Metabolism of Hexoses and Blood Glucose

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Questions and Answers

Which type of hypoglycemia is characterized by a stimulus that leads to low blood sugar levels?

Fasting hypoglycemia

Reactive hypoglycemia

Stimulative hypoglycemia (correct)

Prolonged fasting hypoglycemia

Which condition can cause fasting hypoglycemia due to enhanced insulin production?

Hepatic cirrhosis

Chronic nephritis

Hypothyroidism

Insulinoma (correct)

What is the primary factor that leads to glucosuria?

High insulin levels

Renal threshold for glucose exceeded (correct)

Excessive carbohydrate intake

Defective insulin receptor activity

Which of the following is NOT a cause of stimulative hypoglycemia?

Starvation (A) Signup and view all the answers

Which of the following conditions is categorized under fasting hypoglycemia?

Maple syrup urine disease (A) Signup and view all the answers

What is a common cause of renal glucosuria?

Concomitant diabetes (C) Signup and view all the answers

Which of the following statements about glucosuria is true?

It can indicate renal defects affecting glucose reabsorption. (D) Signup and view all the answers

What causes essential reactive hypoglycemia?

Exaggerated insulin response to carbohydrate ingestion (D) Signup and view all the answers

What is the primary enzyme responsible for converting fructose into fructose-1-P in the liver, kidney, and intestine?

Fructokinase (C) Signup and view all the answers

In which tissues is fructose metabolized by hexokinase instead of fructokinase?

Muscles and adipose tissues (C) Signup and view all the answers

What is the consequence of excess glucose being converted into sorbitol in the lens?

Development of cataracts (C) Signup and view all the answers

What percentage of the total body energy is derived from fructose?

15% (A) Signup and view all the answers

Which product is formed after fructose-1-P undergoes hydrolysis by aldolase B?

Dihydroxyacetone phosphate and glyceraldehyde (C) Signup and view all the answers

Which process occurs after glyceraldehyde is converted into glyceraldehyde-3-P?

Glycolysis (D) Signup and view all the answers

Which of the following is true regarding fructose metabolism in spermatozoa?

Fructose serves as the only source of energy for spermatozoa. (C) Signup and view all the answers

What is the function of triokinase in fructose metabolism?

Converts glyceraldehyde into glyceraldehyde-3-P (A) Signup and view all the answers

Which molecule is responsible for converting galactose to galactose-1-P in the liver?

Galactokinase (D) Signup and view all the answers

What is the normal fasting blood glucose range in mg/dl?

70 - 110 (D) Signup and view all the answers

Which substance is NOT a source of blood glucose?

Nucleotides (A) Signup and view all the answers

What condition follows untreated hypoglycemia?

Permanent cerebral damage (B) Signup and view all the answers

Which enzyme converts glucose-6-P to glucose-1-P?

Phosphoglucomutase (B) Signup and view all the answers

What is produced when UDP-galactose combines with glucose?

Lactose (A) Signup and view all the answers

Which of these is a major role of galactose in the body?

Structure of glycoproteins (D) Signup and view all the answers

Which statement about galactose metabolism is correct?

UDP-galactose is an intermediate in converting galactose to glucose. (B) Signup and view all the answers

Study Notes

Metabolism of Other Hexoses and Blood Glucose

This presentation covers the metabolism of hexoses, specifically fructose and galactose, and blood glucose regulation.
Key molecules discussed include glucose, fructose, galactose, and various enzymes involved in their metabolism.

Fructose Metabolism

Sources of fructose: Sucrose (table sugar), honey, fruits, and vegetables.
Importance of fructose: Contributes 15% of the body's energy, the only source for sperm energy.
Fructose metabolism in liver, kidney, and intestine:
- Tissues contain fructokinase, converting fructose to fructose-1-P.
- Aldolase B then hydrolyzes fructose-1-P into dihydroxyacetone phosphate and glyceraldehyde.
Fructose metabolism pathways are shown schematically.

Fructose Metabolism in Muscles and Adipose Tissues

These tissues lack fructokinase but contain hexokinase.
Hexokinase phosphorylates fructose into fructose-6-phosphate.
This fructose-6-phosphate then enters glycolysis, leading to energy production.

Glucose Conversion to Fructose

In specialised tissues (testes, lens, peripheral nerves, renal glomeruli), glucose is converted into fructose via the enzyme aldolase reductase.
The conversion involves a series of enzymatic reactions.
This conversion is also observed under diabetic conditions, where high glucose levels are associated with sorbitol formation leading to cataracts through increased osmotic pressure.

Galactose Metabolism

Sources of galactose: Dietary lactose (milk sugar).
Importance of galactose: Galactose is incorporated into various structures in the body, including lactose, glycolipids, glycoproteins, and proteoglycans.
Galactose metabolism pathway is detailed, showing galactose transformation within the liver, involving galactokinase, UDP-glucose, and UDP-galactose 4-epimerases steps to eventually integrate into glucose
A separate pathway shows galactose conversion to glucose in the mammary gland

Blood Glucose

Normal blood glucose range: Fasting blood glucose (FBG) is 70-110 mg/dL; 2-hour postprandial (PP) glucose is up to 140 mg/dL.
Sources of blood glucose:
- Dietary carbohydrates (glucose, galactose, and fructose).
- Gluconeogenesis (e.g., amino acids, lactate, glycerol).
- Liver glycogenolysis.

Variations in Blood Glucose: Hypoglycemia

Definition: A condition characterized by decreased plasma glucose below 60 mg/dL.
Symptoms: Faintness, dizziness, lethargy, potentially progressing to coma and death if untreated.
Causes: Two types:
- Stimulatory hypoglycemia: Occurs due to external factors such as drugs (insulin overdose), inborn errors of metabolism (Galactosemia, Hereditary fructose intolerance), post-gastrectomy, excessive alcohol intake, and reactive hypoglycemia.
- Fasting hypoglycemia: Results from problems producing or releasing glucose during a fast—this can result from starvation, malnutrition, Inborn Errors of Metabolism, pancreatic disease, or diseases affecting the liver or endocrine system.

Variations in Blood Glucose: Glucosuria

Definition: Glucose in the urine, detectable by standard methods.
Occurrence: Glucose concentrations in the blood surpassing the renal threshold for glucose (180 mg/dL).
Causes:
- Diabetes mellitus (high blood glucose due to insulin deficiency).
- Renal glucosuria (kidney dysfunction).

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Description

This quiz explores the metabolism of hexoses, focusing on fructose and galactose, along with blood glucose regulation. It covers key pathways, enzymatic processes, and the significance of these sugars in human metabolism. Test your knowledge on how these sugars are utilized in different tissues.