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Molecular and Cellular Princples of Medicine > Unit 6 - Amino Acid Degradation and the Urea Cycle > Flashcards

Unit 6 - Amino Acid Degradation and the Urea Cycle Flashcards

1
Q

what are the most abundant AA in serum? in cells?

A

serum: alanine
cells: glutamate and glutamine

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2
Q

when is the AA gradient highest? (as in, what is most important for maintaining gradient?)

A

glutamate and glutamine (both are collecting sites for ammonia groups)

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3
Q

in a free AA pool of 100 g, how much are glutamate and glutamine, and how much are essential?

A

50% are glutamate and glutamine

10% are essential

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4
Q

what are essential AA? (PVT. TIM HALL)

A 
phenylalamine
valine
threonine
tryptophan
isoleucine
methionine
histidine
argininte (for growth)
lysine
leucine
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5
Q

what 2 AA get essential when other AA are missing?

A

cystine essential if methionine low

tyrosine essential if phenylalanine low

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6
Q

what is endogenous PRO associated with? how much is there?

A

skeleton and other supporting tissues, mostly collagen

-usually 6-7 kg

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7
Q

what is intracellular PRO like?

A

dynamic state with continual turnover

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8
Q

what are GI inputs to AA pool?

A

70-100 g dietary PRO

32-200 g endogenous PRO from sloshed off intestinal cells

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9
Q

how many AA transport systems are there? what are the types? where else are there?

A

5 separate transporters, with overlapping specificity (high redundancy)

  • either Na+ or H+ symporters
  • same transporters are in the kidney
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10
Q

what is Hartnup’s disease? symptoms? treatment? diagnosis?

A

defect in transport system for neutral and aromatic AA from gut and renal tubules (esp. phe and trp)

  • have pellegra symptoms, so treat with niacin
  • will have high levels of neutral and aromatic AA in urine and feces
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11
Q

what is cystinuria? symptoms? treatment? diagnosis?

A

defect in transport system for basic AA and cystine from gut and renal tubules

  • since cys is insoluble, it forms crystals to cause UTI and kidney stones
  • treat with fluids and penicillamine (makes cys soluble)
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12
Q

in a 70 kg man, how much PRO is turned over a day, and how many undergo oxidative degredation?

A

400 g turnover

50 g oxidative degredation and replaced

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13
Q

how can protein breakdown be measured?

A

histidine residues of muscle PRO actomyosin are methylated posttranslationally
-when broken down, 3-methyl histidine is liverated and excreted into urine to be measured

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14
Q

when does catabolism of AA take place and what does this mean?

A

metabolic breakdown of AA to urea and CO2 is continuous drain that is reduced during starvation, but never turned fof

  • thus needs daily intake of A to replenish pools
  • -40-50 g/day
  • 12-20 g/day of N2 is excreted, mostly from urea
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15
Q

positive VS negative N2 balance

A

+ N2 losses are less than intake (growing children, recovering adults)
- N2 losses are more than intake (wasting, starvation); if prolonged, it is fatal if loss reaches 1/3rd total body PRO

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16
Q

what AA are plant PRO usually deficient in?

A

lysine
methionine
tryptophan

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17
Q

what are the 3 ways ammonia is removed?

A
  1. glutamate dehydrogenase
    - NH4+ + NADPH + H+ + a-KG glu + NADP+ + H2O
  2. glutamine synthase
    - glu + ATP + NH4 gln + ADP + Pi
  3. carbamoyl phosphate synthase I and II
    - NH4+ + CO2 + 2 ATP –> carbamoyl phosphate + 2 ADP + Pi
    - -I is in mitochondria for urea cycle
    - -II is in cytoplasm for pyrimidine nt biosynthesis (doesn’t use NH4+)
18
Q

what is the major carrier of NH4+ groups in the blood?

A

glutamine

19
Q

what is the cofactor for all transaminations?

A

pyridoxal phosphate (vit B6)

20
Q

in what reactions does pyridoxine function as a cofactor?

A
  • transaminations (all of them)
  • decarboxylations
  • dehydrations of beta-hydroxyamino acids
  • racemizations of alpha-AA
  • removal of H2S from cysteine
21
Q

what are the free aldehyde VS aminated forms of B6?

A

pyridoxal phosphate = free aldehyde form
pyridoxamine phosphate = aminated form

these are easily interconvertable, and all transaminases have common enzyme mechanism with pyridoxal phosphate

22
Q

what 12 AA can be acted upon by pyridoxal phosphate transaminases?

A

remove alpha-AA groups

  • ala
  • arg
  • asp
  • asn
  • cys
  • ile
  • leu
  • lys
  • phe
  • tryp
  • tyr
  • val
23
Q

what AA are acted upon by asp. transaminase?

A

asp
ala
leu
tyr

24
Q

what AA can easily be made from pyruvate?

A

alanine (ala + a-KG pyruvate + glu)

  • efficient way to send 3 C back into liver to be converted into glucose, while disposing of NH4+ group (from glutamate)
  • so ala is one of most abundant AA in blood
25
Q

what AA do most transaminases converge on?

A

glutamate

  • represents convergence of pathways and method of collecting amino groups in one AA
  • glu then serves as specific donor of amino groups for production of nitrogenous waste products
  • this is why glu is one of most abundant AA in cells
26
Q

where is glutamate dehydrogenase located and why? cofactors? allosteric regulators?

A

in mitochondria (where a-KG is, and NADH is quickly used for E)

  • use either NADH or NADPH (normally high ratios of NADPH : NADP, and low NADH : NAD, so always cofactor available in either direction
  • activated by A/GDP, inhibited by A/GTP
  • -so lowering E charge of cell accelerates AA oxidation
27
Q

where is glutamate mainly deaminated?

A

in the liver

  • if it needs energy, it makes NADH for e- transport
  • if it has E, it will make NADPH for biosynthetic reaction
28
Q

what are AA oxidases?

A

both L and D occur in kidneys and liver

-enzymes use tightly bound flavins as cofactors to convert AA and water into corresponding a-keto acid and NH4+

29
Q

what do dehydratases do? how does this work with serine? what do serine and threonine dehydratase use as cofactors?

A

directly deaminate

  • ser –> dehydrate, then hydrate –> pyruvate + NH3
  • both use pyridoxal phosphate cofactor
30
Q

what does homocysteine desulfhydrase do? what does it use as a cofactor? what happens to the product

A

HC + H2O –> –> H2S + NH3 + alpha-ketobutyrate

  • pyridoxal phosphate cofactor used to remove both NH4 + S
  • a-kb –> PCoA –> mmCoA –B12–> SCoA
31
Q

why does the body convert free NH4+ to urea?

A
  • constant excretion of NH4+ in urine would cause drastic changes in blood pH
  • urea is readily soluble (over 10 M) and easily excreted
  • -carries 2 ammonia groups
  • -non protonatable (10% solution pH 7.2)
  • -low reactivity (inert)
32
Q

what is the stoichiometry for one turn of the urea cycle?

A

CO2 + NH4+ + 3 ATP + asp + 2 H2O urea + 2 ATP + 1 AMP + PPi + fumarate + 2 Pi

33
Q

what urea cycle enzymes are in the mitochondria VS cytosol?

A

mitochondria: carbamoylphosphate synthetase, ornithine transcarbamoylase (steps 1 + 2)
cytosol: argininosuccinate synthetase and lyase, arginase (steps 3 - 5)

34
Q

how is urea cycle regulated? (PRO-free diet VS high-PRO diet)

A

PRO-free: urea excretion accounts for only 60% of total urinary nitrogen, compared to 80% of normal diet
-levels of urea cycle enzymes decrease

high-PRO: (also in starvation) gluconeogenesis from AA is high, so levels of urea cycle enzymes increase several fold (eventually subside if starving, b/c will use fat stores)

35
Q

what is the most common genetic defect regarding the urea cycle?

A

OTC (ornithine transcarbamoylase) deficiency, b/c X-linked

36
Q

for argininosuccinic aciduria…

  • sign
  • blood
  • urine
  • intelligence
  • enzyme defect
A
  • ammonia intoxication
  • NH4 and argininosuccinic acid
  • arginosuccinic acid
  • normal
  • argininosuccinase
37
Q

for hyperammoniemia…

  • sign
  • blood
  • urine
  • intelligence
  • enzyme defect
A
  • ammonia intoxication
  • high NH4+
  • N/A
  • normal intelligence
  • carbamoyl phosphate synthase
38
Q

for arginiemia…

  • sign
  • blood
  • urine
  • intelligence
  • enzyme defect
A
  • ammonia intoxication
  • high NH4 and arginine
  • arg, lys, orn
  • normal
  • arginase
39
Q

for citrullinemia…

  • sign
  • blood
  • urine
  • intelligence
  • enzyme defect
A
  • ammonia intoxication
  • high NH4+ and citrulline
  • citrulline
  • mental retardation
  • arginosuccinate synthetase
40
Q

for hyper-ornithinemia…

  • sign
  • blood
  • urine
  • intelligence
  • enzyme defect
A
  • ammonia intoxication
  • high NH4+, orotate, and ornithine
  • ornithine
  • normal
  • ornithine transcarbamylase
41
Q

how is one treated if one has an inborn error of urea cycle metabolism?

A

low PRO diet supplemented with arginine or citrulline, since arginine becomes essential

  • also give sodium benzoate and sodium phenylacetate to bind up gly and gln to be excreted
  • ammonia levels are lowered to make more nonessential AA

Molecular and Cellular Princples of Medicine (42 decks)

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