Harper's Biochemistry Chapter 20 - The Pentose Phosphate Pathway & Other Pathways of Hexose Metabolism
95 Questions
0 Views

Choose a study mode

Play Quiz
Study Flashcards
Spaced Repetition
Chat to lesson

Podcast

Play an AI-generated podcast conversation about this lesson

Questions and Answers

What role does thiamin diphosphate play in the reaction involving transketolase?

  • It activates transketolase by facilitating substrate binding.
  • It is necessary for the production of NADPH.
  • It acts as a coenzyme required for the transfer of the two-carbon unit. (correct)
  • It serves as a coenzyme providing an energy source.

    • Which tissue is NOT characterized by high activity of the pentose phosphate pathway?

  • Skeletal muscle (correct)
  • Adipose tissue
  • Testis
  • Erythrocytes

    • During the transaldolase reaction, a three-carbon moiety is transferred from which compound?

  • Ribose-5-phosphate
  • Sedoheptulose-7-phosphate (correct)
  • Fructose-6-phosphate
  • Glyceraldehyde-3-phosphate

    • What is a primary product of the pentose phosphate pathway that is utilized in reductive syntheses?

    <p>NADPH</p> Signup and view all the answers

    Which enzyme does NOT require a cofactor in its reaction as described?

    <p>Transaldolase</p> Signup and view all the answers

    In which cellular process is NADPH NOT typically used?

    <p>Glycolysis</p> Signup and view all the answers

    Which compound serves as the donor of glycoaldehyde during the reactions catalyzed by transketolase?

    <p>Xylulose 5-phosphate</p> Signup and view all the answers

    What is the significance of the transketolase reaction in the pentose phosphate pathway?

    <p>It transfers two-carbon units for nucleotide synthesis.</p> Signup and view all the answers

    What is the primary role of NADP+ in the oxidative phase of the pentose phosphate pathway?

    <p>To serve as an electron acceptor in oxidative reactions</p> Signup and view all the answers

    Which of the following enzymes is involved in the conversion of ribulose-5-phosphate back to glucose-6-phosphate?

    <p>Transketolase</p> Signup and view all the answers

    In what way does the pentose phosphate pathway differ from glycolysis?

    <p>It does not produce ATP</p> Signup and view all the answers

    What product is generated from the irreversible oxidative phase of the pentose phosphate pathway?

    <p>Ribulose-5-phosphate</p> Signup and view all the answers

    Which of the following statements about fructose metabolism is false in relation to the pentose phosphate pathway?

    <p>The pentose phosphate pathway only metabolizes glucose.</p> Signup and view all the answers

    What is the role of glucose-6-phosphate dehydrogenase in metabolism?

    <p>It initiates the pentose phosphate pathway.</p> Signup and view all the answers

    What are the end products of the complete oxidation of glucose in the pentose phosphate pathway?

    <p>Carbon dioxide and NADPH</p> Signup and view all the answers

    What condition results from a deficiency in the pentose phosphate pathway?

    <p>Essential pentosuria</p> Signup and view all the answers

    Which metabolite is generated alongside NADPH in the pentose phosphate pathway?

    <p>Xylulose-5-phosphate</p> Signup and view all the answers

    Which of the following describes a key difference between the pentose phosphate pathway and glycolysis?

    <p>Only glycolysis generates ATP directly.</p> Signup and view all the answers

    During the pentose phosphate pathway, how many molecules of carbon dioxide are produced from three molecules of glucose-6-phosphate?

    <p>Three</p> Signup and view all the answers

    What is the primary significance of glucuronic acid synthesis from glucose?

    <p>It plays a critical role in the metabolism of xenobiotics.</p> Signup and view all the answers

    What intermediate is generated after two molecules of glyceraldehyde-3-phosphate in the pentose phosphate pathway?

    <p>Glucose-6-phosphate</p> Signup and view all the answers

    Which enzyme's deficiency is primarily linked to hemolytic anemia?

    <p>Glucose-6-phosphate dehydrogenase</p> Signup and view all the answers

    In the context of the pentose phosphate pathway, what does the term 'xenobiotics' refer to?

    <p>Compounds that are foreign to the body</p> Signup and view all the answers

    What is the role of xylulose-5-phosphate in the metabolic pathways described?

    <p>Activates protein phosphatase that increases fatty acid synthesis.</p> Signup and view all the answers

    Which enzyme catalyzes the conversion of inactive cortisone to active cortisol?

    <p>11-β-hydroxysteroid dehydrogenase-1.</p> Signup and view all the answers

    What is the primary function of the pentose phosphate pathway related to NADPH?

    <p>It produces NADPH for biosynthetic reactions.</p> Signup and view all the answers

    How does ribose-5-phosphate contribute to nucleotide synthesis?

    <p>By serving as a substrate for nucleic acid synthesis.</p> Signup and view all the answers

    Which other molecule besides NADPH is generated during the pentose phosphate pathway?

    <p>Ribulose-5-phosphate.</p> Signup and view all the answers

    What effect does increased fructose-2,6-bisphosphate have on glycolytic activity?

    <p>It enhances glycolytic flux.</p> Signup and view all the answers

    What metabolic change initiates the synthesis of fatty acids according to the pathways described?

    <p>High carbohydrate diet leading to increased NADPH.</p> Signup and view all the answers

    What happens to ribulose-5-phosphate during the non-oxidative phase?

    <p>It is converted into ribose-5-phosphate and xylulose-5-phosphate.</p> Signup and view all the answers

    Which of the following is NOT a characteristic of the enzyme ribulose-5-phosphate 3-epimerase?

    <p>It directly synthesizes ATP.</p> Signup and view all the answers

    What process does trans-ketolase facilitate in carbohydrate metabolism?

    <p>Transfer of carbon units between sugars.</p> Signup and view all the answers

    What is the primary purpose of the pentose phosphate pathway?

    <p>To generate NADPH for biosynthetic reactions.</p> Signup and view all the answers

    Which enzyme is responsible for converting glucose-6-phosphate into 6-phosphogluconolactone?

    <p>Glucose-6-phosphate dehydrogenase</p> Signup and view all the answers

    In the pentose phosphate pathway, what role does NADP+ play?

    <p>It serves as an electron acceptor.</p> Signup and view all the answers

    What is the product of the reaction catalyzed by 6-phosphogluconate dehydrogenase?

    <p>NADPH and CO2</p> Signup and view all the answers

    Which phase of the pentose phosphate pathway generates ribose-5-phosphate?

    <p>Non-oxidative phase</p> Signup and view all the answers

    Which of the following statements is true regarding hexokinase and glucokinase?

    <p>Both catalyze the same reaction but differ in tissue distribution.</p> Signup and view all the answers

    What is the end product of the conversion of xylulose-5-phosphate in the non-oxidative phase of the pentose phosphate pathway?

    <p>Fructose-6-phosphate</p> Signup and view all the answers

    Which cofactor is commonly needed for the activity of enzymes in the pentose phosphate pathway?

    <p>Mg2+</p> Signup and view all the answers

    How is NADPH primarily used in metabolic pathways?

    <p>In reductive biosynthetic reactions.</p> Signup and view all the answers

    Which of the following carbohydrate molecules might be involved in the transketolase reaction?

    <p>Xylulose-5-phosphate</p> Signup and view all the answers

    What is produced during the oxidative phase of the polyol pathway?

    <p>NADPH</p> Signup and view all the answers

    In which tissues is the polyol pathway primarily active?

    <p>Kidneys and peripheral nerves</p> Signup and view all the answers

    Which enzyme catalyzes the conversion of glucose to sorbitol in the polyol pathway?

    <p>Aldose reductase</p> Signup and view all the answers

    Which of the following statements accurately describes the nonoxidative phase of the polyol pathway?

    <p>It produces ribose precursors for nucleotide synthesis.</p> Signup and view all the answers

    What is a primary risk associated with the accumulation of sorbitol in tissues?

    <p>Osmotic damage to cells</p> Signup and view all the answers

    Which enzyme catalyzes the transfer of a three-carbon moiety during the pentose phosphate pathway?

    <p>Transaldolase</p> Signup and view all the answers

    What role does NADPH play in the tissues where the pentose phosphate pathway is active?

    <p>Supporting reductive syntheses</p> Signup and view all the answers

    Which metabolic pathway is implicated in the synthesis of ribose for nucleic acids in most tissues?

    <p>Pentose phosphate pathway</p> Signup and view all the answers

    In which state might the activities of glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase be induced by insulin?

    <p>Fed state</p> Signup and view all the answers

    What type of compound is produced when xylose 5-phosphate acts as a donor of glycoaldehyde in the pentose phosphate pathway?

    <p>Fructose-6-phosphate</p> Signup and view all the answers

    What is the primary function of transketolase in the pentose phosphate pathway?

    <p>Transferring carbon moieties between sugars</p> Signup and view all the answers

    Which substrate serves as a primary source of ribose-5-phosphate for nucleic acid synthesis through the pentose phosphate pathway?

    <p>Glucose-6-phosphate</p> Signup and view all the answers

    The synthesis of which substance is a primary function of NADPH produced in the pentose phosphate pathway?

    <p>Fatty acids</p> Signup and view all the answers

    Which cellular process requires the activity of the pentose phosphate pathway to produce essential metabolites?

    <p>Nucleotide synthesis</p> Signup and view all the answers

    What is the effect of xylulose-5-phosphate on fructose-2,6-bisphosphate levels?

    <p>It increases the levels by activating protein phosphatase.</p> Signup and view all the answers

    What metabolic changes does ribulose-5-phosphate initiate in response to high carbohydrate diets?

    <p>It enhances fatty acid synthesis.</p> Signup and view all the answers

    Which enzyme is responsible for converting ribulose-5-phosphate into its epimer?

    <p>Ribulose-5-phosphate 3-epimerase</p> Signup and view all the answers

    What is a significant function of the enzyme 11-β-hydroxysteroid dehydrogenase-1?

    <p>It reduces cortisone to active cortisol.</p> Signup and view all the answers

    What role does transketolase play in the metabolism of carbohydrates?

    <p>Transfers a two-carbon unit and converts an aldose to a ketose.</p> Signup and view all the answers

    What is the consequence of increased activity of the bifunctional enzyme associated with fructose-2,6-bisphosphate?

    <p>Increased glycolytic flux.</p> Signup and view all the answers

    What product is ultimately generated by the conversion of ribose-5-phosphate in nucleic acid synthesis?

    <p>Ribonucleotides</p> Signup and view all the answers

    Which compound is generated alongside NADPH during the oxidative phase of the pentose phosphate pathway?

    <p>Ribulose-5-phosphate</p> Signup and view all the answers

    What is the primary health risk associated with excessive intake of fructose beyond recommended levels?

    <p>Diabetes</p> Signup and view all the answers

    In which pathway is glucuronic acid synthesized from glucose?

    <p>Uronic acid pathway</p> Signup and view all the answers

    What significant metabolic process does fructose bypass that glucose does not?

    <p>Glycolysis regulatory step catalyzed by phosphofructokinase</p> Signup and view all the answers

    Which of the following enzymes catalyzes the phosphorylation of fructose in the liver, kidney, and intestine?

    <p>Fructokinase</p> Signup and view all the answers

    What is a consequence of high fructose intake on lipid metabolism?

    <p>Increased fatty acid synthesis</p> Signup and view all the answers

    What compound is formed by the action of the enzyme that converts glucose-6-phosphate in the non-oxidative phase of the pentose phosphate pathway?

    <p>Ribulose-5-phosphate</p> Signup and view all the answers

    What is the primary role of NADPH generated from the pentose phosphate pathway?

    <p>Reductive biosynthesis</p> Signup and view all the answers

    What happens to the majority of dietary fructose in the human body?

    <p>Metabolized in the liver</p> Signup and view all the answers

    Which acid is synthesized more regularly from glucose in the uronic acid pathway, except in certain species including humans?

    <p>Ascorbic acid</p> Signup and view all the answers

    What health condition is NOT directly associated with excessive fructose consumption?

    <p>Hypoglycemia</p> Signup and view all the answers

    What metabolic condition can result from excessive fructose intake and its subsequent processing in the liver?

    <p>Hyperuricemia</p> Signup and view all the answers

    Which enzyme deficiency is responsible for essential fructosuria?

    <p>Fructokinase</p> Signup and view all the answers

    How does a high fructose intake directly affect the synthesis of ATP in the liver?

    <p>Decreases ATP synthesis</p> Signup and view all the answers

    What is the role of fructose in the synthesis of very low-density lipoproteins (VLDL)?

    <p>Stimulates the synthesis and secretion of VLDL</p> Signup and view all the answers

    What effect does the consumption of fruits high in pentoses have in individuals with pentosuria?

    <p>Enhanced xylulose excretion</p> Signup and view all the answers

    Which main pathway does fructose use to enter glycolysis in the liver?

    <p>Fructokinase pathway</p> Signup and view all the answers

    What potential effect does acute loading of the liver with glucose have on glucuronate and ascorbate?

    <p>Increases their synthesis</p> Signup and view all the answers

    What is a consequence of the sequestration of inorganic phosphate caused by a high intake of fructose?

    <p>Increased uric acid production</p> Signup and view all the answers

    What notable effect does the metabolism of fructose have on hypertriacylglycerolemia?

    <p>It contributes to increased levels of triacylglycerol</p> Signup and view all the answers

    Which of the following is a consequence of pentosuria after consuming fruits rich in pentoses?

    <p>Increased excretion of pentose metabolites in urine</p> Signup and view all the answers

    Match the following enzymes with their respective functions in the pentose phosphate pathway:

    <p>Transketolase = Transfers two-carbon units Transaldolase = Transfers three-carbon units Glucose-6-phosphate dehydrogenase = Catalyzes first reaction in oxidative phase 6-phosphogluconate dehydrogenase = Generates NADPH and ribulose-5-phosphate</p> Signup and view all the answers

    Match the following tissues with their activity in the pentose phosphate pathway:

    <p>Liver = High activity in metabolism Adipose tissue = Lipogenesis and NADPH generation Skeletal muscle = Low activity Lactating mammary gland = Active in nucleotide synthesis</p> Signup and view all the answers

    Match the products of the pentose phosphate pathway with their roles:

    <p>NADPH = Reductive biosynthesis Ribose-5-phosphate = Nucleotide synthesis Erythrose-4-phosphate = Amino acid synthesis Fructose-6-phosphate = Intermediates for glycolysis</p> Signup and view all the answers

    Match the following reactions with their associated enzymes:

    <p>Formation of ribulose-5-phosphate = 6-phosphogluconate dehydrogenase Transfer of xylose 5-phosphate = Transketolase Conversion of ketose to aldose = Transaldolase Reduction of NADP+ = Glucose-6-phosphate dehydrogenase</p> Signup and view all the answers

    Match the following molecules with their descriptions related to the pentose phosphate pathway:

    <p>Xylulose-5-phosphate = Serves as a donor for glycoaldehyde Glyceraldehyde-3-phosphate = Intermediates involved in synthesis Ribulose-5-phosphate = Essential for nucleotide synthesis Glycolytic intermediates = Involved in energy production</p> Signup and view all the answers

    Match the following enzymes with their respective functions:

    <p>Aldolase A = Found in all tissues Aldolase B = Predominant in liver Galactokinase = Phosphorylates galactose Lactose synthase = Catalyzes formation of lactose</p> Signup and view all the answers

    Match the following sugar derivatives with their roles in the body:

    <p>Galactose = Constituent of glycolipids Glucose = Precursor of amino sugars UDP-galactose = Formation of lactose Sialic acid = Component of glycoproteins</p> Signup and view all the answers

    Match the following terms with their corresponding descriptions:

    <p>Hexosamines = Major amino sugars including glucosamine Galactose-1-phosphate = Reactant in nucleotide synthesis N-acetylneuraminic acid = Principal sialic acid in human tissues Lactose = Sugar found in milk</p> Signup and view all the answers

    Match the following metabolic reactions with their corresponding enzymes:

    <p>Galactose-1-phosphate uridyl transferase = Forms UDP-galactose UDPGal 4-epimerase = Converts UDP-galactose to UDP-glucose Galactose phosphorylase = Catalyzes galactose phosphorylation Glucose-1-phosphate = Used in glycogen synthesis</p> Signup and view all the answers

    Match the following metabolic pathways with their key components:

    <p>Glycolipids = Synthesis requires galactose Proteoglycans = Contain amino sugars Glycoproteins = Involve UDP-galactose Lactose synthesis = Catalyzed by lactose synthase</p> Signup and view all the answers

    Study Notes

    Pentose Phosphate Pathway

    • The pentose phosphate pathway is active in liver, adipose tissue, adrenal cortex, thyroid, erythrocytes, testis and lactating mammary glands.
    • Pathway activity is low in non-lactating mammary glands and skeletal muscle.
    • The pentose phosphate pathway is involved in producing NADPH for reductive biosynthesis, which is used in the biosynthesis of various molecules:
      • Fatty acids
      • Steroids
      • Amino acids through glutamate dehydrogenase
      • Reduced glutathione
    • NADPH is also used by NADPH oxidase in phagocytes and neutrophils to destroy engulfed cells and bacteria using superoxide.
    • Ribose synthesis occurs in virtually all tissues.
    • Ribulose-5-phosphate serves as a substrate in the pentose phosphate pathway and can be converted into xylulose-5-phosphate and ribose-5-phosphate.
    • Genetic deficiency of glucose-6-phosphate dehydrogenase, the first enzyme of the pentose phosphate pathway, causes acute hemolysis of red blood cells.

    The oxidative phase of pentose phosphate pathway

    • The pentose phosphate pathway can be divided into two phases: the oxidative phase and the non-oxidative phase.
    • The oxidative phase involves the production of NADPH and CO2 using glucose-6-phosphate as a substrate.
    • The first step is catalyzed by glucose-6-phosphate dehydrogenase, which is NADP+ dependent and converts glucose-6-phosphate to 6-phosphogluconate.
    • 6-phosphogluconate is then decarboxylated by 6-phosphogluconate dehydrogenase , producing ribulose-5-phosphate.

    The non-oxidative phase of pentose phosphate pathway

    • The non-oxidative phase involves the reversible conversion of ribulose-5-phosphate back to glucose-6-phosphate.
    • This phase involves two key enzymes, transketolase and transaldolase.
    • Transketolase catalyzes the transfer of a 2-carbon unit, from a ketose sugar to an aldose sugar.
    • Transaldolase catalyzes the transfer of a 3-carbon unit, from a ketose sugar to an aldose sugar.
    • The end products of the non-oxidative phase are two molecules of glucose-6-phosphate and one molecule of glyceraldehyde-3-phosphate
    • The pathway can account for the complete oxidation of glucose as two molecules of glyceraldehyde-3-phosphate can regenerate glucose-6-phosphate.

    Coupling the pentose phosphate pathway with glycolysis

    • Xylulose-5-phosphate, a product of the pentose phosphate pathway, has regulatory roles in glycolysis.
    • Xylulose-5-phosphate activates protein phosphatase that dephosphorylates the 6-phosphofructo-2-kinase/fructose 2,6-bisphosphatase bifunctional enzyme.
    • This activation leads to an increase in fructose 2,6-bisphosphate production, which boosts the activity of phosphofructokinase-1, accelerating glycolytic flux.
    • Xylulose-5-phosphate also activates protein phosphatase, initiating the nuclear translocation and DNA binding of carbohydrate response element-binding protein, stimulating fatty acid synthesis.

    Reducing equivalents in reductive syntheses

    • Those tissues specializing in reductive syntheses generate reducing equivalents, such as NADPH, in the pentose phosphate pathway.
    • Glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase are involved in NADPH generation.
    • NADPH is required for hydroxylation (mixed function oxidase) reactions and 11-β-hydroxysteroid dehydrogenase-1.
    • The latter enzyme facilitates the conversion of inactive cortisone to active cortisol in the liver, nervous system, and adipose tissue.

    Pentose Phosphate Pathway & NADPH Generation

    • The pentose phosphate pathway generates NADPH and produces precursors for nucleotide synthesis.
    • NADPH is essential for reductive biosynthesis, like fatty acid and steroid synthesis, and for detoxification reactions.
    • NADPH acts as a reducing agent in the detoxification of reactive oxygen species.

    Non-oxidative Phase of the Pentose Phosphate Pathway

    • The non-oxidative phase includes transketolase and transaldolase reactions.
    • Transketolase uses thiamine diphosphate (Vitamin B1) as a cofactor.
    • Transaldolase transfers a three-carbon dihydroxyacetone unit.

    Regulation of The Pathway

    • Xylulose 5-phosphate activates the protein phosphatase that regulates the 6-phosphofructo-2-kinase/fructose 2,6-bisphosphatase enzyme.
    • This leads to increased formation of fructose 2,6-bisphosphate, which activates phosphofructokinase-1 and increases glycolytic flux.
    • Xylulose 5-phosphate also activates the protein phosphatase involved in nuclear translocation and DNA binding of the carbohydrate response element-binding protein, boosting fatty acid synthesis in response to high-carbohydrate diets.

    Tissues Actively Using NADPH

    • The pentose phosphate pathway is actively functioning in tissues that require NADPH for reductive biosynthesis, including:
      • Liver
      • Adipose tissue
      • Adrenal cortex
      • Thyroid
      • Erythrocytes
      • Testis
      • Lactating mammary gland

    Synthesis of Ribose

    • Ribos is not circulated in the bloodstream.
    • Tissues synthesize the ribose they need for nucleotide and nucleic acid synthesis through the pentose phosphate pathway.

    Fructose Metabolism

    • Fructose enters glycolysis at a different point than glucose, bypassing the regulatory step catalyzed by phosphofructokinase.
    • Fructose metabolism in the liver can increase fatty acid synthesis, triacylglycerol esterification, and VLDL secretion.
    • High fructose intake can contribute to hypertriacylglycerolemia, hypercholesterolemia and hyperuricemia.

    Uronic Acid Pathway

    • Converts glucose to glucuronic acid, ascorbic acid (except in humans), and pentoses.
    • Glucuronic acid is a precursor for proteoglycans and conjugated glucuronides.
    • It represents an alternative pathway for glucose oxidation, not producing ATP.

    Galactose Metabolism

    • Galactose can be converted to glucose through the Leloir pathway.
    • Deficiencies in the Leloir pathway, such as the absence of galactokinase or galactose-1-phosphate uridyl transferase, lead to galactosemia.

    Pentose Phosphate Pathway

    • The pentose phosphate pathway is active in liver, adipose tissue, adrenal cortex, thyroid, erythrocytes, testis, and lactating mammary gland.
    • The pathway is less active in non-lactating mammary gland and skeletal muscle.
    • Tissues with high pathway activity utilize NADPH for reductive processes, including fatty acid and steroid synthesis, amino acid synthesis via glutamate dehydrogenase, and reduced glutathione.
    • The synthesis of glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase can be induced by insulin in a fed state, leading to increased lipogenesis.
    • NADPH is also used by NADPH oxidase in phagocytes and neutrophils for destruction of cells and bacteria using superoxide.

    Ribose Synthesis

    • Ribose synthesis occurs in virtually all tissues.
    • Ribose is not a major component of blood; tissues must synthesize their own ribose for nucleotide and nucleic acid synthesis.
    • This synthesis occurs primarily through the pentose phosphate pathway.

    Galactose

    • Galactose is derived from lactose, the sugar in milk, following intestinal hydrolysis.
    • It is efficiently converted to glucose in the liver after transport by GLUT5.
    • The majority of dietary galactose is metabolized in the liver.
    • Galactose is required for the formation of lactose in lactation and as a constituent of glycolipids, proteoglycans, and glycoproteins.
    • Galactokinase catalyzes the phosphorylation of galactose using ATP as a phosphate donor.
    • Galactose-1-phosphate reacts with UDPGlc to form UDPGal and glucose-1-phosphate, catalyzed by galactose-1-phosphate uridyl transferase.
    • UDPGal is then converted to UDPGlc by UDPGal 4-epimerase, utilizing NAD+ as a coenzyme.
    • UDPGlc is incorporated into glycogen.

    Glucose-6-phosphate Dehydrogenase Deficiency

    • Several mutations in the gene for glucose-6-phosphate dehydrogenase lead to two primary consequences.
    • This deficiency can cause red cell hemolysis, particularly under oxidative stress.
    • Oxidative stress occurs from infection, drugs such as primaquine, sulfonamides, or from eating fava beans, leading to favism.

    Amino Sugars

    • Amino sugars, including glucosamine, galactosamine, mannosamine, and sialic acid, are important components of glycoproteins, glycosphingolipids, and glycosaminoglycans.
    • N-acetylneuraminic acid (NeuAc) is the principal sialic acid found in human tissues.
    • The synthesis of amino sugars involves several steps, including phosphorylation, acetylation, and epimerization.

    Xylulose and Pentosuria

    • Xylulose is a pentose sugar that is typically reduced to xylitol in the body, but a deficiency in xylulose reductase leads to pentosuria.
    • Pentosuria does not have clinical consequences.
    • However, xylulose is a reducing sugar and can produce false positive results with alkaline copper reagents used to test for urinary glucose.
    • Certain drugs, like barbiturates, increase the production of xylulose through the uronic acid pathway.
    • Increased consumption of fruits high in pentoses can also lead to pentosuria.

    Fructose Metabolism

    • Fructose enters glycolysis via fructokinase.
    • The resulting fructose-1-phosphate can lead to phosphate sequestration and reduced ATP synthesis.
    • This can cause hyperuricemia and gout.
    • High oral doses of fructose can cause osmotic diarrhea.
    • Fructose in the liver increases fatty acid and triacylglycerol synthesis as well as VLDL secretion, leading to hypertriacylglycerolemia and increased LDL cholesterol.

    Disorders of Fructose Metabolism

    • A deficiency in hepatic fructokinase leads to essential fructosuria, a benign condition with no symptoms.

    The Polyol Pathway

    • The polyol pathway, not present in the liver, is responsible for fructose production from glucose.
    • The pathway is activated by high glucose concentrations in tissues that are not insulin sensitive.
    • This includes the lens, peripheral nerves, and renal glomeruli, which can lead to sorbitol accumulation and osmotic damage.

    The Uronic Acid Pathway

    • The uronic acid pathway is responsible for the synthesis of glucuronic acid, which is used to conjugate endogenous and exogenous substances for excretion in bile and urine.
    • Fructose bypasses the main regulatory step in glycolysis, catalyzed by phosphofructokinase.
    • It stimulates liver glucose uptake, fatty acid synthesis, and hepatic triacylglycerol secretion.

    Studying That Suits You

    Use AI to generate personalized quizzes and flashcards to suit your learning preferences.

    Quiz Team

    Related Documents

    Harper's Biochemistry Chapter 20 - The Pentose Phosphate Pathway & Other Pathways of Hexose Metabolism.PDF

    Description

    This quiz covers the key functions and significance of the pentose phosphate pathway in various tissues. Learn about its role in producing NADPH and ribose, and the effects of genetic deficiencies associated with this pathway.

    More Like This

    Biochemistry: Pentose Phosphate Pathway
    13 questions

    Biochemistry: Pentose Phosphate Pathway

    SalutaryCherryTree avatar
    SalutaryCherryTree
    NADPH and Pentose Phosphate Pathway
    10 questions

    NADPH and Pentose Phosphate Pathway

    ProperDenver avatar
    ProperDenver
    Pentose Phosphate Pathway Overview
    16 questions

    Pentose Phosphate Pathway Overview

    MerryBoron avatar
    MerryBoron
    Pentose Phosphate Pathway - Lecture 23
    10 questions

    Pentose Phosphate Pathway - Lecture 23

    StylizedVitality6510 avatar
    StylizedVitality6510
    Use Quizgecko on...
    Browser
    Open
    Browser
    Browser