Study Notes on Carbohydrate Metabolism and Metabolic Control
Chapters 14-15: Carbohydrate Metabolism and Metabolic Control
Carbohydrate Metabolic Pathways
Overview of Pathways
Glycolysis: The metabolic pathway that converts glucose into pyruvate, producing ATP and NADH.
Gluconeogenesis: The process of synthesizing glucose from non-carbohydrate precursors.
Pentose Phosphate Pathway: A metabolic pathway parallel to glycolysis that produces NADPH and ribose 5-phosphate.
Glycogen Metabolism: Involves the synthesis (glycogenesis) and breakdown (glycogenolysis) of glycogen.
Extracellular Matrix and Cell Wall Polysaccharides: These are structural polymers composed of glycogen, starch, and sucrose.
Carbohydrate Storage and Oxidation
Storage Forms: Glycogen, starch, and sucrose act as storage entities.
Oxidative Pathways: Include pathways like the pentose phosphate pathway that lead to the oxidation of glucose.
Essential Features of Glycolysis
General Characteristics
Glycolysis is active in most cells, having the largest carbon flux of most metabolic pathways.
It consists of ten enzymatic reactions, which show varying rates in different types of cells.
Divided into two phases:
Preparatory Phase: Converts glucose to two molecules of glyceraldehyde 3-phosphate (G-3-P) while consuming ATP.
Payoff Phase: Converts G-3-P to pyruvate, producing ATP and NADH.
Products of Glycolysis
The end products of glycolysis are pyruvate, ATP, and NADH.
Fates of Pyruvate: Depends on conditions (aerobic vs anaerobic) which can lead to either Acetyl-CoA for the citric acid cycle, lactate, or ethanol production.
Preparatory Phase of Glycolysis
Step 1: Phosphorylation of Glucose
Enzyme: Hexokinase
Reaction: Glucose + ATP → Glucose-6-Phosphate (G6P)
Function: Traps glucose in the cell and lowers intracellular glucose concentration.
Step 2: Isomerization
Enzyme: Phosphohexose Isomerase
Converts Glucose-6-Phosphate to Fructose-6-Phosphate.
Step 3: Second Priming Reaction (Phosphorylation)
Enzyme: Phosphofructokinase-1 (PFK-1)
Involves another ATP to convert Fructose-6-Phosphate to Fructose-1,6-bisphosphate.
Highly regulated and considered the commitment step of glycolysis.
Step 4: Aldol Cleavage
Enzyme: Aldolase
Converts Fructose-1,6-bisphosphate into Glyceraldehyde 3-Phosphate and Dihydroxyacetone phosphate.
Payoff Phase of Glycolysis
Energy Yielding Steps
Step 5: Interconversion of Triose Phosphates
Enzyme: Triose Phosphate Isomerase
Step 6: Oxidative Phosphorylation
Enzyme: Glyceraldehyde 3-Phosphate Dehydrogenase
Converts Glyceraldehyde 3-Phosphate to 1,3-Bisphosphoglycerate, reducing NAD+ to NADH.
This step represents the production of a high-energy mixed anhydride.
Step 7: First ATP-Forming Reaction
Enzyme: Phosphoglycerate Kinase
Converts 1,3-BPG to 3-Phosphoglycerate, generating ATP (substrate-level phosphorylation).
Step 8-10: Further Reactions
Produce additional ATP through substrate-level phosphorylation, finally yielding pyruvate.
Enzymes involved: Phosphoglycerate Mutase, Enolase, and Pyruvate Kinase.
Overall Energy Calculations
The net production of glycolysis involves:
4 ATP formed (2 net gain)
2 NADH produced
Regulation of Glycolysis
Key Regulatory Enzymes
Hexokinase: Inhibited by its product G6P.
PFK-1: The main control point of glycolysis, activated by AMP and fructose-2,6-bisphosphate, inhibited by ATP and citrate.
Pyruvate Kinase: Activated by fructose-1,6-bisphosphate and inhibited by ATP and acetyl-CoA.
Gluconeogenesis
Overview and Importance
Definition: The synthesis of glucose from non-carbohydrate substrates, primarily occurring in the liver.
Daily Requirement: At least 160g needed per day, with 75% utilized by the brain.
Substrates Used: Lactate, glycerol, amino acids, and citric acid cycle intermediates.
Key Enzymes of Gluconeogenesis
Pyruvate Carboxylase: Converts pyruvate to oxaloacetate in mitochondria (requires biotin).
PEP Carboxykinase: Converts oxaloacetate to phosphoenolpyruvate.
Fructose-1,6-bisphosphatase: A key regulatory step that hydrolyzes fructose-1,6-bisphosphate to fructose-6-phosphate.
Regulation of Gluconeogenesis
Requires ATP input to proceed and involves substrate cycling.
Hormonal Control: Insulin and glucagon play critical roles in regulating between glycolysis and gluconeogenesis.
Pentose Phosphate Pathway (PPP)
Functions of the PPP
Generates NADPH for biosynthetic reactions and antioxidant defense.
Produces ribose-5-phosphate necessary for nucleic acid synthesis.
Oxidative and Non-Oxidative Phases
Oxidative Phase: Involves G6P Dehydrogenase and generates NADPH.
Non-Oxidative Phase: Converts pentose phosphates back into intermediates of glycolysis (via several enzymatic steps including Transketolase and Transaldolase).
Glycogen Metabolism
Synthesis and Degradation
Glycogen synthesis involves phosphoglucomutase converting G6P to G1P, which is then activated to UDP-glucose before incorporation into glycogen by glycogen synthase.
Glycogen degradation releases glucose-1-phosphate from glycogen, utilizing glycogen phosphorylase.
Regulation of Glycogen Metabolism
Regulated via hormonal signals (glucagon and insulin) and allosteric effectors affecting glycogen phosphorylase and glycogen synthase.
Hormonal Control of Glycogenolysis and Glycogenesis
Insulin promotes glycogenesis whereas glucagon and epinephrine promote glycogen breakdown.
Energy needs influence the favoring of pathways—high ATP leads to glycolysis being favored while low ATP (high AMP) promotes gluconeogenesis.
Carbohydrate Metabolic Pathways
Overview of Pathways
Glycolysis: The metabolic pathway that converts glucose into pyruvate, producing ATP and NADH. This pathway is central for energy generation in most organisms and cell types, especially under anaerobic conditions.
Gluconeogenesis: The process of synthesizing glucose from non-carbohydrate precursors. Crucial for maintaining blood glucose levels during fasting or prolonged exercise, primarily in the liver and kidneys.
Pentose Phosphate Pathway: A metabolic pathway parallel to glycolysis that produces NADPH and ribose 5-phosphate. NADPH is vital for reductive biosynthesis (e.g., fatty acid synthesis) and counteracting oxidative stress, while ribose 5-phosphate is a precursor for nucleotides.
Glycogen Metabolism: Involves the synthesis (glycogenesis) and breakdown (glycogenolysis) of glycogen. Glycogen serves as the primary glucose storage form in animals, particularly in the liver and muscle.
Extracellular Matrix and Cell Wall Polysaccharides: These are structural polymers composed of glycogen, starch, and sucrose. These complex carbohydrates provide structural integrity and facilitate cell-to-cell communication in various organisms.
Carbohydrate Storage and Oxidation
Storage Forms: Glycogen, starch, and sucrose act as storage entities. Glycogen in animals, starch in plants, and sucrose as a transport sugar.
Oxidative Pathways: Include pathways like the pentose phosphate pathway that lead to the oxidation of glucose. These pathways extract energy or generate essential reducing power and precursors from glucose catabolism.
Essential Features of Glycolysis
General Characteristics
Glycolysis is active in most cells, having the largest carbon flux of most metabolic pathways. This implies that a significant portion of the cell's glucose consumption is channeled through glycolysis to meet immediate energy demands or provide intermediates for other pathways.
It consists of ten enzymatic reactions, which show varying rates in different types of cells.
Divided into two phases:
Preparatory Phase: Converts glucose to two molecules of glyceraldehyde 3-phosphate (G-3-P) while consuming ATP. This phase effectively "primes" the glucose molecule for cleavage, requiring an initial investment of 2 ATP per glucose molecule.
Payoff Phase: Converts G-3-P to pyruvate, producing ATP and NADH. This phase yields a net gain of energy, recovering the invested ATP and generating additional ATP and reducing equivalents.
Products of Glycolysis
The end products of glycolysis are pyruvate, ATP, and NADH.
Fates of Pyruvate: Depends on conditions (aerobic vs anaerobic) which can lead to either Acetyl-CoA for the citric acid cycle, lactate, or ethanol production.
Under aerobic conditions, pyruvate is converted to Acetyl-CoA and enters the citric acid cycle for complete oxidation to and for maximal ATP production.
Under anaerobic conditions (e.g., intense muscle activity or in some microorganisms), pyruvate is converted to lactate (lactic acid fermentation) or ethanol (alcoholic fermentation) to regenerate for glycolysis to continue.
Preparatory Phase of Glycolysis
Step 1: Phosphorylation of Glucose
Enzyme: Hexokinase
Reaction: Glucose + ATP → Glucose-6-Phosphate (G6P) + ADP
Function: Traps glucose in the cell and lowers intracellular glucose concentration. This is an irreversible step catalyzed by hexokinase in most tissues, or glucokinase in the liver and pancreas, which has a higher and is less sensitive to product inhibition.
Step 2: Isomerization
Enzyme: Phosphohexose Isomerase
Converts Glucose-6-Phosphate to Fructose-6-Phosphate. This reversible isomerization primes the molecule for the second phosphorylation step.
Step 3: Second Priming Reaction (Phosphorylation)
Enzyme: Phosphofructokinase-1 (PFK-1)
Involves another ATP to convert Fructose-6-Phosphate to Fructose-1,6-bisphosphate.
Highly regulated and considered the commitment step of glycolysis. This irreversible step is the most important control point in glycolysis, ensuring glucose is committed to breakdown once this step occurs.
Step 4: Aldol Cleavage
Enzyme: Aldolase
Converts Fructose-1,6-bisphosphate into Glyceraldehyde 3-Phosphate (an aldose) and Dihydroxyacetone phosphate (a ketose). This reaction cleaves the 6-carbon sugar into two 3-carbon interconvertible molecules.
Payoff Phase of Glycolysis
Energy Yielding Steps
Step 5: Interconversion of Triose Phosphates
Enzyme: Triose Phosphate Isomerase
Converts Dihydroxyacetone phosphate to Glyceraldehyde 3-Phosphate. This ensures that both 3-carbon products from aldolase can proceed through the subsequent steps of glycolysis.
Step 6: Oxidative Phosphorylation
Enzyme: Glyceraldehyde 3-Phosphate Dehydrogenase
Converts Glyceraldehyde 3-Phosphate to 1,3-Bisphosphoglycerate, reducing to NADH.
This step represents the production of a high-energy mixed anhydride. Specifically, inorganic phosphate is added to G-3-P while it is oxidized, forming an acyl phosphate bond with very high phosphoryl transfer potential.
Step 7: First ATP-Forming Reaction
Enzyme: Phosphoglycerate Kinase
Converts 1,3-BPG to 3-Phosphoglycerate, generating ATP (substrate-level phosphorylation). The high-energy phosphate from 1,3-BPG is directly transferred to ADP, generating ATP. Since there are two 1,3-BPG molecules per glucose, this step produces 2 ATP.
Step 8-10: Further Reactions
Produce additional ATP through substrate-level phosphorylation, finally yielding pyruvate.
Enzymes involved: Phosphoglycerate Mutase, Enolase, and Pyruvate Kinase.
Step 8: Phosphoglycerate Mutase rearranges 3-Phosphoglycerate to 2-Phosphoglycerate.
Step 9: Enolase dehydrates 2-Phosphoglycerate to Phosphoenolpyruvate (PEP), forming another high-energy phosphate compound.
Step 10: Pyruvate Kinase transfers the phosphate from PEP to ADP, generating the second ATP per G-3-P molecule (2 ATP per glucose) and yielding pyruvate. This is another highly regulated and irreversible step.
Overall Energy Calculations
The net production of glycolysis involves:
4 ATP formed (2 net gain). 2 ATP consumed in the preparatory phase, 4 ATP produced in the payoff phase.
2 NADH produced. Each NADH can yield approximately 2.5 ATP through oxidative phosphorylation in the electron transport chain under aerobic conditions, thus contributing significantly to overall energy yield.
Regulation of Glycolysis
Key Regulatory Enzymes
Hexokinase: Inhibited by its product G6P. This feedback inhibition prevents the cell from accumulating excess G6P when glucose is abundant or when downstream pathways are saturated.
PFK-1: The main control point of glycolysis, activated by AMP and fructose-2,6-bisphosphate, inhibited by ATP and citrate.
High ATP levels signal ample energy, inhibiting PFK-1 to slow down glucose breakdown.
High AMP levels signal low energy, activating PFK-1 to accelerate ATP production.
Citrate, an intermediate of the citric acid cycle, also indicates sufficient energy, signaling PFK-1 to slow down.
Fructose-2,6-bisphosphate is a potent allosteric activator, linking PFK-1 activity to hormonal signals (insulin/glucagon).
Pyruvate Kinase: Activated by fructose-1,6-bisphosphate and inhibited by ATP and acetyl-CoA. This feed-forward activation by an earlier glycolytic intermediate (FBP) ensures that sugars committed to glycolysis proceed efficiently. Inhibition by ATP and Acetyl-CoA (products of downstream pathways) prevents overproduction of pyruvate when energy is high.
Gluconeogenesis
Overview and Importance
Definition: The synthesis of glucose from non-carbohydrate substrates, primarily occurring in the liver. A less significant amount also occurs in the kidney.
Daily Requirement: At least 160g needed per day, with 75% utilized by the brain. The brain relies almost exclusively on glucose for energy, making gluconeogenesis critical during prolonged fasting to supply glucose.
Substrates Used: Lactate, glycerol, amino acids, and citric acid cycle intermediates. Fatty acids generally cannot be converted to glucose in animals, as their breakdown yields acetyl-CoA, which cannot be net converted to oxaloacetate.
Key Enzymes of Gluconeogenesis
Pyruvate Carboxylase: Converts pyruvate to oxaloacetate in mitochondria (requires biotin). This enzyme replenishes oxaloacetate, a citric acid cycle intermediate, which is then used for glucose synthesis.
PEP Carboxykinase (PEPCK): Converts oxaloacetate to phosphoenolpyruvate (PEP). This step occurs in either the mitochondria or cytosol, depending on the species and conditions, and is crucial for bypassing the irreversible pyruvate kinase step of glycolysis.
Fructose-1,6-bisphosphatase: A key regulatory step that hydrolyzes fructose-1,6-bisphosphate to fructose-6-phosphate. This bypasses the irreversible PFK-1 step of glycolysis and is a major control point for gluconeogenesis.
Glucose-6-Phosphatase: Hydrolyzes glucose-6-phosphate to glucose. This final step, primarily in the liver and kidney, allows free glucose to be released into the bloodstream, bypassing the hexokinase step.
Regulation of Gluconeogenesis
Requires ATP input to proceed and involves substrate cycling. It is an energy-intensive process, consuming 4 ATP, 2 GTP, and 2 NADH per glucose molecule synthesized.
Hormonal Control: Insulin and glucagon play critical roles in regulating between glycolysis and gluconeogenesis.
Glucagon (released during low blood glucose) promotes gluconeogenesis by increasing the synthesis and activity of key gluconeogenic enzymes and decreasing glycolytic enzyme activity.
Insulin (released during high blood glucose) promotes glycolysis and inhibits gluconeogenesis.
Pentose Phosphate Pathway (PPP)
Functions of the PPP
Generates NADPH for biosynthetic reactions and antioxidant defense. NADPH is essential for fatty acid and steroid synthesis and for reducing reactive oxygen species (ROS) via glutathione reductase.
Produces ribose-5-phosphate necessary for nucleic acid synthesis. Ribose-5-phosphate is a precursor for RNA, DNA, ATP, NADH, , and coenzyme A.
Oxidative and Non-Oxidative Phases
Oxidative Phase: Involves G6P Dehydrogenase and generates NADPH. This irreversible phase commits glucose-6-phosphate to the PPP, producing 2 NADPH molecules per glucose-6-phosphate.
Non-Oxidative Phase: Converts pentose phosphates back into intermediates of glycolysis (via several enzymatic steps including Transketolase and Transaldolase). This phase is reversible and allows the interconversion of 3-carbon, 4-carbon, 5-carbon, 6-carbon, and 7-carbon sugars to meet the cell's needs for either ribose-5-phosphate or glycolytic intermediates.
Glycogen Metabolism
Synthesis and Degradation
Glycogen synthesis involves phosphoglucomutase converting G6P to G1P, which is then activated to UDP-glucose before incorporation into glycogen by glycogen synthase. Glycogen synthase is the key enzyme for adding glucose units to the growing glycogen chain.
Glycogen degradation releases glucose-1-phosphate from glycogen, utilizing glycogen phosphorylase. Glycogen phosphorylase cleaves glycosidic bonds to release G1P. A debranching enzyme is also required to handle branch points.
Regulation of Glycogen Metabolism
Regulated via hormonal signals (glucagon and insulin) and allosteric effectors affecting glycogen phosphorylase and glycogen synthase.
Glycogen phosphorylase is activated by AMP (low energy) and phosphorylation (glucagon/epinephrine) and inhibited by ATP and G6P.
Glycogen synthase is activated by G6P (high glucose availability) and dephosphorylation (insulin) and inhibited by phosphorylation (glucagon/epinephrine).
Hormonal Control of Glycogenolysis and Glycogenesis
Insulin promotes glycogenesis whereas glucagon and epinephrine promote glycogen breakdown.
Insulin's actions lower blood glucose levels (e.g., by increasing glucose uptake and promoting glycogen synthesis).
Glucagon and epinephrine's actions raise blood glucose levels (e.g., by promoting glycogen breakdown and gluconeogenesis).
Energy needs influence the favoring of pathways:
When ATP levels are high, catabolic pathways like glycolysis are inhibited, and anabolic pathways like glycogenesis or gluconeogenesis (depending on the tissue and other signals) are favored.
*When ATP levels are low (high AMP), catabolic pathways such as glycolysis and
Carbohydrate Metabolic Pathways
Overview of Pathways
Glycolysis: The metabolic pathway that converts glucose into pyruvate, producing ATP and NADH. This pathway is central for energy generation in most organisms and cell types, especially under anaerobic conditions.
Gluconeogenesis: The process of synthesizing glucose from non-carbohydrate precursors. Crucial for maintaining blood glucose levels during fasting or prolonged exercise, primarily in the liver and kidneys.
Pentose Phosphate Pathway: A metabolic pathway parallel to glycolysis that produces NADPH and ribose 5-phosphate. NADPH is vital for reductive biosynthesis (e.g., fatty acid synthesis) and counteracting oxidative stress, while ribose 5-phosphate is a precursor for nucleotides.
Glycogen Metabolism: Involves the synthesis (glycogenesis) and breakdown (glycogenolysis) of glycogen. Glycogen serves as the primary glucose storage form in animals, particularly in the liver and muscle.
Extracellular Matrix and Cell Wall Polysaccharides: These are structural polymers composed of glycogen, starch, and sucrose. These complex carbohydrates provide structural integrity and facilitate cell-to-cell communication in various organisms.
Carbohydrate Storage and Oxidation
Storage Forms: Glycogen, starch, and sucrose act as storage entities. Glycogen in animals, starch in plants, and sucrose as a transport sugar.
Oxidative Pathways: Include pathways like the pentose phosphate pathway that lead to the oxidation of glucose. These pathways extract energy or generate essential reducing power and precursors from glucose catabolism.
Essential Features of Glycolysis
General Characteristics
Glycolysis is active in most cells, having the largest carbon flux of most metabolic pathways. This implies that a significant portion of the cell's glucose consumption is channeled through glycolysis to meet immediate energy demands or provide intermediates for other pathways.
It consists of ten enzymatic reactions, which show varying rates in different types of cells.
Divided into two phases:
Preparatory Phase: Converts glucose to two molecules of glyceraldehyde 3-phosphate (G-3-P) while consuming ATP. This phase effectively "primes" the glucose molecule for cleavage, requiring an initial investment of 2 ATP per glucose molecule.
Payoff Phase: Converts G-3-P to pyruvate, producing ATP and NADH. This phase yields a net gain of energy, recovering the invested ATP and generating additional ATP and reducing equivalents.
Products of Glycolysis
The end products of glycolysis are pyruvate, ATP, and NADH.
Fates of Pyruvate: Depends on conditions (aerobic vs anaerobic) which can lead to either Acetyl-CoA for the citric acid cycle, lactate, or ethanol production.
Under aerobic conditions, pyruvate is converted to Acetyl-CoA and enters the citric acid cycle for complete oxidation to and for maximal ATP production.
Under anaerobic conditions (e.g., intense muscle activity or in some microorganisms), pyruvate is converted to lactate (lactic acid fermentation) or ethanol (alcoholic fermentation) to regenerate for glycolysis to continue.
Preparatory Phase of Glycolysis
Step 1: Phosphorylation of Glucose
Enzyme: Hexokinase
Reaction: Glucose + ATP → Glucose-6-Phosphate (G6P) + ADP
Function: Traps glucose in the cell and lowers intracellular glucose concentration. This is an irreversible step catalyzed by hexokinase in most tissues, or glucokinase in the liver and pancreas, which has a higher and is less sensitive to product inhibition.
Step 2: Isomerization
Enzyme: Phosphohexose Isomerase
Converts Glucose-6-Phosphate to Fructose-6-Phosphate. This reversible isomerization primes the molecule for the second phosphorylation step.
Step 3: Second Priming Reaction (Phosphorylation)
Enzyme: Phosphofructokinase-1 (PFK-1)
Involves another ATP to convert Fructose-6-Phosphate to Fructose-1,6-bisphosphate.
Highly regulated and considered the commitment step of glycolysis. This irreversible step is the most important control point in glycolysis, ensuring glucose is committed to breakdown once this step occurs.
Step 4: Aldol Cleavage
Enzyme: Aldolase
Converts Fructose-1,6-bisphosphate into Glyceraldehyde 3-Phosphate (an aldose) and Dihydroxyacetone phosphate (a ketose). This reaction cleaves the 6-carbon sugar into two 3-carbon interconvertible molecules.
Payoff Phase of Glycolysis
Energy Yielding Steps
Step 5: Interconversion of Triose Phosphates
Enzyme: Triose Phosphate Isomerase
Converts Dihydroxyacetone phosphate to Glyceraldehyde 3-Phosphate. This ensures that both 3-carbon products from aldolase can proceed through the subsequent steps of glycolysis.
Step 6: Oxidative Phosphorylation
Enzyme: Glyceraldehyde 3-Phosphate Dehydrogenase
Converts Glyceraldehyde 3-Phosphate to 1,3-Bisphosphoglycerate, reducing to NADH.
This step represents the production of a high-energy mixed anhydride. Specifically, inorganic phosphate is added to G-3-P while it is oxidized, forming an acyl phosphate bond with very high phosphoryl transfer potential.
Step 7: First ATP-Forming Reaction
Enzyme: Phosphoglycerate Kinase
Converts 1,3-BPG to 3-Phosphoglycerate, generating ATP (substrate-level phosphorylation). The high-energy phosphate from 1,3-BPG is directly transferred to ADP, generating ATP. Since there are two 1,3-BPG molecules per glucose, this step produces 2 ATP.
Step 8-10: Further Reactions
Produce additional ATP through substrate-level phosphorylation, finally yielding pyruvate.
Enzymes involved: Phosphoglycerate Mutase, Enolase, and Pyruvate Kinase.
Step 8: Phosphoglycerate Mutase rearranges 3-Phosphoglycerate to 2-Phosphoglycerate.
Step 9: Enolase dehydrates 2-Phosphoglycerate to Phosphoenolpyruvate (PEP), forming another high-energy phosphate compound.
Step 10: Pyruvate Kinase transfers the phosphate from PEP to ADP, generating the second ATP per G-3-P molecule (2 ATP per glucose) and yielding pyruvate. This is another highly regulated and irreversible step.
Overall Energy Calculations
The net production of glycolysis involves:
4 ATP formed (2 net gain). 2 ATP consumed in the preparatory phase, 4 ATP produced in the payoff phase.
2 NADH produced. Each NADH can yield approximately 2.5 ATP through oxidative phosphorylation in the electron transport chain under aerobic conditions, thus contributing significantly to overall energy yield.
Regulation of Glycolysis
Key Regulatory Enzymes
Hexokinase: Inhibited by its product G6P. This feedback inhibition prevents the cell from accumulating excess G6P when glucose is abundant or when downstream pathways are saturated.
PFK-1: The main control point of glycolysis, activated by AMP and fructose-2,6-bisphosphate, inhibited by ATP and citrate.
High ATP levels signal ample energy, inhibiting PFK-1 to slow down glucose breakdown.
High AMP levels signal low energy, activating PFK-1 to accelerate ATP production.
Citrate, an intermediate of the citric acid cycle, also indicates sufficient energy, signaling PFK-1 to slow down.
Fructose-2,6-bisphosphate is a potent allosteric activator, linking PFK-1 activity to hormonal signals (insulin/glucagon).
Pyruvate Kinase: Activated by fructose-1,6-bisphosphate and inhibited by ATP and acetyl-CoA. This feed-forward activation by an earlier glycolytic intermediate (FBP) ensures that sugars committed to glycolysis proceed efficiently. Inhibition by ATP and Acetyl-CoA (products of downstream pathways) prevents overproduction of pyruvate when energy is high.
Gluconeogenesis
Overview and Importance
Definition: The synthesis of glucose from non-carbohydrate substrates, primarily occurring in the liver. A less significant amount also occurs in the kidney.
Daily Requirement: At least 160g needed per day, with 75% utilized by the brain. The brain relies almost exclusively on glucose for energy, making gluconeogenesis critical during prolonged fasting to supply glucose.
Substrates Used: Lactate, glycerol, amino acids, and citric acid cycle intermediates. Fatty acids generally cannot be converted to glucose in animals, as their breakdown yields acetyl-CoA, which cannot be net converted to oxaloacetate.
Key Enzymes of Gluconeogenesis
Pyruvate Carboxylase: Converts pyruvate to oxaloacetate in mitochondria (requires biotin). This enzyme replenishes oxaloacetate, a citric acid cycle intermediate, which is then used for glucose synthesis.
PEP Carboxykinase (PEPCK): Converts oxaloacetate to phosphoenolpyruvate (PEP). This step occurs in either the mitochondria or cytosol, depending on the species and conditions, and is crucial for bypassing the irreversible pyruvate kinase step of glycolysis.
Fructose-1,6-bisphosphatase: A key regulatory step that hydrolyzes fructose-1,6-bisphosphate to fructose-6-phosphate. This bypasses the irreversible PFK-1 step of glycolysis and is a major control point for gluconeogenesis.
Glucose-6-Phosphatase: Hydrolyzes glucose-6-phosphate to glucose. This final step, primarily in the liver and kidney, allows free glucose to be released into the bloodstream, bypassing the hexokinase step.
Regulation of Gluconeogenesis
Requires ATP input to proceed and involves substrate cycling. It is an energy-intensive process, consuming 4 ATP, 2 GTP, and 2 NADH per glucose molecule synthesized.
Hormonal Control: Insulin and glucagon play critical roles in regulating between glycolysis and gluconeogenesis.
Glucagon (released during low blood glucose) promotes gluconeogenesis by increasing the synthesis and activity of key gluconeogenic enzymes and decreasing glycolytic enzyme activity.
Insulin (released during high blood glucose) promotes glycolysis and inhibits gluconeogenesis.
Pentose Phosphate Pathway (PPP)
Functions of the PPP
Generates NADPH for biosynthetic reactions and antioxidant defense. NADPH is essential for fatty acid and steroid synthesis and for reducing reactive oxygen species (ROS) via glutathione reductase.
Produces ribose-5-phosphate necessary for nucleic acid synthesis. Ribose-5-phosphate is a precursor for RNA, DNA, ATP, NADH, , and coenzyme A.
Oxidative and Non-Oxidative Phases
Oxidative Phase: Involves G6P Dehydrogenase and generates NADPH. This irreversible phase commits glucose-6-phosphate to the PPP, producing 2 NADPH molecules per glucose-6-phosphate.
Non-Oxidative Phase: Converts pentose phosphates back into intermediates of glycolysis (via several enzymatic steps including Transketolase and Transaldolase). This phase is reversible and allows the interconversion of 3-carbon, 4-carbon, 5-carbon, 6-carbon, and 7-carbon sugars to meet the cell's needs for either ribose-5-phosphate or glycolytic intermediates.
Glycogen Metabolism
Synthesis and Degradation
Glycogen synthesis involves phosphoglucomutase converting G6P to G1P, which is then activated to UDP-glucose before incorporation into glycogen by glycogen synthase. Glycogen synthase is the key enzyme for adding glucose units to the growing glycogen chain.
Glycogen degradation releases glucose-1-phosphate from glycogen, utilizing glycogen phosphorylase. Glycogen phosphorylase cleaves glycosidic bonds to release G1P. A debranching enzyme is also required to handle branch points.
Regulation of Glycogen Metabolism
Regulated via hormonal signals (glucagon and insulin) and allosteric effectors affecting glycogen phosphorylase and glycogen synthase.
Glycogen phosphorylase is activated by AMP (low energy) and phosphorylation (glucagon/epinephrine) and inhibited by ATP and G6P.
Glycogen synthase is activated by G6P (high glucose availability) and dephosphorylation (insulin) and inhibited by phosphorylation (glucagon/epinephrine).
Hormonal Control of Glycogenolysis and Glycogenesis
Insulin promotes glycogenesis whereas glucagon and epinephrine promote glycogen breakdown.
Insulin's actions lower blood glucose levels (e.g., by increasing glucose uptake and promoting glycogen synthesis).
Glucagon and epinephrine's actions raise blood glucose levels (e.g., by promoting glycogen breakdown and gluconeogenesis).
Energy needs influence the favoring of pathways:
When ATP levels are high, catabolic pathways like glycolysis are inhibited, and anabolic pathways like glycogenesis or gluconeogenesis (depending on the tissue and other signals) are favored.
When ATP levels are low (high AMP), catabolic pathways such as glycolysis and glycogenolysis are promoted to generate ATP.
Questions for Professor
Could you elaborate on the specific differences in hexokinase vs. glucokinase regulation and their physiological implications in different tissues?
What are the major allosteric effectors of PFK-1 and pyruvate kinase beyond ATP, AMP, citrate, and fructose-1,6-bisphosphate, and how do they fine-tune glycolysis?
How does the Cori cycle effectively link anaerobic glycolysis in muscle with gluconeogenesis in the liver?
Besides ATP, GTP, and NADH, what other energy costs or regulatory inputs are there for gluconeogenesis?
Can you explain the cellular conditions that would favor the flux of glucose-6-phosphate into the Pentose Phosphate Pathway versus glycolysis?
What are some clinical implications of defects in glycogen metabolism enzymes (e.g., glycogen storage diseases)?
How exactly do insulin and glucagon, as peptide hormones, transduce their signals inside a cell to affect the activity of specific enzymes like glycogen phosphorylase and glycogen synthase?
What is the overall energy balance (ATP per glucose) when considering complete oxidation of glucose through glycolysis, citric acid cycle, and oxidative phosphorylation under ideal aerobic conditions?
Are there any other minor pathways for carbohydrate metabolism that are important under specific physiological conditions, and how do they integrate with the major pathways discussed?