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Biochem review USMLE > amino acid metabolism > Flashcards

amino acid metabolism Flashcards

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USMLE® Step 1 flashcards
1
Q

essential amino acids

A 
Phenylalanine
Valine
Threonine
Tryptophan
Isoleucine
Methionine
Histidine
Leucine
lysine
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2
Q

mnemonic for essential amino acids

A

PVT TIM HaLL

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3
Q

fate of excess nitrogen from amino acid catabolism

A

converted to urea that is then excreted by the kidneys

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4
Q

rate limiting enzyme in urea cycle

A

CPS I

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5
Q

2 enzymes in the urea cycle that work inside the mitochondria

A

CPS I

ornithine transcarbamoylase

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6
Q

site of urea cycle

A

liver

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7
Q

in what molecule is NH3 transported in the blood?

A

alanine

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8
Q

what occurs in muscle cells to produce alanine? (2 steps)

A

NH3+ a-ketoglutarate = glutamate(NH3)

glutamate(NH3)+pyruvate = alanine (NH3) (by ALT)

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9
Q

what happens to alanine after it reaches the liver?

A

alanine (NH3) + a-ketoglutarate = glutamate (NH3) –> enters urea cycle

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10
Q

acquired cause of hyperammonemia

genetic cause of hyperammonemia

A

genetic- urea cycle enzyme deficiencies

acquired- liver disease

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11
Q

why is hyper ammonia harmful?

A

excess NH4 leads to increased glutamate production (which depletes a-ketoglutarate, thereby stopping the TCA cycle)

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12
Q

treatments of hyperammonemia (4)

A

limit protein in diet
benzoate/phenylbutyrate- bind amino acid/excretion
lactulose- acidifies GI tract to trap NH4+ for excretion
biotin- stimulates OTC

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13
Q

inheritance of ornithine transcarbamoylase deficiency

A

X-linked recessive

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14
Q

consequences of ornithine transcarbamoylase deficiency

A

unable to convert carbamoyl phosphate + ornithine –> citrulline, leads to accumulation of orotic acid

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15
Q

symptoms of ornithine transcarbamoylase deficiency

A

increased orotic acid in blood
decreased BUN
increased ammonia

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16
Q

difference between presentation of ornithine transcarbamoylase deficiency and orotic aciduria

A

ornithine transcarbamoylase deficiency - decreased BUN, increased ammonia
orotic aciduria- no increased in ammonia

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17
Q

tryptophan derivatives (2)

A

Niacin –> NAD+/NADP+ using B6

serotonin –> melanin using BH4

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18
Q

histidine derivative (1)

A

histamine using B6

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19
Q

glycine derivative (1)

A

glycine + succinyl-CoA –> dALA by ALA synthase in heme synthesis (B6 cofactor)

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20
Q

arginine derivatives (3)

A

urea
NO
Creatine

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21
Q

glutamate derivatives (2)

A

GABA (with B6)

glutathione

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22
Q

starting amino acid in catecholamine synthesis

A

phenylalanine

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23
Q

enzyme/cofactor responsible for conversion of phenylalanine –> tyrosine

A

phenylalanine hydroxylase

BH4

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24
Q

enzyme/cofactor responsible for conversion of tyrosine –> dopa

A

tyrosine hydroxylase

BH4

25
Q

enzyme/cofactor responsible for conversion of dopa –> dopamine

A

dopa decarboxylase

B6

26
Q

inhibitor of dopa decarboxylase

A

carbidopa (decreases peripheral conversion)

27
Q

what vitamin should be avoided in pts with PD on levodopa?

A

B6- will increase peripheral conversion of dopa to dopamine = decreased therapeutic benefit and increased SE

28
Q

enzyme/cofactor responsible for conversion of dopamine –> norepinephrine

A

dopamine b-hydroxylase

vitamin c

29
Q

enzyme/cofactor responsible for conversion of norepinephrine, site

A

PNMT
SAM
only in adrenal medulla

30
Q

urine byproduct of dopamine

A

HVA

31
Q

urine byproduct of NE

A

VMA

32
Q

urine byproduct of epinephrine

A

metanephrine, HVA

33
Q

urine byproduct of serotonin

A

5-HIAA

34
Q

stimulates the conversion of norepinephrine to epinephrine

A

cortisol

35
Q

2 deficiencies that can lead to phenylketonuria

A

1) phenylalanine hydroxylase

2) BH4

36
Q

amino acid that becomes essential in PKU

A

tyrosine

37
Q

3 substances decreased in PKU

A

melanin
catecholamines
thyroxine

38
Q

presentation of PKU (6)

A

musty body odor, mental retardation, growth retardation, fair skin, eczema, seizures

39
Q

treatment

A

decrease Phe and increase tyrosine in diet

40
Q

what does screening for PKU detect? when done?

A

detects presence of phenylketones, done 2-3 days after birth, once effects of maternal phenylalanine hydroxylase has been diminished

41
Q

what is maternal PKU?

A

occurs when mothers with PKU do not follow appropriate diet during pregnancy, fetus = microcephaly, congenital heart defects, metal/growth retardation

42
Q

cause of alkaptonuria

A

deficiency of homogentisic acid oxidase (involved in conversion of tyrosine –> fumarate)

43
Q

symptoms of alkaptonuria

A

accumulation of homogentisic acid leads to black urine, brown pigmentation and arthralgias (homogentisic acid is toxic to cartilage)

44
Q

3 causes of albanism

A

1) failure of neural crest cell migration
2) tyrosinase def (=decreased melanin synthesis)
3) defective tyrosine transporters

45
Q

risk assoc with albanism

A

increased skin CA

46
Q

albanism exhibits what inheritance phenomenon

A

locus heterogenity- different defects on different genes produce the same phenotype

47
Q

what is ocular albanism?

A

X-linked recessive, only affects eyes

48
Q

2 fates of homocysteine

A

cystathionine (cystathionine synthase, B6)

methionine (homocysteine methyltransferase, B12)

49
Q

3 causes of homocysteinuria

A
  • cystathionine synthase deficiency
  • decreased affinity of cystathionine synthase for B6
  • homocysteine methyltransferase deficiency
50
Q

symptoms of homocystinuria

A

increased homocysteine in urine
marfanoid- tall, kyphosis, lens subluxation
acclerated atherosclerosis (CVA, MI)

51
Q

treatment of cystathionine synthase deficiency

A

increase cysteine in diet
decrease methionine in diet
increase B12 in diet

52
Q

treatment of decreased affinity of cystathionine synthase for B6

A

increase B6

53
Q

cause/effect of cysteinuria

A

defect in renal tubular amino acid transporter of the PCT= excess excretion of cysteine, ornithine, lysine, arginine

54
Q

symptoms of cysteinuria

A

cystine stones (hexagonal)

55
Q

treatment of cysteinuria

A

alkalinization of urine, acetazolamide

56
Q

defect of maple syrup urine disease

A

decreased a-ketoacid dehydrogenase = block degradation of branched chain amino acids

57
Q

amino acids affected in maple syrup urine disease

A

isoleucine
leucine
valine

58
Q

symptoms of maple syrup urine disease

A

maple syrup odor to urine
CNS defects
death

59
Q

what is the cause/effect of Hartnup disease?

A

defective absorption of neutral amino acids int he intestine leads to tryptophan deficiency = decreased Niacin synthesis = pellagra

Biochem review USMLE (11 decks)

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