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Medical Biochemistry > Ch. 14: Alt Pathways of Glucose Metabolism & Hexose Interconversions > Flashcards

Ch. 14: Alt Pathways of Glucose Metabolism & Hexose Interconversions Flashcards

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USMLE® Step 1 flashcards
1
Q

What is glucuronic acid? Essential part of what?

A

Glucose w/oxidized C6

GAGs

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2
Q

What reactions are glucuronic acid needed for?

A

Highly polar ➡️ detox rxns of insoluble compounds (bilirubin, steroids, drugs)

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3
Q

Enzymes used to synthesize glucuronic acid?

A

1) UDPG-dehydrogenase (UDP glucose ➡️ UDP glucuronic acid)

2) Glucuronyl-transferase (UDP glucuronic acid ➡️ glucuronides)

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4
Q

2 main products from conversion of D-glucuronic acid?

A

L-ascorbic acid (not in humans)

D-Xylulose 5-P (➡️ PPP)

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5
Q

Disorders of glucuronic acid metabolism:

L-gulonolactone oxidase deficiency?

A

Unable to convert L-gulonolactone ➡️ L-ascorbic acid

Requires Vitamin C in diet

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6
Q

Disorders of glucuronic acid metabolism:

NADP+ dependent xylitol dehydrogenase deficiency?

A

No conversion of L-xylulose ➡️ xylitol

Causes essential pentosuria (⬆️ L-xylulose in urine)

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7
Q

Why is metabolism of fructose important?

A

Unable to regulate fructose blood levels

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8
Q

Major pathway of dietary fructose metabolism?

A

D-fructose ➡️ fructose 1-P (fructokinase)

Fructose 1-P ➡️ D-glyceraldehyde (aldolase B)

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9
Q

Minor pathway of dietary fructose metabolism?

A

D-fructose ➡️ fructose 6-P (hexokinase)

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10
Q

Blocking which enzyme occurs in essential fructosuria? Hereditary fructose intolerance?

A

Fructokinase

Aldolase B

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11
Q

Which enzymes convert D-glucose ➡️ D-sorbitol ➡️ D-fructose? Cofactors for each rxn?

A

1) Aldose reductase, NADPH (when ⬆️ glucose)

2) Sorbitol dehydrogenase, NAD+

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12
Q

Characteristics of essential fructosuria?

A

Benign

⬆️ fructose in urine

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13
Q

Characteristics of hereditary fructose intolerance?

A

Traps fructose 1-P in cells

Hypoglycemia, jaundice, hepatomegaly, liver failure, hyperuricemia, lactic acidemia

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14
Q

Steps in metabolism of galactose? Enzymes at each step?

A

Galactose ➡️ galactose 1-P (galactokinase)

Galactose 1-P ➡️ glucose 1-P or UDP galactose (galactose 1-P uridyl transferase)

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15
Q

Disorders of galactose metabolism:

Galactosemia? Characteristics?

A

Galactose 1-P uridyl transferase deficiency

Galactosemia, galactosuria, liver damage, cataracts, intellectual disability

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16
Q

Disorders of galactose metabolism:

Galactokinase deficiency, characteristics?

A

Galactosemia, galactosuria

Galactose ➡️ ⬆️ galactitol (aldose reductase
Can cause cataracts

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17
Q

How is lactose synthesized?

A

UDP-galactose + glucose ➡️ lactose (lactose synthase)

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18
Q

Which 2 proteins make up lactose synthase?

A

Beta-D-galactosyltransferase (protein A)

alpha-lactalbumin (protein B)

19
Q

Metabolism of amino sugars:

How is fructose 6-P ➡️ glucosamine 6-P?

A

Aminotransferase, with gln ➡️ glu

20
Q

Metabolism of amino sugars:

End products of metabolism of glucosamine 6-P?

A

GAGs (chondroitins, heparin)
glycoproteins
Sialic acid
Gangliosides

21
Q

Where does pentose phosphate pathway occur?

A

Cytosol

22
Q

3 function of PPP?

A

1) supply NADPH
2) supply ribulose 5-P
3) generate glycolysis intermediates

23
Q

PPP:

Steps from glucose 6-P ➡️ ribulose 5-P? Enzymes for each step?

A

1) Glucose 6-P ➡️ 6-phosphogluconate (G6P dehydrogenase)
2) 6-phosphogluconate ➡️ ribulose 5-P (6-phosphogluconate dehydrogenase)

1 NADPH generated at each step

24
Q

2 diff pathways for ribulose 5-P?

A

1) ➡️ ribose 5-P (ribose 5-P isomerase) ➡️ nucleic acid biosynthesis
2) ➡️ xylulose 5-P ➡️ glycolysis intermediates

25
Q

What is the important fxn of NADPH?

A

Provides reducing power ➡️ detox harmful by-products

26
Q

3 examples of harmful oxidized by-products in mitochondria?

A

Superoxide
Hydrogen peroxide
Hydroxyl radical

27
Q

3 enzymes NADPH brings back to normal state in mitochondria? (After they handle ROS)

A

Superoxide dismutase
Catalase
Glutathione peroxidase

28
Q

What enzyme is important in liver for detox of drugs?

A

Cytochrome P450 reductase (uses FAD, FMN)

29
Q

General pathway for phagocytic cell to attack bacteria?

A

1) Attachment of pathogen to phagocytic cell (IgG + cell receptor)
2) Ingestion of pathogen ➡️ lysosome + phagosome = phagolysosome
3) Pathogen destroyed w/ROS

30
Q

How is nitric oxide (NO) synthesized? What is needed for the rxn?

A

L-arginine ➡️ L-citrulline (NO synthase)

Needs NADPH & BH4

31
Q

What is the most common inherited enzyme deficiency known to cause human disease? What disease results from this deficiency?

A

G6PD deficiency

Hemolysis (occurs from impaired ability of erythrocytes to produce NADPH ➡️ can’t recycle oxidized enzymes)

32
Q

What are causes of oxidant stress in erythrocytes ➡️ increases demand for NADPH?

A

Certain drugs
Infections
Fava beans

33
Q

What are Heinz bodies?

A

Aggregates of damaged RBCs

34
Q

What happens to G6PD function as RBCs age?

Compare normal enzyme to G6PD Mediterranean & A-

A

RBC age ➡️ G6PD fxn ⬇️

Normal: Oldest cells still have enough G6PD activity to protect against oxidative damage &a hemolysis

Abnormal: ⬇️ in G6PD A-, ⬇️⬇️ in G6PD Mediterranean

35
Q

Which nucleotides can sugars be coupled to ➡️ activated sugars?

A

UDP
GDP
CMP

36
Q

Which activated sugars make up the sugar-nucleotide pool? (Involved in glycoconjugate biosynthesis)

A

UDP-GlcNAc, UDP-Gal, UDP-GlcUA,
UDP-GalNAc, UDP-xylose, CMP-sialic acid,
GDP-fucose, GDP-mannose

37
Q

4 sugar-nucleotide pool defects?

A

1) Type I CHO-deficient glycoprotein syndrome (CDGs I)
2) Gal-1-P uridyltransferase deficiency
3) Gal-4 epimerase deficiency
4) Leukocyte adhesion deficiency type II (LAD II)

38
Q

5 steps of N-linked glycosylation?

A

1) pro synthesis starts (ribosomes) ➡️ polypeptide extruded into RER
2) branched oligosaccharides synthesized on dolichol pyrophosphate
3) oligosaccharide transferred to asparagine residue of pro
4) as pro moves through RER, CHO chain trimmed
5) Further trimming and/or adding monosaccharides in Golgi

39
Q

What is I-cell disease?

A

Deficiency in enzyme needed to phosphorylate mannose of glycoprotein (usually used to target glycoproteins to be sent to lysosomes ➡️ instead, sent to surface)

40
Q

Characteristics of I-cell disease?

A

Skeletal abnormalities
Restricted joint movement
Coarse facial features
Severe psychomotor impairment

41
Q

What does GPI stand for? Where are GPI-anchored proteins?

A

Glycosylphosphatidylinositol

Anchored to cell membrane

42
Q

3 stages of peptidoglycan synthesis?

A

1) Synthesis of N-Acetylmuramic acid (MurNAc) peptide chain
2) Synthesis of GlcNAc-MurNAc repeating units
3) Cross-linking of polymer side chains (via transpeptidase)

43
Q

How do penicillins & cephalosporins function?

A

Inhibit transpeptidation rxn in cross-linking (acts as suicide substrate)

Has similar structure to substrate for enzyme ➡️ forms inactive, stable penicilloyl-enzyme complex

Medical Biochemistry (23 decks)

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