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Questions and Answers
What primarily distinguishes glycogen from fat as an energy storage molecule in animals?
Which statement about α-granules in the liver is true?
What is the primary function of glycogen stored in the liver?
During glycogenesis, what is required for the synthesis of glycogen?
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How does the branched structure of glycogen benefit its function in glucose storage?
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What enzyme deficiency is primarily responsible for the accumulation of galactose and subsequent galactosemia?
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What is the primary treatment approach for galactokinase deficiency?
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Which pathway primarily utilizes fructose in the body?
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What contributes to the development of cataracts in the lens regarding galactose accumulation?
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Which enzyme has a low affinity for fructose, leading to its limited phosphorylation?
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What is the primary dietary source of galactose?
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Which enzyme is responsible for converting galactose to galactose-1-phosphate?
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What is the significance of the enzyme galactose 1-phosphate uridyltransferase?
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What condition arises from the deficiency of galactose 1-phosphate uridyltransferase?
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What is the end product of the reaction catalyzed by UDP-hexose 4-epimerase?
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What metabolic pathway becomes active in the liver after 4 to 5 years of life?
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What is the effect of increased galactose levels in the blood?
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Why is galactose not considered an essential nutrient?
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What is the active form of skeletal glycogen phosphorylase?
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What triggers the conversion of phosphorylase b to phosphorylase a in muscle?
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Which substance signals for muscle contraction by activating phosphorylase b kinase?
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What does phosphoprotein phosphatase 1 (PP1) do to glycogen phosphorylase?
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How does glucose affect glycogen phosphorylase a activity in the liver?
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What is the role of glycogen synthase kinase 3 (GSK3)?
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What happens when insulin is present regarding GSK3 activity?
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What effect does elevated levels of AMP in the muscle have on glycogen metabolism?
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Which process does glucagon stimulate in the liver?
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What must occur before GSK3 can phosphorylate glycogen synthase?
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What is the role of aldose reductase in the sorbitol pathway?
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Which tissue types exhibit high activity of aldose reductase in uncontrolled diabetes?
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What consequence does the accumulation of sorbitol in cells primarily lead to?
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What is the defect in essential fructosuria?
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What happens in hereditary fructose intolerance?
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What test results would indicate essential fructosuria?
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Which statement about amino sugars is correct?
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What is the primary treatment approach for essential fructosuria?
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Study Notes
Galactose Metabolism
- Dietary Source: Milk and milk products are the primary source of galactose.
- Hydrolysis: Lactase, found in the intestinal mucosal cells, breaks down lactose into galactose and glucose.
- Lysosomal Degradation: Lysosomal degradation of glycoproteins and glycolipids also produces galactose.
- Liver Metabolism: Almost all galactose metabolism occurs in the liver.
- Galactose Tolerance Test: This test assesses liver function by measuring the body's ability to process galactose.
- UDP-Galactose: UDP-galactose is the active form of galactose used in various synthetic reactions.
Galactose Metabolism Steps
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Step 1: Galactokinase Reaction
- Galactokinase phosphorylates galactose to galactose-1-phosphate.
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Step 2: Galactose-1-Phosphate Uridyltransferase
- This is the rate-limiting enzyme in galactose metabolism.
- Galactose-1-phosphate reacts with UDP-glucose to form UDP-galactose and glucose-1-phosphate.
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Step 3: Epimerase Reaction
- UDP-hexose 4-epimerase converts UDP-galactose to UDP-glucose.
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Step 4: Alternate Pathway
- Galactose 1-phosphate pyrophosphorylase, active after 4-5 years of age, directly produces UDP-galactose.
- UDP-galactose can be epimerized to UDP-glucose.
Disorders of Galactose Metabolism
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Classical Galactosemia:
- Deficiency of galactose 1-phosphate uridyltransferase.
- Rare, congenital disease in infants.
- Inherited as an autosomal recessive disorder.
Salient Features of Classical Galactosemia
- Galactose-1-Phosphate Accumulation: Galactose-1-phosphate accumulates in the liver, inhibiting galactokinase and glycogen phosphorylase.
- Hypoglycemia: Galactose cannot be converted to glucose, leading to hypoglycemia.
- Insulin Secretion: Elevated galactose levels increase insulin secretion, further lowering blood glucose.
- Galactosemia and Galactosuria: Impaired metabolism results in increased galactose in the blood (galactosemia) and urine (galactosuria).
- Bilirubin Metabolism: Bilirubin uptake and conjugation are reduced.
Galactokinase Deficiency
- Enzyme Deficiency: Defect in the enzyme galactokinase.
- Galactosemia & Galactosuria: Results in galactosemia and galactosuria.
- Dulcitol Formation: Dulcitol (galactitol) is formed.
- Rare Cataracts: Cataract development is uncommon.
- Treatment: Dietary removal of galactose and lactose.
Fructose Metabolism
- Dietary Sources: Fruits, fruit juices, honey, and sucrose are dietary sources of fructose.
- Hydrolysis: Sucrase hydrolyzes sucrose in the intestine.
- Absorption: Fructose is absorbed by facilitated transport and delivered to the liver via the portal blood.
- Conversion to Glucose: The majority of fructose is converted to glucose.
Biomedical Importance of Fructose
- Energy Source: Fructose is easily metabolized and provides energy.
- Seminal Fluid: Seminal fluid is high in fructose, which spermatozoa use for energy.
- Diabetic Cataracts: Fructose metabolism through the sorbitol pathway might contribute to cataract formation in diabetics.
Fructose Metabolism Steps
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Phosphorylation:
- Hexokinase has a low affinity for fructose.
- Fructokinase, present in the liver, kidney, muscles, and intestine, phosphorylates fructose to fructose-1-phosphate.
- Aldolase B Cleavage: Aldolase B cleaves fructose-1-phosphate into glyceraldehyde and dihydroxyacetone phosphate (DHAP).
- Glyceraldehyde Phosphorylation: Triokinase phosphorylates glyceraldehyde to glyceraldehyde-3-phosphate.
- Glycolysis and Gluconeogenesis: DHAP and glyceraldehyde-3-phosphate enter glycolysis or gluconeogenesis.
Sorbitol/Polyol Pathway
- Glucose to Fructose: Converts glucose to fructose via sorbitol.
- Increased Activity in Uncontrolled Diabetes: The sorbitol pathway is more active in uncontrolled diabetes.
- Aldose Reductase: Aldose reductase reduces glucose to sorbitol using NADPH.
- Sorbitol Dehydrogenase: Sorbitol dehydrogenase oxidizes sorbitol to fructose using NAD+.
Sorbitol Pathway in Diabetes Mellitus
- Increased Glucose Entry: In uncontrolled diabetes, high amounts of glucose enter cells independent of insulin.
- Aldose Reductase Activity: Cells with high intracellular glucose levels (lens, retina, nerve cells, kidney) have high aldose reductase activity and NADPH supply.
- Sorbitol Accumulation: Efficient conversion of glucose to sorbitol occurs due to high aldose reductase activity.
- Low Sorbitol Dehydrogenase Activity: Sorbitol dehydrogenase activity is low or absent in these cells.
- Sorbitol Accumulation: Sorbitol cannot easily exit the cell membrane and accumulates.
- Osmotic Effects: Sorbitol's hydrophilic nature causes osmotic effects, leading to cell swelling.
- Pathological Changes: Sorbitol accumulation contributes to diabetic complications.
Defects in Fructose Metabolism
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Essential Fructosuria:
- Deficiency of hepatic fructokinase.
- Fructose is not converted to fructose 1-phosphate.
- Fructose is excreted in urine.
- Benedicts and Seliwanoff's tests are positive in urine.
- Treatment: Limiting dietary fructose.
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Hereditary Fructose Intolerance:
- Autosomal recessive disorder.
- Deficiency of aldolase-B.
- Fructose-1-phosphate cannot be metabolized.
- Fructose-1-phosphate accumulates and inhibits glycogen phosphorylase.
- Liver glycogen accumulation and hypoglycemia occur.
Symptoms of Hereditary Fructose Intolerance
- Vomiting:
- Loss of Appetite:
- Hepatomegaly:
- Jaundice:
- Possible Death: If liver damage progresses.
- Fructose in Urine:
- Treatment: Restricting dietary fructose.
Amino Sugars
- Hydroxyl Group Replacement: One or more hydroxyl groups are replaced by amino groups in monosaccharides.
- Examples: D-glucosamine, D-galactosamine, mannoseamine, sialic acid.
- Components: Constituents of glycosaminoglycans (GAGs), glycolipids, and glycoproteins.
- Presence in Oligosaccharides and Antibiotics: Found in some oligosaccharides and antibiotics.
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Description
This quiz covers key aspects of galactose metabolism, including its dietary sources, hydrolysis, and liver functions. It also explains the steps involved in galactose metabolization, such as the roles of galactokinase and galactose-1-phosphate uridyltransferase. Test your knowledge on this essential metabolic pathway!
