Fructose and Hexose Metabolism
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Questions and Answers

What is the initial product formed from the conversion of fructose in the liver?

  • Fructose-1-P (correct)
  • Dihydroxyacetone phosphate
  • Glyceraldehyde
  • Fructose-6-P

Which enzyme is responsible for converting glyceraldehyde into glyceraldehyde-3-P?

  • Fructokinase
  • Hexokinase
  • Aldolase B
  • Triokinase (correct)

How is fructose metabolized in muscles and adipose tissues?

  • Through fructokinase
  • Only partially metabolized
  • It is directly converted into glucose
  • Through hexokinase (correct)

What unique role does fructose serve in the human body?

<p>It is the only energy source for spermatozoa (D)</p> Signup and view all the answers

What metabolic pathway does glyceraldehyde-3-P enter after its formation?

<p>Glycolysis (B)</p> Signup and view all the answers

Which process occurs due to excess glucose in diabetes mellitus?

<p>Formation of sorbitol causing cataracts (D)</p> Signup and view all the answers

What is the outcome of combining dihydroxyacetone phosphate with glyceraldehyde-3-P?

<p>Formation of glucose (A)</p> Signup and view all the answers

Which enzyme is deficient in muscles and adipose tissues for fructose metabolism?

<p>Fructokinase (A)</p> Signup and view all the answers

What is the primary source of galactose in the diet?

<p>Lactose (A)</p> Signup and view all the answers

Which enzyme is responsible for the conversion of galactose to galactose-1-P in the liver?

<p>Galactokinase (D)</p> Signup and view all the answers

Which of the following conditions can lead to fasting hypoglycemia?

<p>Liver cirrhosis (C), Galactosemia (D)</p> Signup and view all the answers

What is a common cause of stimulative hypoglycemia?

<p>Insulin overdose (A)</p> Signup and view all the answers

What essential biochemical process occurs in the liver involving galactose?

<p>Conversion to glucose (C)</p> Signup and view all the answers

Which of the following does NOT contribute to glucosuria?

<p>Excessive insulin production (A)</p> Signup and view all the answers

Which product is formed directly from the combination of glucose and UDP-galactose?

<p>Lactose (C)</p> Signup and view all the answers

What is the normal fasting blood glucose level range?

<p>70 - 110 mg/dl (B)</p> Signup and view all the answers

What is the renal threshold for glucose concentration in blood?

<p>180 mg/dl (A)</p> Signup and view all the answers

What condition is defined by plasma glucose levels dropping below 60 mg/dl?

<p>Hypoglycemia (C)</p> Signup and view all the answers

Which disorder is associated with defective glucose production leading to hypoglycemia?

<p>Chronic pancreatitis (B), Hypothyroidism (D)</p> Signup and view all the answers

Which substance is NOT a direct source of blood glucose?

<p>Acetic acid (C)</p> Signup and view all the answers

What is a characteristic of essential reactive hypoglycemia?

<p>Exaggerated insulin response (B)</p> Signup and view all the answers

Which of the following can occur during pregnancy, affecting glucose levels?

<p>Increased insulin resistance (A)</p> Signup and view all the answers

Which process describes the conversion of glycogen back to glucose?

<p>Glycogenolysis (C)</p> Signup and view all the answers

What condition is categorized under stimulative hypoglycemia due to metabolic errors?

<p>Maple syrup urine disease (A)</p> Signup and view all the answers

Flashcards

Galactose Metabolism

The process of converting and using galactose in the body

Galactose Sources (Dietary)

Galactose primarily comes from lactose (milk sugar), which breaks down into glucose and galactose.

Galactose's Role

Galactose is a component of lactose, glycolipids, glycoproteins, and proteoglycans.

Galactose to Glucose Conversion (Liver)

The liver converts galactose to glucose using enzymes like galactokinase and galactose-1-phosphate uridyltransferase.

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Blood Glucose (Normal)

Normal fasting blood glucose (FBG) is 70-110 mg/dL; 2-hour post-prandial (after-meal) is up to 140 mg/dL.

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Blood Glucose Sources

Blood glucose comes from dietary carbs (glucose, galactose, fructose), gluconeogenic substances, and liver glycogen.

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Hypoglycemia

Lower than normal blood glucose (below 60 mg/dL).

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Hypoglycemia Symptoms

Symptoms include faintness, dizziness, lethargy, and potentially coma.

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Glucose to Galactose Conversion (Mammary Gland)

Mammary glands convert glucose to galactose to synthesize lactose.

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Lactose Synthase

Enzyme responsible for lactose synthesis.

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Fructose Metabolism

The process of breaking down fructose for energy in the body.

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Fructose Sources

Fructose comes from sucrose (table sugar), fruits, and honey, among other sources.

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Fructose in Liver/Kidney/Intestine

Fructose is phosphorylated by fructokinase to fructose-1-phosphate then cleaved by aldolase B to form dihydroxyacetone phosphate and glyceraldehyde.

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Fructose in Muscles/Adipose Tissues

Hexokinase phosphorylates fructose to fructose-6-phosphate, which enters glycolysis.

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Fructose in Testes/Lens/Nerves/Glomeruli

Glucose is converted to fructose via the sorbitol pathway using aldose reductase and dehydrogenase.

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Sorbitol Pathway

A pathway that converts glucose to sorbitol and then to fructose.

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Fructose-1-P

Intermediate compound in fructose metabolism, critical for energy production and glucose synthesis

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Hexokinase

Enzyme that phosphorylates fructose to fructose-6-phosphate

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Aldolase B

Enzyme that cleaves fructose-1-phosphate into two smaller sugars

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Diabetes Mellitus and Sorbitol Pathway

High glucose levels lead to an overproduction of sorbitol, potentially causing cataracts and complications in specific tissue types

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Hypoglycemia Causes

Low blood sugar, categorized into stimulative and fasting types.

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Stimulative Hypoglycemia

Hypoglycemia caused by factors that stimulate the body to produce excessive insulin or reduce glucose production.

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Fasting Hypoglycemia

Hypoglycemia during fasting due to failure to maintain blood glucose which is usually regulated for those periods.

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Stimulus for Stimulative Hypoglycemia

External factors like drugs, inborn errors of metabolism, post-gastric surgery, alcohol consumption, or idiopathic conditions might disrupt normal glucose regulation causing hypoglycemia.

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Fasting Hypoglycemia Causes

Conditions like starvation, malnutrition, inborn errors of metabolism (e.g., dicarboxylic aciduria, maple syrup urine disease), pancreatic diseases (e.g., insulinoma), and defective glucose production due to liver/kidney/endocrine diseases can cause fasting hypoglycemia.

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Glucosuria Definition

Glucose present in urine, exceeding the kidney's ability to reabsorb glucose.

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Glucosuria Causes

Caused by diabetes mellitus (high blood sugar), renal glucosuria (low renal threshold), certain diseases (like Fanconi's syndrome/pregnancy), or drugs.

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Study Notes

Metabolism of Other Hexoses and Blood Glucose

  • Topic: Metabolism of other hexoses and blood glucose.
  • Location: Medical Biochemistry and Molecular Biology Department, Faculty of Medicine - Menoufia University

Fructose Metabolism

  • Sources of fructose: Sucrose, diet (honey, fruits, vegetables)
  • Importance of fructose: 15% of total body energy, only source of energy for spermatozoa.
  • Fructose metabolism in the liver, kidney, and intestine:
    • Contains fructokinase, which converts fructose into fructose-1-P.
    • Fructose-1-P is hydrolyzed by aldolase B to form dihydroxyacetone phosphate and glyceraldehyde.

Fructose Metabolism in Muscles and Adipose Tissues

  • These tissues lack fructokinase but contain hexokinase.
  • Fructose is phosphorylated by hexokinase to fructose-6-phosphate.
  • Enters glycolysis, generating 2 pyruvate molecules and energy.

Glyceraldehyde

  • Triokinase converts glyceraldehyde to glyceraldehyde-3-P.
  • Glyceraldehyde-3-P enters glycolysis and combines with dihydroxyacetone phosphate to yield glucose.

Glucose Conversion to Fructose in Specific Tissues

  • In testes, lenses, peripheral nerves, and renal glomeruli, glucose is converted to fructose via aldolase reductase and sorbitol dehydrogenase.
  • In diabetes mellitus, excess glucose is converted into sorbitol in the lens, which can cause cataracts.

Galactose Metabolism

  • Sources of galactose: Dietary milk (lactose)
  • Importance of galactose: Structural component of lactose, glycolipids, glycoproteins, and proteoglycans.
  • Galactose conversion to glucose in the liver:
    • Galactose is phosphorylated by galactokinase into galactose-1-phosphate.
    • Galactose-1-phosphate is transformed into UDP-galactose and glucose using uridyl transferase and UDP-galactose-4-epimerase.

Conversion of Glucose to Galactose in Mammary Gland

  • Glucose, using hexokinase, is transformed to glucose-6-phosphate, and then glucose-1-phosphate.
  • Glucose-1-phosphate is converted to UDP-glucose and using uridyl transferase eventually resulting in UDP-galactose via UDP-galactose 4-epimerase forming UDP-glucose and lactose.

Normal Blood Glucose Levels

  • Fasting blood glucose (FBG): 70-110 mg/dL
  • 2-hour postprandial glucose (2-hr PP): up to 140 mg/dL
  • Sources of blood glucose: Dietary carbohydrates (glucose, galactose, fructose), gluconeogenic substances (e.g., amino acids, lactate, glycerol), liver glycogen (glycogenolysis).

Variations in Blood Glucose: Hypoglycemia

  • Definition: Plasma glucose below 60 mg/dL.
  • Symptoms: Faintness, dizziness, lethargy, progressing to coma and permanent cerebral damage if untreated.
  • Causes: Stimulative hypoglycemia (drugs, inborn errors of metabolism), post-gastrectomy, excess alcohol intake, deficient glucose production (endocrine disease, liver disease, renal disease) and fasting hypoglycemia (starvation, malnutrition, inborn errors of metabolism, pancreatic disease).

Glucosuria

  • Definition: Glucose in urine detectable by ordinary methods.
  • Occurrence: When venous blood glucose exceeds the renal threshold (180 mg/dL).
  • Causes: Diabetes mellitus, Renal glucosuria (low renal threshold, inherited defects, drugs, pregnancy, Fanconi's syndrome).

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Related Documents

Lecture 5: Metabolism of Other Hexoses and Blood Glucose PDF

Description

This quiz explores the metabolism of hexoses, specifically focusing on fructose. It covers the pathways of fructose metabolism in various organs, its role in energy production, and how it interacts with other metabolic processes, including glycolysis. Test your knowledge on the biochemical transformations of fructose and its significance in human physiology.

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