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USMLE Biochemistry > Ammonia > Flashcards

Ammonia Flashcards

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USMLE® Step 1 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
Q

What is Urea synthesized from?

A
  • NH4+
  • CO2
  • Aspartate
26
Q

What enzyme is involved in the rate-limiting step of the urea cycle?

A

Carbamoyl Phosphate Synthetase I

27
Q

Give the chemical equation for the rate limiting step of the urea cycle (Carbamoyl Phosphate Synthetase I)

A

NH4+ + CO2
-> Carbamoyl Phosphate

2 ATP -> 2ADP

28
Q

What is the allosteric activator of Carbamoyl Phosphate Synthetase I?

A

N-acetylglutamate

- Used to regulate urea cycle

29
Q

What is N-acetylglutamate (allosteric activator of Carbamoyl Phosphate Synthetase I) synthesized from?

A
  • Glutamate

- Acetyl CoA

30
Q

What is N-acetylglutamate (allosteric activator of Carbamoyl Phosphate Synthetase I) increased by?

A

Increased protein (fed state)

31
Q

What is Carbamoyl Phosphate Synthetase II involved in?

A

Pyrimidine Synthesis

- Glutamate -> Carbamoyl Phosphate

32
Q

What is the second reaction of the Urea cycle?

What enzyme creates Citrulline?

A

Ornithine Transcarbamylase

33
Q

What is Citrulline created from?

A

Carbamoyl Phospahte + Ornithine

  • Ornithine Transcarbamylase
34
Q

The first 2 reactions of the Urea cycle occur where?

A

Mitochondria

35
Q

Where is Ornithine created?

A

Cytosol

36
Q

Where is Citrulline created? (and where does it go)

A

Mitochondria

- Goes to cytosol

37
Q

What is Citrulline converted to in the Urea cycle?

A

Argininosuccinate (+ ATP + Aspartate)

38
Q

What is Argininosuccinate converted into in the urea cycle?

A

Arginine (then converted to urea)

+ Fumarate

39
Q

What makes Citrulline a non-standard amino acid?

A
  • Not encoded by the genome
  • Incorporated into proteins via post-translational modification
  • More incorporation in inflammation
40
Q

What are anti-citrulline antibodies found in?

A

Rheumatoid arthritis

41
Q

Anti-Cyclic Citrullinated peptide antiBs (anti-CCP) are found in what patients?

A

Up to 80% of patients with RA

42
Q

What molecule is depleted in hyperammonemia?

What other cycle does this disrupt

A

alpha-ketoglutarate (TCA cycle)

43
Q

Hyperammonemia results in what in the CNS?

A
  • Cerebral edema
  • Tremor (asterixis)
  • Memory impairment
  • Slurred speech
  • Vomiting
  • Can progress to coma
44
Q

How is hyperammonemia treated?

A
  • Low protein diet

- Lactulose

45
Q

How does lactulose treat hyperammonemia?

A
  • 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
Q

What treatments are used for hyperammonemia in enzyme deficiencies?

A

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
Q

What is the most common urea cycle disorder?

A

OTC deficiency

48
Q

How is OTC deficiency inherited?

A

X-linked recessive

49
Q

What substances are increased in conc. in OTC deficiency?

A
  • Carbamoyl phosphate
  • Ammonia
  • Orotic acid (derived from carbamoyl phosphate)
50
Q

When do OTC deficiencies present?

A

Infancy or childhood (depending on severity)

51
Q

What are common symptoms of OTC deficiency?

A
  • Somnolence, poor feeding
  • Seizures
  • Vomiting
  • Lethargy
  • Coma
52
Q

What can ditinguish OTC deficiency from orotic aciduria?

A

Only OTC has increased ammonia levels

53
Q

What enzyme is deficient in orotic aciduria?

A

Uridine monophosphate synthase (UMPS)

54
Q

What is citrullinemia a deficiency of (enzyme)?

A

Argininosuccinate synthase

55
Q

What are the levels of different substances in citrullinemia?

A
  • Elevated citrulline
  • Low arginine
  • Hyperammonemia
56
Q

Urea cycle disorders generally present with what?

A

Hyperammonemia

- Build up of urea cycle intermediates

57
Q

How are urea cycle disorders inherited?

A

Aut recessive

- Except OTC deficiency

USMLE Biochemistry (44 decks)

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