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Foundations 1 > Nitrogen Metabolism > Flashcards

Nitrogen Metabolism Flashcards

1
Q

Nitrogen balance in different populations

A

Normal adult: in = out

Growing child, adolescent, pregnancy: in > out

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2
Q

What sources contribute to and remove from the amino acid pool?

A

In: body protein degradation, dietary protein, synthesis of non-essential AAs

Out: body protein synthesis, catabolism, biosynthesis of non-protein nitrogenous tissue constituents (purines + pyrimidines)

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3
Q

What is the role of glutamate in urea production?

A
  • Collects N from other AAs via transamination reactions

- Oxidative deamination of glu => NH4+ => urea cycle

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4
Q

Transamination reaction

A
  • AA + a-ketoglutarate => a-keto acid + glu
  • requires coenzyme pyridoxal phosphate (PLP or B6) for nitrogen transfer
  • all AAs except K, T, P, HO-P use aminotransferases
  • ex: asp aminotransferase (AST); ala aminotransferase (ALT)
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5
Q

Glutamate dehydrogenase

A
  • Oxidatively deaminates glu to form a-KG + NH4 using NAD

- Reductively aminates a-KG to form glu using NADPH + NH3

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6
Q

What is the purpose of the glucose/alanine cycle?

A
  • Transfer N from muscle to the liver
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7
Q

Describe the steps of the glucose/alanine cycle.

A

In muscle:

  1. Glycolysis converts glucose to pyruvate and lactate
  2. Glu generated from BCAAs by BCAA aminotransferase
  3. Pyruvate converted to ala by ALT using glu
  4. Ala moves from the muscle to the liver in the blood
  5. ALT converts ala back to pyruvate using a-KG
  6. Glu generated from aminotransferase reaction returned to a-KG + NH3 by GDH
  7. NH3 enters the urea cycle
  8. C skeletons recycled: pyruvate converted back to glucose, which returns to the muscle
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8
Q

When and where does the purine nucleotide cycle occur?

A

In skeletal muscle during prolonged exercise requiring ATP production; allows AMP recycling

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9
Q

What are the steps in the purine nucleotide cycle?

A
  1. ATP => ADP + Pi during exercise
  2. ADP => AMP by adenylate kinase
  3. AMP => IMP + NH3 by AMP deaminase
  4. NH3 used to make gln
  5. BCAA used to make asp
  6. IMP + asp => adenylosuccinate
  7. adenylosuccinate => fumarate + AMP
  8. AMP continues to cycle and fumarate goes into the TCA cycle
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10
Q

In what form is ammonia transported to the liver from the peripheral tissues?

A

gln

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11
Q

What are the steps in ammonia transport from peripheral tissues to the liver?

A
  1. oxidative amination of a-KG by GDH forms glu
  2. glu converted to gln by gln synthase, requiring NH4+ and ATP
  3. gln diffuses from peripheral tissues to liver in the blood
  4. gln converted back to glu by glutaminase, releasing NH4+
  5. glu converted to a-KG by GDH, releasing NH4+
  6. NH4+ enters the urea cycle
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12
Q

What are the amino acid sources of N for the urea cycle?

A

ser, thr, glu, his, asp, gln

some times emma has no questions

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13
Q

What are two non-amino acid sources of N for the urea cycle?

A
  • Fungal/bacterial action in the gut

- Purine nucleotide cycle in muscle

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14
Q

What are the steps of the urea cycle?

A

Mitochondria:

  1. CO2 + H20 => HCO3-
  2. CPSI condenses bicarbonate and ammonia to form carbamoyl phosphate, using 2 ATP
  3. Ornithine transcarbamoylase transfers a carbamoyl from carbamoyl phosphate to ornithine forming citrulline, and a phosphate is released
  4. Citrulline transported to the cytosol
  5. Citrulline and asp combined by arginosuccinate synthetase to form arginosuccinate, requiring 2 ATP equivalents
  6. Arginosuccinate lyase breaks arginosuccinate into fumarate + arg
  7. Arginase hydrolyzes arg to form urea and ornithine
  8. Ornithine is transported back into the mitochondria
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15
Q

What reaction is catalyzed by carbamoyl phosphate synthetase I (CPSI)?

A

HCO3- + NH4+ + 2ATP => carbamoyl phosphate + 2ADP + 2 Pi

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16
Q

What reaction is catalyzed by ornithine transcarbamoylase?

A

Carbamoyl is transferred from carbamoyl phosphate to ornithine forming citrulline and a phosphate is released

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17
Q

What reaction is catalyzed by arginosuccinate synthetase?

A

Citrulline and asp are combined to form arginosuccinate, requiring 2 ATP equivalents

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18
Q

What reaction is catalyzed by arginosuccinate lyase?

A

Arginosuccinate is broken into fumarate + arg

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19
Q

What reaction is catalyzed by arginase?

A

Arg is hydrolyzed to form urea and ornithine

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20
Q

How is CSPI regulated?

A

Allosteric activation by N-acetyl glutamate (glu + acetyl CoA => NAG; reaction activated by arg)

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21
Q

What is the Krebs bicycle?

A

Linking of urea cycle and TCA cycle through arginosuccinate

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22
Q

What are the 9 essential AAs?

A

phe, val, thr, trp, ile, met, his, leu, lys

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23
Q

Which AAs are conditionally essential?

A

Y (made from F) and C (needs sulfur from M)

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24
Q

What are the main steps in the degradation of BCAAs?

A
  1. Transamination to a-keto acid
  2. Oxidative dexcarboxylation by a-keto acid DH generating NADH
  3. glucogenic (val, ile) => gluconeogenesis and ketogenic (ile, leu) => ketogenesis
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25
Q

What enzyme is defective in maple syrup urine disease?

A

a-keto acid DH involved in BCAA degradation

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26
Q

What are key metabolic intermediates involved in serine synthesis and degradation?

A
  • synthesized from 3-PG

- breakdown generates 2-PG

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27
Q

How is glycine synthesized?

A
  • from serine: requires PLP and FH4
  • from threonine: requires PLP
  • from glyoxylate: transamination reaction with alanine
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28
Q

What are the possible degradation pathways for glycine?

A
  • Complete oxidation to CO2 and NH4+
  • Oxidation to glyoxylate and then oxalate (main pathway)
  • Oxidation to glyoxylate and then CO2
29
Q

What causes kidney stones?

A

Oxalate is relatively insoluble => high concentrations form crystals

30
Q

What is hyperglycinemia?

A
  • Inborn error of metabolism where glycine cannot be broken down
  • Fatal by age 3
31
Q

What are the key precursors in cysteine synthesis?

A
  • serine + sulfur from methionine

- reactions require PLP

32
Q

What are the key products of cysteine breakdown?

A
  • reactions require PLP
  • glu => nh4 => urea
  • sulfate => PAPS (sulfurs added to carbs)
33
Q

What are the key intermediates and enzymes in phe/tyr breakdown.

A
  • phenylalanine hydroxylase converts phe to tyr
  • downstream reactions require PLP
  • products = fumarate and aceotacetate
34
Q

What enzyme is defective in PKU?

A

phenylalanine hydroxylase

35
Q

What are the nitrogenous bases?

A

Adenine, guanine, cytosine, thymine

36
Q

Purines

A

Adenine and guanine

37
Q

Pyrimidines

A

Cytosine and thymine

38
Q

Nucleoside

A
  • Nitrogenous base + sugar

- adenosine, guanosine, cytidine, thymidine

39
Q

Nucleotide

A

nitrogenous base + sugar + phosphate(s)

40
Q

Which amino acids are involved in nucleic acid synthesis?

A

Purines: G, D, Q
Pyrimidines: D, Q

41
Q

What are the key steps in purine synthesis?

A
  1. R5P generated from the PPP
  2. R5P phosphorylated (2x) by PRPP synthetase to form PRPP.
  3. gly + gln + asp + 3 phosphates => IMP
  4. IMP + GTP + asp => AMP and IMP + ATP + gln => GMP
  5. NMPs phosphorylated to make NDPs
  6. NDPs phosphorylated to make NTPs
  7. NDPs => dNDPs => dNTPs
42
Q

How are dNTPs generated from NDPs?

A
  1. NDP reduced to dNDP by ribonucleotide reductase with oxidation of thioredoxin
  2. Phosphorylation of dNDP to dNTP
43
Q

How is the reduction of ribose to deoxyribose regulated via thioredoxin?

A
  • RR inhibited by dATP
  • thioredoxin must be re-reduced by thioredoxin reductase to be used by RR
  • thioredoxin reductase requires NADPH from the PPP
44
Q

How are the generation of AMP and GMP from IMP co-regulated?

A

AMP formation uses GTP and GMP formation uses ATP => reciprocal regulation to balance synthesis

45
Q

What enzymes in purine synthesis are regulated?

A
  • PRPP synthetase inhibited by diphosphates
  • glutamine phosphoribosyl amidotransferase (in IMP synthesis) inhibited by all nucleotides
  • IMP DH and adenylosuccinate synthetase (IMP => GMP and AMP) inhibited by monophosphates
46
Q

Purine salvage

A

Adenosine is the only nucleoside that can be converted directly from its free base in the salvage pathway w/o intermediates

47
Q

When is the purine nucleotide cycle used?

A

Recycles bases in muscle during exercise to produce more ATP

48
Q

What are the steps in the purine nucleotide cycle?

A
  • AMP => IMP + NH3
  • IMP + asp + GTP => adenylosuccinate + GDP
  • adenylosuccinate => AMP + fumarate
  • fumarate => TCA
49
Q

What is the general pathway for purine degradation?

A

nucleotide monophosphate => nucleoside => nitrogenous base => => uric acid

50
Q

What causes gout?

A

Uric acid buildup in joints

51
Q

What are sources of excess dietary purines?

A

Meat, seafood, fructose, alcohol

52
Q

What enzyme is inhibited to treat gout?

A

xanthine oxidase

53
Q

What reactions does xanthine oxidase catalyze?

A

Purine degradation:

  • hypoxanthine => xanthine
  • xanthine => uric acid
54
Q

What reaction does CPS-II catalyze and how is it regulated?

A
  • Pyrimidine synthesis: gln + CO2 + 2ATP => carbamoyl phosphate
  • Inhibited by UTP and activated by PRPP
55
Q

What is the general pathway for pyrimidine synthesis?

A
  • carbamoyl phosphate => => UMP => UDP => dUTP or UTP
  • UTP => CTP => CDP => dCDP => dCTP or dCMP
  • dCMP => dUMP
  • dUDP => dUMP => dTMP => dTDP
56
Q

What steps in pyrimidine nucleotide biosynthesis are regulated?

A 
CPSII:
- activated by PRPP
- inhibited by purines
- inhibited by UTP
PRPP biosynthesis:
- inhibited by TDP
pyrimidine biosynthesis:
- inhibited by CTP
57
Q

What are the key intermediates in pyrimidine salvage?

A
  • cytosine => uracil
  • uracil and thymine enter pathway
  • conversion to alanine and aminoisobutyrate
  • conversion to succinyl coA => TCA
58
Q

What is the purpose of the one carbon pool?

A

transfer of one carbon groups to sulfur or carbon

59
Q

What are the elements of the one carbon pool and how reduced are they?

A
  • formyl tetrahydrofolate
  • methylene tetrahydrofolate
  • methyl tetrahydrofolate (cannot be reoxidized to other forms!)
60
Q

What are some sources of 1 carbon units?

A

ser, gly, his, formaldehyde, formate

61
Q

What are some recipeints of 1 carbon units?

A

dTMP, ser, purines, B12

62
Q

How is folate reduced to FH4?

A
  1. dietary folate is poly-glutamated
  2. reduction of folate by DHFR produces dihydrofolate and requires NADPH
  3. reduction of dihydrofolate by DHFR produces tetrahydrofolate and requires NADPH
63
Q

What is the methyl trap hypothesis?

A

B12 deficiency or enzyme defect in conversion of homocysteine to methionine => all THF is trapped in the methyl form and is useless ( ~THF deficiency)

64
Q

How are FH4, B12, and SAM connected?

A
  1. Methionine + 3 ATP equivalents generate activated methyls (SAM)
  2. SAM methylates a precursor, becoming SAH
  3. SAH broken into adenosine and homocysteine
  4. Homocysteine converted into methionine, requiring transfer of methyl from B12
  5. B12 re-methylated by FH4-methyl
65
Q

What two reactions require vitamin B12?

A
  1. methylation of homocysteine to form methionine

2. rearrangement of methylmalonyl CoA (product of BCAA degradation) to form succinyl CoA - B12 carries the methyl

66
Q

What reaction generates SAM?

A

methionine adenosyltransferase transfers an adenosine from ATP to methionine, activating the methionine

67
Q

What are some reactions that require SAM?

A

Generation of: epinephrine, creatine, methylated nucleotides, phosphatidylcholine, melatonin, methylated histones

68
Q

What are the causes of hyperhomocysteinemia?

A

B12 deficiency

enzyme defects causes homocysteine buildup:

  • enzyme converting homocysteine to met
  • THFR, which reduces methyl-FH4 used in that reaction
  • alternate pathway used to generate cys from homocysteine, but cys buildup will inhibit that pathway
69
Q

What are the symptoms of hyperhomocysteinemia?

A

neurologic issues and cardiac disease

Foundations 1 (26 decks)

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