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Carbohydrate metabolism (final exam) Flashcards

Metabolism of monosaccharides and disaccharides, pentose-phosphate pathway

1
Q

What are the major sources of dietary fructose?

A
  • Sucrose
  • Free monosaccharide in many fruits, in honey, and in high-fructose corn syrup
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2
Q

What distinguishes uptake of fructose into cells from uptake of glucose?

A

It is not insulin-dependent

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3
Q

What is the major metabolic fate of fructose?

A

Conversion to DHAP, GA3P, and glyceraldehyde for use in glycolysis/gluconeogenesis, as well as other pathways

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4
Q

What is the first step of fructose metabolism?

A

(1) fructose + ATP → fructose-1-phosphate + ADP, OR
(2) fructose + ATP → fructose-6-phosphate + ADP

(1): catalyzed by fructokinase
(2): catalyzed by hexokinase

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5
Q

What are the two enzymes that catalyze the phosphorylation of fructose?

A
  • Hexokinase
  • Fructokinase
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6
Q

What are the differences between fructokinase and hexokinase in the phosphorylation of fructose?

A

Fructokinase

  • Produces fructose-1-phosphate
  • Found in the liver, kidney, and small intestinal mucosa

Hexokinase

  • Produces fructose-6-phosphate
  • Has a high Km, so it does not function unless intracellular fructose levels are high
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7
Q

What is the fate of fructose-1-phosphate?

A

fructose-1-phosphate ⇌ glyceraldehyde + dihydroxyacetone phosphate
Catalyzed by *aldolase B *

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8
Q

What is the fate of fructose-6-phosphate?

A

Continues through glycolysis normally

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9
Q

What are the different aldolases?

A
  • Aldolase A: found in most tissues; cleaves F-1,6-BP only
  • Aldolase B: found in the liver; cleaves F-1,6-BP AND F-1-P
  • Aldolase C: found in the brain; cleaves F-1,6-BP only
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10
Q

Which is faster: glycolysis, or fructose metabolism (which eventually merges with glycolysis)?

A

Fructose metabolism, because the trioses formed from fructose-1-phosphate bypass PFK-1, the rate-limiting step of glycolysis

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11
Q

What are the features of essential fructosuria?

A
  • Deficiency in fructokinase
  • Autosomal recessive
  • Benign condition
  • Fructose accumulates in the urine
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12
Q

Deficiency in which enzyme results in essential fructosuria?

A

Fructokinase

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13
Q

Deficiency in which enzyme results in hereditary fructose intolerance (HFI)?

A

Aldolase B

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14
Q

What are the features of hereditary fructose intolerance (HFI)?

A
  • Autosomal recessive
  • Absence of aldolase B
  • Symptoms begin when baby is weaned off milk and is fed food containing sucrose or fructose
  • F-1P begins to accumulate, resulting in a drop in levels of ATP and Pi
  • As ATP levels fall, AMP levels rise, and AMP is eventually degraded, resulting in hyperuricemia and lactic acidosis
  • Decreased availability of hepatic ATP affects gluconeogenesis and plasma protein synthesis and may lead to liver failure
  • Symptoms: severe hypoglycemia, vomiting, jaundice, hemorrhage, hepatomegaly, renal dysfunction, hyperuricemia, lacticacidemia
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15
Q

How is hereditary fructose intolerance treated?

A

Sucrose, sorbitol, and fructose must be removed from the diet to prevent liver failure

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16
Q

How is glucose converted to sorbitol?

A

glucose + NADPH + H+ → sorbitol + NADP+
Catalyzed by aldose reductase

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17
Q

Where in the body is sorbitol produced?

A
  • Lens
  • Retina
  • Schwann cells of peripheral nerves
  • Liver
  • Kidney
  • Placenta
  • RBCs
  • Ovaries and seminal vesicles
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18
Q

How is sorbitol converted to fructose?

A

sorbitol + NAD+ (or NADP+) → fructose + NADH (or NADPH)
Catalyzed by sorbitol dehydrogenase

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19
Q

Where in the body is sorbitol reduced to fructose?

A
  • Liver
  • Ovaries
  • Seminal vesicles
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20
Q

How is sorbitol implicated in cell swelling?

A
  • During hyperglycemia, glucose enters the cells containing aldose reductase freely, as they do not use the insulin-sensitive GLUT-4
  • High intracellular glucose levels accompanied with adequate NAPH/NADH cause aldose reductase to produce a high amount of sorbitol
  • Sorbitol cannot pass through cell membranes and is trapped inside the cell, causing uptake of whater by osmosis
  • Water retention occurs
  • This problem is exacerbated when the tissue does not contain sorbitol dehydrogenase, e.g. the lens, nerves, retina, and kidneys
  • Some of the pathologic changes in diabetes can be attributed to this phenomenon, e.g. cataracts, peripheral neuropathy, microvascular problems leading to nephropathy and retinopathy
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21
Q

What are the major sources of galactose?

A
  • Milk and milk products
  • Degradation of complex carbohydrates, e.g. glycoproteins and glycolipids in cell membranes
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22
Q

What is the first step of galactose metabolism?

A

galactose + ATP → galactose-1-phosphate + ADP
Catalyzed by galactokinase

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23
Q

How is galactose-1-phosphate converted to UDP-galactose?

A

UDP-glucose + galactose-1-phosphate ⇌ UDP-galactose + glucose-1-phosphate
Catalyzed by galactose-1-phosphate uridyltransferase (GALT)

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24
Q

What are the fates of UDP-galactose?

A
  • Converted to UDP-glucose by UDP-hexose 4-epimerase for use in glucose metabolism
  • Donor of galactose units in synthetic pathways of lactose, glycoproteins, glycolipids, and glycosaminoglycans
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25
What are the disorders of fructose metabolism?
* Essential fructosemia (fructokinase deficiency) * Hereditary fructose intolerance (aldolase B deficiency)
26
What are the disorders of galactose metabolism?
* Galactokinase deficiency * Classic galactosemia (GALT deficiency)
27
What are the features of galactokinase deficiency?
* Elevation of galactose in blood (galactosemia) and urine (galactosuria) * Causes galactitol accumulation if galactose is present in the diet, leading to cataracts * Treated by dietary restriction
28
What are the features of classic galactosemia?
* GALT deficiency * Autosomal recessive * Causes galactosemia, galactosuria, vomiting, diarrhea, and jaundice * Accumulation of Gal-1P and galactitol in nerve, lens, liver, and kidney tissue causes liver damage, severe mental retardation, and cataracts * Treated by rapid diagnosis and removal of galactose (and therefore lacose) from the diet * Despite adequate treatment, patients are at risk for developmental delays and premature ovarian failure
29
What is the structure of lactose?
Gal-β(1→4)-Glc
30
Where in the cell is lactose synthesized?
Golgi apparatus
31
What is the enzyme that is used to synthesize lactose?
Lactose synthase (UDP-galactose:glucose galactosyltransferase), composed of: * Protein A (β-ᴅ-galactosyltransferase), an enzyme * Protein B (α-lactalbumin), which is not an enzyme
32
How is lactose synthesis confined to the mammary glands?
Synthesis of α-lactalbumin (protein B) only occurs in response to prolactin in the lactating mammary glands. Protein B forms a complex with the enzyme, protein A, changing the specificity of the transferase so lactose is synthesized instead of N-acetyllactosamine
33
What is the activity of lactose synthase in tissues other than lactating mammary glands?
UDP-galactose + N-acetyl-ᴅ-glucosamine → N-acetyllactosamine + UDP * Contains the same β(1→4) bond * N-acetyllactosamine is a component of N-linked glycoproteins
34
How is glucuronate produced?
(1) glucose-1-phosphate ⇌ UDP-glucose (2) UDP-glucose + NAD+ → UDP-glucuronic acid + NADH (1): catalyzed by *pyrophosphorylase* (2): catalyzed by *UDP-glucose dehydrogenase*
35
What is the fate of UDP-glucuronic acid?
* Conversion to UDP-xylose for use in the pentose phosphate pathway * Precursor for glycosaminoglycans
36
What are the main products of the pentose phosphate pathway?
* NADPH for reductive biosynthesis * Ribose-5-phosphate for nucleic acid biosynthesis * Glycolytic intermediates (as byproducts)
37
Where in the body is the irreversible, oxidative portion of the pentose phosphate pathway particularly important?
* Liver, lactating mammary glands, adipose tissue: active in fatty acid biosynthesis * Testes, ovaries, placenta, adrenal cortex: active in biosynthesis of steroid hormones * Erythrocytes: use NADPH to keep glutathione reduced
38
What is the first step of the pentose phosphate pathway?
glucose-6-phosphate + NADP+ + H2O ⇌ 6-phosphogluconate + NADPH + H+ Catalyzed by *glucose-6-phosphate dehydrogenase* (G6PD)
39
What is the second step of the pentose phosphate pathway?
6-phosphogluconate + NADP+ ⇌ ribulose-5-phosphate + NADPH + H+ + CO2 Catalyzed by *6-phosphogluconate dehydrogenase*
40
What is the slow, rate-determining step of the pentose phosphate pathway?
The first step, catalyzed by G6PD (oxidation of glucose)
41
How is glucose-6-phosphate dehydrogenase regulated?
**Activators** Insulin: upregulates expression of G6PD in the well-fed state **Inhibitors** NADPH
42
How does insulin function as an activator of the pentose phosphate pathway?
Upregulates expression of G6PD in the well-fed state
43
How many molecules of glucose-6-phosphate are needed to complete the reversible, nonoxidative reactions of the pentose phosphate pathway?
Three: * One to produce ribose-5-phosphate for the first transfer reaction * One to produce xylulose-5-phosphate for the first transfer reaction * One to produce an additional xylulose-5-phosphate for the final transfer reaction
44
What are the enzymes used in the reversible, nonoxidative reactions of the pentose phosphate pathway?
* Transketolase: transfers 2-carbon units * Transaldolase: transfers 3-carbon units * A ribulose epimerase * A ribulose isomerase
45
What is the sequence of **transfer** reactions in the reversible, nonoxidative phase of the pentose phosphate pathway?
[ketose + aldose ⇌ aldose + ketose] (number of carbons equals the position of the phosphate) * xylulose-5-phosphate + ribose-5-phosphate ⇌ glyceraldehyde-3-phosphate + sedoheptulose-7-phosphate (∆2C) * sedoheptulose-7-phosphate + glyceraldehyde-3-phosphate ⇌ erythrose-4-phosphate + fructose-6-phosphate (∆3C) * xylulose-5-phosphate + glyceraldehyde-3-phosphate ⇌ glyceraldehyde-3-phosphate + fructose-6-phosphate (∆2C)
46
How is ribulose-5-phosphate modified in the pentose phosphate pathway?
* Isomerized to ribose-5-phosphate by an *isomerase* * Epimerized to xylulose-5-phosphate (a C3 epimer) by an *epimerase*
47
Using G6P to represent all hexose sugar units, what is the stoichiometry of the pentose phosphate pathway?
3G6P + 6NADP+ ⇌ 2G6P + GA3P + 6NADPH + 3CO2 If we progress through the reactions twice, we obtain: 6G6P + 12NADP+ ⇌ 4G6P + 2GA3P + 12NADPH + 6CO2 Two molecules of GA3P can be simplified to glucose (as would happen in gluconeogenesis), therefore: 6G6P + 12NADP+ ⇌ 5G6P + 12NADPH + 6CO2 tldr; **for every 6 molecules of glucose, 5 are preserved as hexoses, and 1 is lost as 6CO2**
48
Where in the structure of NADPH is the phosphate?
2' of the adenylyl ribose
49
What is the ratio of NADP+ to NADPH in hepatocytes?
1:10 (0.1)
50
What is the ratio of NAD+ to NADH in hepatocytes?
1000:1
51
What are the uses of NADPH in the cell?
* Reductive biosynthesis * Reduction of ROS * Coenzyme for cytochrome P450 * Production of ROS in the oxidative burst of phagocytes * Synthesis of NO
52
What are the sources of ROS in the cell?
* Byproducts of aerobic metabolism: * Produced accidentally by CoQ in the ETC * Produced intentionally in the oxidative burst of phagocytes * Produced accidentally by the cytochrome P450 system * Ionizing radiation
53
What kinds of ROS are produced by CoQ in the ETC?
O2∙–
54
What kinds of ROS are produced in the oxidative burst?
* O2∙– * H2O2 * OH∙ * HOCl
55
What kinds of ROS are produced by ionizing radiation?
OH∙
56
What kinds of molecules are damaged by ROS?
* DNA * Proteins * Unsaturated lipids
57
Which pathologic processes have ROS been implicated in?
* Reperfusion injury * Cancer * Inflammatory disease * Aging
58
What are the intracellular mechanisms of neutralizing ROS?
* Glutathione peroxidase * Catalase * Superoxide dismutase * Antioxidant chemicals
59
What is the structure of glutathione?
Gly-Cys-Glu
60
How does glutathione neutralize ROS?
2G-SH + H2O2 → G-S–S-G + H2O Catalyzed by *glutathione peroxidase*
61
How is reduced glutathione regenerated after its oxidation?
G-S–S-G + NADPH → 2G-SH + NADP+ Catalyzed by *glutathione reductase*
62
Which chemicals are antioxidants?
* Ascorbate (vitamin C) * Vitamin E * Carotenoids
63
What is the general reaction of cytochrome P450?
R-H + O2 + NADPH → R-OH + H2O + NADP+, where R may be a steroid, drug, or other chemical
64
What are the different cytochrome P450 systems?
* Mitochondrial * Microsomal
65
What is the function of the mitochondrial cytochrome P450 system?
Synthesis of steroids, bile acids, and the active forms of vitamin D
66
What is the function of the microsomal cytochrome P450 system?
* Detoxification of foreign compounds * Activation or inactivation of drugs * Solubilization for excretion in the urine or feces
67
What are the enzymes used in the oxidative burst?
* NADPH oxidase: produces superoxide * Superoxidase dismutase: produces hydrogen peroxide * Myeloperoxidase: uses hydrogen peroxide to produce HOCl * Nitric oxide synthase: produces NO for conversion to ONOO–
68
What are the isozymes of NO synthase?
* Endothelial (eNOS): constitutive in endothelial cells * Neural (nNOS): constitutive in neural tissue * Inducible (iNOS): induced in macrophages in response to cytokines or bacterial endotoxins
69
How is NO synthesized?
ʟ-arginine + O2 + NADPH → ʟ-citrulline + NO + NADP+ Catalyzed by *NO synthase*
70
What are the functions of NO?
* Smooth muscle relaxant * Prevents platelet aggregation * Functions as a neurotransmitter in the brain * Medidates tumoricidal and bactericidal actions of macrophages
71
How does NO relax smooth muscles?
* NO is produced in the endothelial cells by *eNOS* * NO diffuses into vascular smooth muscle cells and activates cytosolic guanylyl cyclase * cGMP is produced * cGMP activates protein kinase G * Protein kinase G phosphorylates Ca2+ channels, causing decreased calcium entry * Decreased Ca2+ levels decrease smooth muscle contraction
72
How is G6PD deficiency inherited?
X-linked
73
What kind of mutation causes G6PD deficiency?
* Usually a point mutation leading to missense * It is caused by one of more than 400 different mutations, not all of which lead to disease
74
What is the epidemiology of G6PD deficiency?
200–400 million people are affected worldwide. Prevalent in: * Middle East * Tropical Africa * Southeast Asia * Mediterranean
75
What are the symptoms of G6PD deficiency?
* Hemolytic anemia * Resistance to falciparum malaria
76
What are the precipitating factors in G6PD deficiency?
* Oxidant drugs (AAA): * Antibiotics: e.g. sulfamethoxazole * Antimalarials: e.g. primaquine * Antipyretics: e.g. acetanilid * Favism due to vicine and covicine in fava beans (doesn't affect all patients) * Infection and generation of an oxidative burst * Neonatal jaundice
77
What is the pathogenesis of hemolytic anemia in G6PD deficiency?
* The deficiency leads to low NADPH * Low NADPH means oxidized glutathione cannot be reduced, leading to accumulation of ROS * Glutathione also functions in keeping the thiol (—SH) groups of proteins in the reduced state, e.g. in hemoglobin, so loss of glutathione activity causes denaturation * The RBC becomes more rigid
78
What are the variants of G6PD deficiency?
* B is the wild type * Mediterranean variant B– (class II) is due to a point mutation at bp 563 (C→T) with severe, episodic hemolytic anemia * African variant A– (class III) is due to 2 point mutations with moderate disease * African variant A (class IV) causes normal activity * Class I has very severe, chronic hemolytic anemia

Metabolism (6 decks)

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