Podcast
Questions and Answers
Match the following monosaccharides with their metabolic pathways:
Match the following monosaccharides with their metabolic pathways:
Fructose = Converted to fructose-1-phosphate Galactose = Converted to galactose-1-phosphate Mannose = Metabolized via hexokinase pathway Glucose = Undergoes glycolysis directly
Match the following enzymes with their corresponding substrates or products:
Match the following enzymes with their corresponding substrates or products:
Fructokinase = Fructose Galactokinase = Galactose-1-phosphate Aldolase-B = Fructose-1-phosphate UDP-glucose pyrophosphorylase = Glucose-1-phosphate
Match the following disorders with their causes:
Match the following disorders with their causes:
Fructosuria = Deficiency of fructokinase Hereditary Fructose Intolerance = Deficiency of aldolase-B Galactosemia = Absence of galactose-1-phosphate uridyltransferase Fasting hypoglycemia = Accumulation of fructose-1-phosphate
Match the following glycolytic intermediates with their respective transformations:
Match the following glycolytic intermediates with their respective transformations:
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Match the following sources with the type of sugar they provide:
Match the following sources with the type of sugar they provide:
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Match the following key enzymes with their corresponding functions in the Citric Acid Cycle:
Match the following key enzymes with their corresponding functions in the Citric Acid Cycle:
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Match the following metabolites with their roles in metabolism:
Match the following metabolites with their roles in metabolism:
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Match the following conditions with their associated metabolic consequences:
Match the following conditions with their associated metabolic consequences:
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Match the following anaplerotic reaction catalysts with their corresponding substrates:
Match the following anaplerotic reaction catalysts with their corresponding substrates:
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Match the following metabolic signals with their regulatory effects on citric acid cycle enzymes:
Match the following metabolic signals with their regulatory effects on citric acid cycle enzymes:
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Study Notes
Metabolism of Other Monosaccharides
- Other monosaccharides besides glucose are crucial in vertebrates, including fructose, galactose, and mannose.
- These are common in oligosaccharides and polysaccharides, and act as energy sources, converted into glycolytic intermediates.
Fructose Metabolism
- Dietary Sources: Fruit, honey, and sucrose.
- Role in Diet: A significant carbohydrate source after glucose.
- Liver Conversion: Fructose is converted to fructose-1-phosphate by fructokinase in the liver.
- Glycolytic Pathway Entry: Before entering the glycolytic pathway, fructose-1-phosphate is split into dihydroxyacetone phosphate (DHAP) and glyceraldehyde by fructose-1-phosphate aldolase (Aldolase-B).
- DHAP Conversion: DHAP is converted to glyceraldehyde-3-phosphate by triose phosphate isomerase.
- Faster Metabolism: Bypasses regulatory steps involved in glucose metabolism, leading to faster fructose metabolism compared to glucose.
- Fructosuria: Inherited, asymptomatic disorder due to fructokinase deficiency, resulting in fructose appearing in urine.
- Hereditary Fructose Intolerance: Inherited disorder due to fructose-1-phosphate aldolase (aldolase-B) deficiency.
- Metabolic Effects: Accumulation of fructose-1-phosphate inhibits glycogen phosphorylase (glycogenolysis) leading to fasting hypoglycemia, especially after fructose ingestion.
- (If you asked me):* What are the dietary sources of fructose?
- (Answer):* Fruit, honey, and sucrose.
- (If you asked me):* How is fructose metabolized differently compared to glucose, and why?
- (Answer):* Fructose bypasses the regulatory steps in glucose metabolism, thus leading to faster metabolism compared to glucose.
Galactose Metabolism
- Similar Structure: Galactose and glucose are C4 epimers.
- Initial Conversion: Galactose is converted into galactose-1-phosphate by galactokinase.
- UDP-Galactose Formation: Galactose-1-phosphate is then transformed into UDP-galactose.
- Galactose-1-phosphate uridyltransferase: A crucial enzyme during fetal development and childhood for this conversion.
- Galactosemia: An inherited disorder caused by the absence of galactose-1-phosphate uridyltransferase.
- UDP-Glucose Formation: UDP-galactose is isomerized to UDP-glucose by UDP-galactose-4-epimerase.
- Glycolytic Pathway Entry: UDP-glucose is used in glycogen synthesis or converted to glucose-1-phosphate by UDP-glucose pyrophosphorylase, which then enters the glycolytic pathway after conversion to glucose-6-phosphate.
- Galactosemia Symptoms: Accumulation of galactose, galactose-1-phosphate, and galactitol (sugar alcohol derivative) lead to liver damage, cataracts, and mental retardation.
- Treatment: Early diagnosis and a galactose-free diet are the only effective treatments.
- (If you asked me):* What is the function of galactose-1-phosphate uridyltransferase?
- (Answer):* It converts galactose-1-phosphate to UDP-galactose, a critical step in galactose metabolism.
Citric Acid Cycle
- Function: Aerobic organisms use this cycle to release chemical energy from acetyl-CoA (a product of carbohydrate, lipid, and some amino acid breakdown).
- Acetyl-CoA Synthesis: From pyruvate, fatty acid catabolism, and certain amino acid reactions.
- Energy Transfer: Carbon atoms are oxidized to CO2; high-energy electrons are transferred to NAD+ and FAD forming reduced coenzymes NADH and FADH2, respectively.
- Net Reaction: Acetyl-CoA + 3 NAD+ + FAD + GDP + Pi + 2 H2O → 2 CO2 + 3 NADH + FADH2 + CoASH + GTP + 3 H+
- (If you asked me):* What is the overall purpose of the citric acid cycle?
- (Answer):* To release chemical energy stored in acetyl-CoA, producing CO2 and reduced coenzymes (NADH and FADH2).
Conversion of Pyruvate to Acetyl-CoA
- Location: Mitochondrial matrix.
- Mechanism: Catalyzed by the pyruvate dehydrogenase complex, a multienzyme structure with three activities: pyruvate dehydrogenase, dihydrolipoyl transacetylase, and dihydrolipoyl dehydrogenase.
- Coenzymes Required: Thiamine pyrophosphate (TPP), lipoic acid, NAD+, FAD, and CoASH.
- NADH Production: One molecule of NADH is produced for each pyruvate molecule converted.
- (If you asked me):* What are the three enzymes component of the pyruvate dehydrogenase complex?
- (Answer):* Pyruvate dehydrogenase, dihydrolipoyl transacetylase, and dihydrolipoyl dehydrogenase.
Succinate Thiokinase
- Reaction: Cleavage of succinyl-CoA to succinate, coupled to GDP phosphorylation into GTP (convertible to ATP).
- Enzyme: Succinate thiokinase is the enzyme catalyzing this reaction in mammals.
- (If you asked me):* What high energy compound is generated within the citric acid cycle and how? and what is the function of this compund.
- (Answer):* GTP is generated through succinyl-CoA cleavage. This is convertible to ATP and provides energy for various cellular processes.
Citric Acid Cycle Reactions (details)
- Citrate Formation: Acetyl-CoA combines with oxaloacetate to form citrate catalyzed by citrate synthase
- Isocitrate Formation: Citrate is reversibly converted into isocitrate by aconitase.
- α-Ketoglutarate Formation: Isocitrate is oxidized to α-ketoglutarate via isocitrate dehydrogenase, creating first NADH and CO2
- Succinyl-CoA Formation: α-ketoglutarate is converted to succinyl-CoA by the α-ketoglutarate dehydrogenase complex, releasing second NADH and CO2.
- Succinate Formation: Succinyl-CoA is cleaved to succinate by succinate thiokinase, coupling GDP to GTP (later ATP).
- Fumarate Formation: Succinate dehyrogenase oxidizes succinate into fumarate, creating FADH2.
- L-Malate Formation: Fumarase hydrates fumarate to L-malate.
- Oxaloacetate Formation: Malate dehydrogenase oxidizes L-malate to oxaloacetate, forming the third NADH produced in the citric acid cycle
Amphibolic Citric Acid Cycle
- Dual Function: Can function in both anabolic and catabolic pathways.
- Catabolism in Citric Acid Cycle: Acts to oxidize acetyl groups to CO2 and convert energy to reduced coenzymes.
- Anabolism in Citric Acid Cycle: Intermediates like α-Ketoglutarate are vital for biosynthetic pathways.
- Anaplerotic Reactions: Replenishment reactions for consumed intermediates.
- Example: Pyruvate carboxylase reaction, which is catalyzed by pyruvate carboxylase and plays critical role replenishing oxaloacetate when acetyl-CoA concentration is high.
- Pyruvate Carboxylase Deficiency: A fatal disease caused by a missing/defective enzyme in pyruvate to oxaloacetate conversion, characterized by mental retardation and metabolic issues especially in amino acids. A distinct characteristic of this disease is lactic aciduria.
Citric Acid Cycle Regulation
- Key Enzymes: Citrate synthase, isocitrate dehydrogenase, and α-ketoglutarate dehydrogenase are key regulatory enzymes.
- Stimulation/Inhibition: Citrate synthase is stimulated by high acetyl-CoA and oxaloacetate, inhibited by succinyl-CoA and citrate.
- Isocitrate Dehydrogenase Regulation: Stimulated by high ADP and NAD+, inhibited by high ATP and NADH.
- α-Ketoglutarate Dehydrogenase Regulation: Regulated closely due to various metabolic roles. Stimulated under low energy conditions to retain α-Ketoglutarate within the cycle and inhibited when NADH rises, thus directing α-ketoglutarate towards biosynthetic reactions.
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Description
This quiz explores the metabolism of other monosaccharides, focusing particularly on fructose. Learn about fructose sources, its conversion in the liver, and its unique metabolic pathway compared to glucose. It also highlights the significance of fructose in energy production in vertebrates.
