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#‼️BIOCHEMISTRY - PLE TOPNOTCH 2020 > NITROGEN METABOLISM > Flashcards

NITROGEN METABOLISM Flashcards

1
Q

LEAD inactivates what enzymes in heme synthesis?

A

ALA dehydratase and ferrochelatase​

  • ↑ urinary ALA and free erythrocyte porphyrins
  • Microcytic, hypochromic anemia with basophilic stippling of RBCs
  • Headache, memory loss
  • Peripheral neuropathy claw hand, wrist-drop
  • Nausea, abdominal pain, diarrhea
  • Lead lines in gums, lead deposits in epiphyses
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2
Q

Inherited defect of renal tubular amino acid transporter for Cystine, Ornithine, Lysine and Arginine in the PCT of the kidneys

A

CYSTINURIA​

  • Clinical Presentation:
    • Excess cystine in urine can lead to precipitation of cystine kidney stones and cause staghorn calculi
  • Treatment:
    • Acetazolamide to alkalinize the urine
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3
Q

FIRST PHASE OF AA CATABOLISM

  1. TRANSAMINATION
  2. OXIDATIVE DEAMINATION
A
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4
Q

Most common heme defect in Children

A

Congenital erythropoietic porphyria

Enzyme: Uroporphyrinogen III cosynthase

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5
Q

Most severe enzyme defect in the urea cycle

A

Carbamoyl phosphate synthetase-I deficiency

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6
Q

Sphingosine, purines, thymine

A

Serine

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7
Q
  • Defect in fumarylacetoacetate hydrolase
  • Accumulation of fumarylacetoacetate leads to liver and kidney failure, nervous system disorders, cabbage-like ordor, and increased risk for liver cancer
A

TYROSINEMIA TYPE I

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8
Q

Enzyme deficient in Acute intermittent porphyria

A

hydroxymethylbilane synthase (uroporphyrinogen I synthase)

  • Accumulation of PBG and δ-ALA in the urine
  • Episodes of abdominal pain, psychiatric symptoms, peripheral neuropathies
  • Urine darkens on exposure to light and air
  • Treat with high dose of glucose (mild attack) or hematin (severe attack)
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9
Q

Histamine

A

Histidine

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10
Q

UREA CYCLE occurs ONLY to where?

A

LIVER

o It occurs in both mitochondria and cytosol

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11
Q

Carnitine

A

Lysine

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12
Q

STEPS in HEME DEGRADATION

A

1. Formation of bilirubin

  • After approximately 120 days, RBCs are taken up and degraded by the reticuloendothelial system, particularly in the liver and spleen
  • Heme → Biliverdin (green) → Bilirubin (red orange)
  • Enzyme: Heme oxygenase system of reticuloendothelial cells (relases carbon monoxide)

2. Uptake of bilirubin by the liver

  • Bilirubin transported to the liver in the blood by binding to albumin
  • In the liver, bilirubin binds to intracellular proteins, particularly to ligandin

3. Formation of bilirubin diglucoronide

  • Bilirubin is conjugated to two molecules of glucoronic acid
  • Enzyme: Bilirubin glucuronyltransferase
  • Deficient in Crigler-Najjar I and II, and Gilbert syndrome

4. Secrection of bilirubin into bile

  • Bilirubin diglucuronide is transported into the bile canaliculi and then into the bile
  • Susceptible to impairment in liver disease

5. Formation of urobilins in the intestine

  • In the gut, bilirubin → urobilinogen (colorless)
  • Intestinal bacteria then oxidize urobilinogen → stercolin (brown)
  • Some urobilinogen is reabsorbed from the blood and enters the portal circulation
  • Remaining urobilinogen is transported by the blood to the kidney, where it is converted to urobilin (yellow)
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13
Q

Congenital deficiency of homogentisate oxidase in the degradative pathway of tyrosine, leading to build up of homogentisic acid

*Homogentisate oxidase converts homogentisic acid to maleylacetoacetate.

A

ALKAPTONURIA​

  • Clinical Presentation:
    • Alkapton bodies cause urine to turn black on standing
    • Connective tissue is dark (ochronosis)
    • Benign disease but may have debilitating arthralgias
  • Treatment:
    • Aimed at reducing pigment deposition
    • Reduce phenylalanine and tyrosine in the diet
    • Vitamin C for older children and adults
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14
Q

UREA CYCLE DEFECTS

A
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15
Q

Rate-limiting step of HEME SYNTHESIS

A

Reaction: Glycine + Succinyl CoA → δ-Aminolevulinic Acid

Enzyme: ALA synthase

Co-factor: Pyridoxine

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16
Q

HEME SYNTHESIS

A

1. δ-Aminolevulinic Acid is synthesized

  • This is the rate-limiting step
  • Reaction: Glycine + Succinyl CoA → δ-Aminolevulinic Acid
  • Enzyme: ALA synthase
  • Co-factor: Pyridoxine

2. Porphobilinogen is formed

  • Condensation of two molecules of ALA by zinc-containing ALA dehydratase
  • Inhibited by heavy metal ions (e.g., lead) that replace the zinc

3. The first porphyrin (uroporphyrinogen) is formed from four PBGs

4. Porphyrins are decarboxylated and oxidized

5. Protoporphyrin IX binds iron, forming heme

  • Introduction of iron (as Fe 2+ ) into protoporphyrin IX occurs spontaneously, but the rate is enhanced by ferrochelatase, that is also inhibited by lead
  • Vitamin C increases the uptake of iron from the intestinal tract
  • Ceruloplasmin is involved in the oxidation of iron
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17
Q

enzyme deficient in Porphyria cutanea tarda

* Photosensitivity: cutaneous fragility and blistering of the hands, forearms, and sometimes, the face

A

uroporphyrinogen decarboxylase (UPD)

18
Q
  • Heme, purines, creatine, glutathione
  • Conjugated to bile acids, drugs, and other metabolites
A

Glycine

19
Q
  • Blocked degradation of branched amino acids (isoleucine, valine, leucine) due to a deficiency in α-ketoacid dehydrogenase complex​
A

MAPLE SYRUP URINE DISEASE​

20
Q

Creatine, polyamines, nitric oxide

A

Arginine

21
Q

TRANSAMINATION

A

Amino acids transfer their α-amino group to α-ketoglutarate, resulting in the formation of glutamate

Exceptions: lysine, threonine, proline, hydroxyproline

22
Q

Rate-limiting step of UREA CYCLE

A

NH 3 + CO 2 → Carbamoyl phosphate

Enzyme: Carbamoyl phosphate synthetase I

Allosteric Activator: N-acetylglutamate

23
Q

What are the substrates for UREA CYCLE?

A

NH3 , aspartate, CO2

24
Q
  • Defect in methionine degradation leads to:
    • High plasma and urinary levels of homocysteine and methionine
    • Low levels of cysteine
  • Most common deficiency: cystathionine β-synthase
A

HOMOCYSTINURIA

  • Clinical Presentation:
    • Ectopia lentis (displacement is downward)
    • Faulty bone development and osteoporosis
    • Mental retardation
    • Tendency to form thrombi
    • Myocardial infarction
    • Stroke in children and young adults
25
Q

Coenzyme A, taurine, glutathione

A

Cysteine

26
Q

Results from ↓ phenylalanine hydroxylase or ↓ tetrahydrobiopterin​

A

PHENYLKETONURIA

  • Tyrosine becomes essential and phenylalanine accumulates
  • Clinical presentation:
    • Accumulation of phenylketones
      • Phenylpyruvate (characteristic odor to the urine)
      • Phenyllactate
      • Phenylacetate
    • Severe mental retardation, failure to walk or talk, seizures
    • Fair skin and eczema
    • Musty body odor
27
Q

Purely Ketogenic AA

A

“KL”

  • Lysine
  • Leucine
28
Q

Catecholamine Synthesis

A
29
Q

OXIDATIVE DEAMINATION OF AA CATABOLISM

A
30
Q

Defective melanin synthesis from tyrosine, which may be from:

o Absence of the copper-requiring enzyme tyrosinase

o Defective tyrosine transporters

A

ALBINISM

31
Q

Serotonin, niacin, melatonin

A

Tryptophan

32
Q

• Catecholamines, thyroid hormones, melanin

A

Tyrosine

33
Q

BOTH Glucogenic and Ketogenic AA

A

“WIFY”

• Phenylalanine

• Tyrosine

• Isoleucine

• Tryptophan

34
Q
  • Defect in tyrosine aminotransferase, which catalyzes transamination of tyrosine
  • Affects the eyes, skin, and mental development
A

TYROSINEMIA TYPE II​

35
Q

S-adenosylmethionine, creatine, polyamines

A

Methionine

36
Q
  • Methylmalonyl CoA mutase deficiency leads to defect in the conversion of methylmalonyl CoA to succinyl CoA
    • Involves the metabolism of amino acids that are converted to propionyl CoA: isoleucine, valine, threonine, and methionine
    • To a lesser extent, odd-chain fatty acid and cholesterol degradation
A

METHYLMALONIC ACIDEMIA

  • Clinical Presentation:
    • Seizure, encephalopathy, and stroke at the age of 1 month to 1 year
    • Hypotonia, lethargy, failure to thrive, hepatosplenomegaly, and monilial infections
  • Treatment:
    • Protein-restricted diet (0.5-1.5 g/kg/d) with L-carnitine and cobalamin supplementation
37
Q
  • Defective histidase, which converts histidine to urocanate
  • Autosomal recessive disorder
  • Earlier cases associated with intellectual disability, but now considered benign
A

HISTIDINEMIA

38
Q

Most common enzyme defect in the urea cycle

A

Ornithine transcarbomylase deficiency

39
Q

GABA, glutathione

A

Glutamate

40
Q

UREA CYCLE

A
41
Q

TRANSPORT OF AMMONIA FROM PERIPHERAL TISSUES

A
  • THROUGH GLUTAMINE
    • In most tissues, glutamate combines with ammonia to form glutamine
    • Enzyme: Glutamine synthetase
    • Glutamine is transported in the blood and may be deaminated to release ammonia in two organs:
      • Liver: in response to high protein intake
      • Kidneys: in response to metabolic acidosis
  • Enzyme: Glutaminase
  • THROUGH ALANINE​
    • ​In muscle, pyruvate is transaminated to alanine
    • Alanine is transported to the liver where it is converted back to pyruvate

#‼️BIOCHEMISTRY - PLE TOPNOTCH 2020 (13 decks)

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