location

66
Q

The α-NH3 groups are generated in all tissues, but are assimilated into urea only in the

A

Liver

67
Q

Only the liver expresses which three enzymes?

A

Arginase, glutaminase, and asparaginase

68
Q

The two nitrogen’s in urea are obtained from

A

Ammonia and aspartate

69
Q

Both nitrogens were originally present in

A

Glutamate

70
Q

The addition of nitrogen occurs in which two distinct intracellular regions of the liver cell?

A

Mitochondria and cytosol

71
Q

The carbon is obtained from CO2, which is transported as soluble

A

Bicarbonate

72
Q

Two key metabolites in the urea cycle are

A

Ornithine and citrulline

73
Q

Converts arginine to ornithine + urea, and is present only in the liver

A

Arginase

74
Q

What are the three key steps in nitrogen regulation?

A

Glutamine synthase, Alanine aminotransferase, and glutaminase

75
Q

When there is a sudden increase in the levels of amino acids, the cellular capacity to assimilate nitrogen into urea can become

A

Overwhelmed

76
Q

Since ammonia is highly toxic (due to CNS toxicity), a temporary mechanism functions to sequester excess nitrogen. I.e. NH3 is added to

-serves as a temporary storehouse for excess nitrogen

A

Glutamate (generates glutamine)

77
Q

This process requires

A

ATP

78
Q

Urea is synthesized only in the liver, and several enzymes can release NH3 from compounds that are transported to this organ. What releaes NH3 from glutamine

A

Glutaminase

79
Q

Glutaminase specifically releases NH3 from glutamine, while oxidative deamination of glutamate also yields free

A

NH3 + α-ketoglutarate

80
Q

Glutaminase (unlike glutamate dehydrogenase), is only expressed in the

A

Liver

81
Q

Pyruvate that is released from alanine can be used for the synthesis of glucose, and returned to muscle (and other) tissues for energy production. This is termed the

A

Glucose-Alanine cycle

82
Q

Rapid increase in intracellular ammonia can be temporarily suppressed by the conversion of

A

Glutamate into glutamine

83
Q

Bacteria in the gut, and gastrointestinal bleeding can increase the blood levels of

A

Ammonia

84
Q

Activated to convert α-ketoglutarate to glutamate when ammonia levels rise rapidly

A

Glutamate dehydrogenase

85
Q

However, this results in a proportionate reduction in the level of a key TCA intermediate, which leads to reduced

A

Energy production

86
Q

Reduced function of the kidney can result in elevated levels of urea in the blood. An increased level of urea can then cross the intestinal cell wall and become cleaved by

A

Bacterial urease

87
Q

Bacterial urease releases

A

H2O and ammonia

88
Q

Ammonia can rapidly diffuse back into the blood and cause

A

Hyperammonemia

89
Q

The deamination of glutamine in the liver, by glutaminase, generates glutamate and NH3. NH3 is condensed with CO2 to form

A

Carbonoyl Phosphate

90
Q

NH3 is condensed with CO2 to form carbamoyl phosphate by carbamoyl phosphate synthase I in the

A

Mitochondrial matrix

91
Q

NH3 is condensed with CO2 to form carbamoyl phosphate by carbamoyl phosphate synthase I in the mitochondrial matrix. This represents the first assimilation of

A

Nitrogen into urea

92
Q

The addition of NH3 to oxaloacetate (via glutamate), provides a source for the

-occurs in cytosol of liver cells

A

2nd nitrogen into urea

93
Q

The essential co-factor in all aminotransferases, and participates directly in the removal of alpha amino groups from amino acids

A

Pyridoxil Phosphate

94
Q

Derived from vitamin B6, and forms a Schiff base with an epsilon amino group in a lysine residue in the aminotransferases

A

Pyridoxil

95
Q

Pyridoxil is derived from vitamin B6, and forms a Schiff base with an epsilon amino group in a lysine residue in the

A

Aminotransferase

96
Q

However, when an aminotransferase binds an amino acid, the alpha-amino group in the amino acid forms a transient Schiff base with

A

Pyridoxal phosphate

97
Q

The key enzyme, and the rate-limiting step, in the Urea Cycle

A

Carbamoyl phosphate synthase I

98
Q

This rate-limiting step is positively regulated by

A

N-acetylglutamate

99
Q

The synthesis of N-acetylglutamate is stimulated by the presence of

A

Arginine

100
Q

The initial step in urea synthesis occurs in the mitochondria, and requires

A

2 ATP’s per urea

GI (59 decks)

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