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Questions and Answers
What is the primary organ where gluconeogenesis occurs?
What is the primary organ where gluconeogenesis occurs?
Which of the following substances can be a starting material for gluconeogenesis?
Which of the following substances can be a starting material for gluconeogenesis?
Which enzyme is responsible for converting pyruvate to oxaloacetate in gluconeogenesis?
Which enzyme is responsible for converting pyruvate to oxaloacetate in gluconeogenesis?
Which of the following tissues primarily relies on glucose as an energy source?
Which of the following tissues primarily relies on glucose as an energy source?
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Which of the following enzymes is NOT part of the gluconeogenesis pathway?
Which of the following enzymes is NOT part of the gluconeogenesis pathway?
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What is one of the key functions of gluconeogenesis in the body?
What is one of the key functions of gluconeogenesis in the body?
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In which cellular compartment does the conversion of phosphoenol pyruvate to oxaloacetate occur?
In which cellular compartment does the conversion of phosphoenol pyruvate to oxaloacetate occur?
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What role does lactic acid play in gluconeogenesis?
What role does lactic acid play in gluconeogenesis?
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What is the product of the conversion of fructose 1,6 biphosphate and water?
What is the product of the conversion of fructose 1,6 biphosphate and water?
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Which gluconeogenic substance is specifically mentioned to occur only in ruminants?
Which gluconeogenic substance is specifically mentioned to occur only in ruminants?
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How many ATP molecules are consumed during the conversion of 2 molecules of pyruvate to glucose?
How many ATP molecules are consumed during the conversion of 2 molecules of pyruvate to glucose?
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What role does glucagon play in gluconeogenesis?
What role does glucagon play in gluconeogenesis?
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What is the function of fructose 1,6 biphosphatase in gluconeogenesis?
What is the function of fructose 1,6 biphosphatase in gluconeogenesis?
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Which hormone is known to repress gluconeogenesis by affecting enzyme synthesis?
Which hormone is known to repress gluconeogenesis by affecting enzyme synthesis?
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What is the final product formed from glycerol during gluconeogenesis?
What is the final product formed from glycerol during gluconeogenesis?
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What modification occurs to pyruvate kinase in response to glucagon?
What modification occurs to pyruvate kinase in response to glucagon?
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Study Notes
Gluconeogenesis Definition
- Gluconeogenesis is the synthesis of glucose or glycogen from non-carbohydrate sources
- Sources include lactic acid, pyruvic acid, propionic acid, glycerol, and amino acids
Gluconeogenesis Site
- Primarily occurs in the liver (90%) and kidneys (10%)
- Cellular site includes cytoplasm and mitochondria
Importance of Gluconeogenesis
- Provides blood glucose during fasting (more than 18 hours) when glycogen stores are depleted
- Only energy source for nervous tissue, RBCs, and skeletal muscles during exercise
- Precursor of milk sugar (lactose) in mammary glands
- Clears waste products like lactic acid from blood
Steps in Gluconeogenesis
- Reverses glycolysis, excluding three irreversible steps
- Overcomes these steps using different enzymes
- Pyruvate kinase is replaced with pyruvate carboxylase (mitochondrial) and phosphoenolpyruvate carboxykinase
- Phosphofructokinase-1 is replaced with fructose 1,6-bisphosphatase
- Hexokinase/glucokinase is replaced with glucose-6-phosphatase
Conversion of Pyruvate to Phosphoenolpyruvate
- Pyruvate, initially in the mitochondria, must be transported into the cytoplasm
- Then:
- Pyruvate carboxylase first converts pyruvate to oxaloacetate. (Requires CO2, ATP, Mn2+, biotin)
- In the cytoplasm, oxaloacetate is converted to phosphoenolpyruvate by phosphoenolpyruvate carboxykinase (Requires GTP)
Conversion of Fructose 1,6-bisphosphate to Fructose 6-Phosphate
- Achieved by the enzyme fructose 1,6-bisphosphatase
Conversion of Glucose-6-Phosphate to Glucose
- Utilized by the enzyme glucose-6-phosphatase
Gluconeogenic Substances
- Amino acids are key gluconeogenic substances
- Pyruvate and lactate
- Propionate (only in ruminants – converted to succinyl CoA)
- Glycerol (derived from lipids, converted to glycerol-3-phosphate, then dihydroxyacetone phosphate, which then enters the common pathway)
Energy Cost of Gluconeogenesis
- 6 ATP molecules and 2 NADH+H+ are required to convert 2 pyruvate molecules into glucose
Regulation of Gluconeogenesis
- Gluconeogenesis and glycolysis are regulated to prevent simultaneous activation:
- Key regulatory enzymes: pyruvate carboxylase, phosphoenolpyruvate carboxykinase, fructose 1,6-bisphosphatase, glucose-6-phosphatase
- Regulated by changing enzyme synthesis rates or covalent modification, or allosteric regulation
Regulation - Changes in Enzyme Synthesis
- Glucocorticoids (e.g., cortisol) stimulate gluconeogenesis by inducing key enzyme synthesis and increasing protein catabolism
- Insulin inhibits gluconeogenesis by repressing key enzyme synthesis
Regulation - Covalent Modification
- Glucagon and epinephrine stimulate gluconeogenesis by phosphorylating pyruvate kinase (inactivating it) and thus inhibiting glycolysis
Regulation - Allosteric Regulation
- Acetyl CoA is an allosteric activator of pyruvate carboxylase, stimulating gluconeogenesis
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Description
This quiz covers the key aspects of gluconeogenesis, including its definition, sites of occurrence, importance, and the steps involved. Understand how glucose is synthesized from non-carbohydrate sources and learn the role of this process in maintaining blood glucose levels during fasting.
