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USMLE Biochemistry > Ammonia > Flashcards

Ammonia Flashcards

1
Q

What does the removal of an amino group of amino acid create?

A
  • NH3

- a-keto acid

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2
Q

Can amino acids be kept in a storage form?

A

No

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3
Q

What is the first step of amino acid breakdown?

A

Removal of nitrogen by transamination

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4
Q

What is ammonia converted to in the liver?

A

Urea

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5
Q

What is the amino group passed to in the first step of amino acid breakdown?

A

a-ketoglutarate

- Creating glutamate

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6
Q

What does the amino acid become when an amino group is removed?

A

a-keto acid

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7
Q

What is the function of aminotransferases?

A

Transfer nitrogen from amino acids to glutamate

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8
Q

What do aminotransferases require as a cofactor?

A

B6

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9
Q

Give to examples of aminotransferases which may also be used as liver function tests?

A
  • ALT - Alanine aminotransferase

- Aspartate aminotransferase (AST)

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10
Q

What does AST take an amino acid from and what does it give it to to yield?

A

AST takes an amino group from aspartate and transfers it to a-ketoglutarate to create Glutamate

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11
Q

What does aspartate become after it has been deaminated by AST?

A

Oxaloacetate

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12
Q

What are the 2 methods by which nitrogen can be transferred from glutamate to liver for excretion in urea cycle?

A
  1. Glutamine synthesis

2. Alanine cycle

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13
Q

What amino acid transfers Nitrogen to the liver for excretion?

A

Glutamine

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14
Q

What enzyme converts glutamate to glutamine?

A

Glutamate synthetase

- Adds another N from ammonia

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15
Q

What enzyme converts Glutamine to Glutamate through the removal of an N

A

Glutaminase

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16
Q

What does Glutamate Dehydrogenase convert Glutamate into?

- Removes another NH4+

A

a-ketoglutarate

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17
Q

Where is glutaminase found?

A

Liver

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18
Q

What alternate pathway is used by muscles to transfer nitrogen to the liver?

A

Alanine Cycle

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19
Q

What does ALT do in the Alanine cycle?

A

Passes NH2 from Glutamate to Pyruvate
- Creating Alanine

Also the reverse reaction

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20
Q

What is the structure of Alanine?

A

Pyruvate with an amino group

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21
Q

What is the purpose of alanine?

A

Can shuttle N to liver from muscle

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22
Q

a-ketoglutarate + NH2 will yield what amino acid?

  • Catalyzed by ALT
A

Glutamate

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23
Q

What does Glutamate dehydrogenase in the liver do? (as part of the of alanine cycle)

A

Converts Glutamate into a-ketoglutarate and NH4+

24
Q

What will mitochondrial disorders (e.g Pyruvate carboxylase and dehydrogenase deficiency) result in elevated levels of?

A
  • Alanine

- Lactate

25
What is Urea synthesized from?
- NH4+ - CO2 - Aspartate
26
What enzyme is involved in the rate-limiting step of the urea cycle?
Carbamoyl Phosphate Synthetase I
27
Give the chemical equation for the rate limiting step of the urea cycle (Carbamoyl Phosphate Synthetase I)
NH4+ + CO2 -> Carbamoyl Phosphate 2 ATP -> 2ADP
28
What is the allosteric activator of Carbamoyl Phosphate Synthetase I?
N-acetylglutamate | - Used to regulate urea cycle
29
What is N-acetylglutamate (allosteric activator of Carbamoyl Phosphate Synthetase I) synthesized from?
- Glutamate | - Acetyl CoA
30
What is N-acetylglutamate (allosteric activator of Carbamoyl Phosphate Synthetase I) increased by?
Increased protein (fed state)
31
What is Carbamoyl Phosphate Synthetase II involved in?
Pyrimidine Synthesis | - Glutamate -> Carbamoyl Phosphate
32
What is the second reaction of the Urea cycle? | What enzyme creates Citrulline?
Ornithine Transcarbamylase
33
What is Citrulline created from?
Carbamoyl Phospahte + Ornithine - Ornithine Transcarbamylase
34
The first 2 reactions of the Urea cycle occur where?
Mitochondria
35
Where is Ornithine created?
Cytosol
36
Where is Citrulline created? (and where does it go)
Mitochondria | - Goes to cytosol
37
What is Citrulline converted to in the Urea cycle?
Argininosuccinate (+ ATP + Aspartate)
38
What is Argininosuccinate converted into in the urea cycle?
Arginine (then converted to urea) | + Fumarate
39
What makes Citrulline a non-standard amino acid?
- Not encoded by the genome - Incorporated into proteins via post-translational modification - More incorporation in inflammation
40
What are anti-citrulline antibodies found in?
Rheumatoid arthritis
41
Anti-Cyclic Citrullinated peptide antiBs (anti-CCP) are found in what patients?
Up to 80% of patients with RA
42
What molecule is depleted in hyperammonemia? | What other cycle does this disrupt
alpha-ketoglutarate (TCA cycle)
43
Hyperammonemia results in what in the CNS?
- Cerebral edema - Tremor (asterixis) - Memory impairment - Slurred speech - Vomiting - Can progress to coma
44
How is hyperammonemia treated?
- Low protein diet | - Lactulose
45
How does lactulose treat hyperammonemia?
- Synthetic disaccharide (laxative) - Colon breakdown by bacteria to fatty acids - Lowers colonic pH; favors formation of NH4+ over NH3 - NH4+ not absorbed -> trapped in colon - Decreasing ammonia plasma conc.
46
What treatments are used for hyperammonemia in enzyme deficiencies?
Ammonium detoxicants - Na+ phenylbutyrate (oral) - Na+ phenylacetate-Na+ benzoate (IV) - Conjugate with glutamine - Excreted in urine -> removal of nitrogen / ammonia Arginine supplementation - Urea cycle disorders make arginine essential
47
What is the most common urea cycle disorder?
OTC deficiency
48
How is OTC deficiency inherited?
X-linked recessive
49
What substances are increased in conc. in OTC deficiency?
- Carbamoyl phosphate - Ammonia - Orotic acid (derived from carbamoyl phosphate)
50
When do OTC deficiencies present?
Infancy or childhood (depending on severity)
51
What are common symptoms of OTC deficiency?
- Somnolence, poor feeding - Seizures - Vomiting - Lethargy - Coma
52
What can ditinguish OTC deficiency from orotic aciduria?
Only OTC has increased ammonia levels
53
What enzyme is deficient in orotic aciduria?
Uridine monophosphate synthase (UMPS)
54
What is citrullinemia a deficiency of (enzyme)?
Argininosuccinate synthase
55
What are the levels of different substances in citrullinemia?
- Elevated citrulline - Low arginine - Hyperammonemia
56
Urea cycle disorders generally present with what?
Hyperammonemia | - Build up of urea cycle intermediates
57
How are urea cycle disorders inherited?
Aut recessive | - Except OTC deficiency

USMLE Biochemistry (44 decks)

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