Glycolysis

Glycolysis, PPP, Cori Cycle

Glycolysis Overview

  • First Stage of Glucose Metabolism:

    • Glycolysis is the first biochemical pathway discovered.

    • It involves the breakdown of glucose from polysaccharides and disaccharides.

  • Anaerobic Process:

    • Glycolysis yields only two molecules of ATP.

  • Aerobic Oxidation:

    • Complete oxidation of glucose to carbon dioxide and water (involving glycolysis, citric acid cycle, and oxidative phosphorylation) can yield energy equivalent to 32 ATP molecules.

  • Fuel Utilization in Physical Exercise:

    • During strenuous exercise, the body utilizes carbohydrates, fats, and proteins aerobically for fuel.

    • For short bursts of energy (e.g., sprinting), carbohydrates are metabolized faster than they can be processed aerobically, leading to glycolysis where pyruvate is the end product.

Details of Glycolysis

  • Conversion Process:

    • One molecule of glucose is converted to two molecules of pyruvate.

    • Under anaerobic conditions, pyruvate is converted to lactic acid and exported from muscle to liver.

  • Aerobic Conditions:

    • Main purpose is to feed pyruvate into the citric acid cycle, producing considerably more ATP.

    • Pyruvate is oxidized to CO2 and H2O during aerobic respiration.

  • Anaerobic Conditions:

    • Lactic acid is produced, especially in muscles.

    • NADH produced during glycolysis is reoxidized to NAD+ in reactions involving pyruvate.

  • Alcoholic Fermentation in Yeast:

    • Involves conversion of pyruvate to ethanol.

Importance of Glucose and Its Different Fates

  • Glucose and Structural Polymers:

    • Glucose is critical for the synthesis of glycogen, starch, sucrose, and various structural polymers.

  • Metabolic Pathways:

    • Oxidation via pentose phosphate pathway (PPP) leads to the production of ribose 5-phosphate.

  • Key Components:

    • The main glycolytic pathway and alternative fates of pyruvate (e.g., aerobic metabolism, anaerobic metabolism).

Overall Pathway in Glycolysis

  • Initial Steps:

    • Glucose (6-carbon molecule) is converted to fructose-1,6-bisphosphate, eventually leading to the production of two molecules of pyruvate (3-carbon molecules).

    • The pathway consists of many steps where the metabolites of glucose are oxidized. Each step is catalyzed by a specific enzyme.

  • ATP Utilization:

    • In the initial steps, one ATP molecule is hydrolyzed for each glucose molecule.

    • Two molecules of ATP are produced in later reactions through phosphorylation of ADP, leading to a net gain of two ATP molecules per glucose processed.

  • Configuration of Sugars:

    • All sugars in this pathway have D configuration.

Breakdown of Glucose to Pyruvate

  • Fates of Pyruvate:

    1. Aerobic Metabolism:

    • Pyruvate loses one carbon (CO2) and forms acetyl-CoA, which enters the citric acid cycle.

    1. Anaerobic Metabolism:

    • (a) In organisms that undergo alcoholic fermentation, pyruvate loses CO2 and is converted to acetaldehyde, then to ethanol.

    • (b) Common fate includes reduction to lactate.

    • Anaerobic metabolism serves as the sole energy source for red blood cells and some bacteria.

  • Net Reaction Summary:

    • extGlucose+2extADP+2extPi<br>ightarrow2extPyruvate+2extATPext{Glucose} + 2 ext{ADP} + 2 ext{Pi} <br>ightarrow 2 ext{Pyruvate} + 2 ext{ATP}

Phases of Glycolysis

  • Phase 1: (

    • Involves phosphorylation, isomerization, cleavage, and interconversion of 3-carbon compounds.

    • Two ATP molecules are consumed in this phase.

  • Phase 2:

    • ATP is produced, highlighting the shift from energy investment to energy payoff from the glycolytic pathway.

Importance of Phosphorylated Intermediates

  • Characteristics and Functions:

    • Each intermediate in the pathway is phosphorylated.

    • Functions:

    1. Prevents transport out of the cell, since the plasma membrane lacks transporters for phosphorylated sugars.

    2. High energy phosphate compounds donate phosphoryl groups to ADP to form ATP.

    3. Binding of phosphate groups lowers activation energy for enzymatic reactions, increasing specificity.

The Glycolytic Enzymatic Steps

  • Step 1: Phosphorylation of glucose to glucose-6-phosphate:

    • Enzyme: Hexokinase (low Km, irreversible, rate-limiting step).

    • Source of phosphate: ATP.

    • Hexokinase activity is allosterically inhibited by glucose-6-phosphate.

  • Step 2: Isomerization of glucose-6-phosphate to fructose-6-phosphate:

    • Enzyme: Glucose-Phosphate Isomerase.

    • Conversion involves reduction and oxidation at respective carbon groups leading to ring opening and closure.

  • Step 3: Phosphorylation of fructose-6-phosphate to fructose-1,6-bisphosphate:

    • Enzyme: Phosphofructokinase (committed step, highly regulated by ATP and AMP).

    • Source of phosphate: ATP.

  • Step 4: Cleavage of fructose-1,6-bisphosphate:

    • Enzyme: Aldolase, resulting in two interconvertible 3-carbon fragments.

  • Step 5: Isomerization of dihydroxyacetone phosphate (DHAP) to glyceraldehyde-3-phosphate (G3P):

    • Enzyme: Triosephosphate Isomerase.

    • Second molecule of G3P produced.

  • Steps 6 to 10: ATP Production and Finalization:

    • Each subsequent step involves various enzymes (such as glyceraldehyde-3-phosphate dehydrogenase, phosphoglycerate kinase, and pyruvate kinase), facilitating production and conversion processes with net ATP generation in the payoff phase.

    • The final reaction entails conversion of phosphoenolpyruvate to pyruvate, producing ATP in the process.

Control Points in Glycolysis

  • Key Control Points:

    • Phosphofructokinase, pyruvate kinase, and hexokinase act as regulatory points for the pathway.

  • Glucose Uptake Mechanisms:

    • Transporters (GLUT family) facilitate uptake, dependent on insulin in specific tissues (GLUT4 in muscle, cardiac tissue).

Type I Diabetes and Its Effects

  • Insulin Regulation:

    • In Type I Diabetes, lack of insulin affects glucose uptake through GLUT4, leading to elevated blood glucose and the breakdown of fats.

    • Symptoms include ketoacidosis due to accumulation of fatty acid oxidation products (e.g., acetoacetate).

Feeder Pathways for Glycolysis

  • Entry of Dietary Sugars:

    • Various dietary polysaccharides and disaccharides (e.g., lactose, sucrose) undergo hydrolysis to monosaccharides for glycolytic entry.

Alcoholic Fermentation and Anaerobic Pathways

  • Lactate Formation:

    • Enzyme: Lactate Dehydrogenase. Recycles NADH and leads to muscle fatigue.

  • Alcoholic Fermentation:

    • Produces ethanol and CO2, regulated by crucial enzymes.

Gluconeogenesis: The Reverse of Glycolysis

  • Pathway Overview:

    • The conversion of pyruvate back to glucose, which involves bypassing three irreversible glycolytic steps with distinct enzymes.

  • Energy Requirements:

    • Requires 4 ATP, 2 GTP, and 2 NADH, signifying high energy cost but critical for glucose homeostasis during fasting or intense exercise.

The Cori Cycle

  • Division of Labor:

    • Cycling of lactate produced in muscles via glycolysis is transported to the liver where gluconeogenesis occurs, ensuring glucose availability for energy-demanding tissues.

  • Energy Regulation:

    • Ensures that glucose metabolism pathways are independently controlled and energetically feasible via hormonal modulation.

Pentose Phosphate Pathway (PPP)

  • General Overview:

    • Divided into oxidative reactions (producing NADPH) and nonoxidative reactions (producing ribose for nucleotide synthesis).

  • Control:

    • G6PDH acts as a control point, with shifts depending on cellular needs for NADPH versus energy.

Conclusion

  • The intricate interplay between glycolysis, gluconeogenesis, and the pentose phosphate pathway highlights the regulatory mechanisms that maintain cellular energy homeostasis, adapt to metabolic needs, and respond energetically to fluctuations in nutrient availability.

Glycolysis, PPP, Cori Cycle---## Glycolysis Overview

  • First Stage of Glucose Metabolism:

    • Glycolysis is the first biochemical pathway discovered.

    • It involves the breakdown of glucose from polysaccharides and disaccharides.

  • Anaerobic Process:

    • Glycolysis yields only two molecules of ATP.

  • Aerobic Oxidation:

    • Complete oxidation of glucose to carbon dioxide and water (involving glycolysis, citric acid cycle, and oxidative phosphorylation) can yield energy equivalent to 32 ATP molecules.

  • Fuel Utilization in Physical Exercise:

    • During strenuous exercise, the body utilizes carbohydrates, fats, and proteins aerobically for fuel.

    • For short bursts of energy (e.g., sprinting), carbohydrates are metabolized faster than they can be processed aerobically, leading to glycolysis where pyruvate is the end product.

---## Details of Glycolysis

  • Conversion Process:

    • One molecule of glucose is converted to two molecules of pyruvate.

    • Under anaerobic conditions, pyruvate is converted to lactic acid and exported from muscle to liver.

  • Aerobic Conditions:

    • Main purpose is to feed pyruvate into the citric acid cycle, producing considerably more ATP.

    • Pyruvate is oxidized to CO2 and H2O during aerobic respiration.

  • Anaerobic Conditions:

    • Lactic acid is produced, especially in muscles.

    • NADH produced during glycolysis is reoxidized to NAD+ in reactions involving pyruvate.

  • Alcoholic Fermentation in Yeast:

    • Involves conversion of pyruvate to ethanol.

---## Importance of Glucose and Its Different Fates

  • Glucose and Structural Polymers:

    • Glucose is critical for the synthesis of glycogen, starch, sucrose, and various structural polymers.

  • Metabolic Pathways:

    • Oxidation via pentose phosphate pathway (PPP) leads to the production of ribose 5-phosphate.

  • Key Components:

    • The main glycolytic pathway and alternative fates of pyruvate (e.g., aerobic metabolism, anaerobic metabolism).

---## Overall Pathway in Glycolysis

  • Initial Steps:

    • Glucose (6-carbon molecule) is converted to fructose-1,6-bisphosphate, eventually leading to the production of two molecules of pyruvate (3-carbon molecules).

    • The pathway consists of many steps where the metabolites of glucose are oxidized. Each step is catalyzed by a specific enzyme.

  • ATP Utilization:

    • In the initial steps, one ATP molecule is hydrolyzed for each glucose molecule.

    • Two molecules of ATP are produced in later reactions through phosphorylation of ADP, leading to a net gain of two ATP molecules per glucose processed.

  • Configuration of Sugars:

    • All sugars in this pathway have D configuration.

---## Breakdown of Glucose to Pyruvate

  • Fates of Pyruvate:

    1. Aerobic Metabolism:

    • Pyruvate loses one carbon (CO2) and forms acetyl-CoA, which enters the citric acid cycle.

    1. Anaerobic Metabolism:

    • (a) In organisms that undergo alcoholic fermentation, pyruvate loses CO2 and is converted to acetaldehyde, then to ethanol.

    • (b) Common fate includes reduction to lactate.

    • Anaerobic metabolism serves as the sole energy source for red blood cells and some bacteria.

  • Net Reaction Summary:
    extGlucose+2extADP+2extPi<br>ightarrow2extPyruvate+2extATPext{Glucose} + 2 ext{ADP} + 2 ext{Pi} <br>ightarrow 2 ext{Pyruvate} + 2 ext{ATP}

---## Phases of Glycolysis

  • Phase 1:

    • Involves phosphorylation, isomerization, cleavage, and interconversion of 3-carbon compounds.

    • Two ATP molecules are consumed in this phase.

  • Phase 2:

    • ATP is produced, highlighting the shift from energy investment to energy payoff from the glycolytic pathway.

---## Importance of Phosphorylated Intermediates

  • Characteristics and Functions:

    • Each intermediate in the pathway is phosphorylated.

  • Functions:

    1. Prevents transport out of the cell, since the plasma membrane lacks transporters for phosphorylated sugars.

    2. High energy phosphate compounds donate phosphoryl groups to ADP to form ATP.

    3. Binding of phosphate groups lowers activation energy for enzymatic reactions, increasing specificity.

---## The Glycolytic Enzymatic Steps

  • Step 1: Phosphorylation of glucose to glucose-6-phosphate:

    • Enzyme: Hexokinase (low Km, irreversible, rate-limiting step).

    • Source of phosphate: ATP.

    • Hexokinase activity is allosterically inhibited by glucose-6-phosphate.

  • Step 2: Isomerization of glucose-6-phosphate to fructose-6-phosphate:

    • Enzyme: Glucose-Phosphate Isomerase.

    • Conversion involves reduction and oxidation at respective carbon groups leading to ring opening and closure.

  • Step 3: Phosphorylation of fructose-6-phosphate to fructose-1,6-bisphosphate:

    • Enzyme: Phosphofructokinase (committed step, highly regulated by ATP and AMP).

    • Source of phosphate: ATP.

  • Step 4: Cleavage of fructose-1,6-bisphosphate:

    • Enzyme: Aldolase, resulting in two interconvertible 3-carbon fragments.

  • Step 5: Isomerization of dihydroxyacetone phosphate (DHAP) to glyceraldehyde-3-phosphate (G3P):

    • Enzyme: Triosephosphate Isomerase.

    • Second molecule of G3P produced.

  • Steps 6 to 10: ATP Production and Finalization:

    • Each subsequent step involves various enzymes (such as glyceraldehyde-3-phosphate dehydrogenase, phosphoglycerate kinase, and pyruvate kinase), facilitating production and conversion processes with net ATP generation in the payoff phase.

    • The final reaction entails conversion of phosphoenolpyruvate to pyruvate, producing ATP in the process.

---## Control Points in Glycolysis

  • Key Control Points:

    • Phosphofructokinase, pyruvate kinase, and hexokinase act as regulatory points for the pathway.

  • Glucose Uptake Mechanisms:

    • Transporters (GLUT family) facilitate uptake, dependent on insulin in specific tissues (GLUT4 in muscle, cardiac tissue).

---## Type I Diabetes and Its Effects

  • Insulin Regulation:

    • In Type I Diabetes, lack of insulin affects glucose uptake through GLUT4, leading to elevated blood glucose and the breakdown of fats.

    • Symptoms include ketoacidosis due to accumulation of fatty acid oxidation products (e.g., acetoacetate).

---## Feeder Pathways for Glycolysis

  • Entry of Dietary Sugars:

    • Various dietary polysaccharides and disaccharides (e.g., lactose, sucrose) undergo hydrolysis to monosaccharides for glycolytic entry.

---## Alcoholic Fermentation and Anaerobic Pathways

  • Lactate Formation:

    • Enzyme: Lactate Dehydrogenase. Recycles NADH and leads to muscle fatigue.

  • Alcoholic Fermentation:

    • Produces ethanol and CO2, regulated by crucial enzymes.

---## Gluconeogenesis: The Reverse of Glycolysis

  • Pathway Overview:

    • The conversion of pyruvate back to glucose, which involves bypassing three irreversible glycolytic steps with distinct enzymes.

  • Energy Requirements:

    • Requires 4 ATP, 2 GTP, and 2 NADH, signifying high energy cost but critical for glucose homeostasis during fasting or intense exercise.

---## Pentose Phosphate Pathway (PPP)

  • General Overview:

    • Divided into oxidative reactions (producing NADPH) and nonoxidative reactions (producing ribose for nucleotide synthesis).

  • Control:

    • G6PDH acts as a control point, with shifts depending on cellular needs for NADPH versus energy.

---## Conclusion

  • The intricate interplay between glycolysis, gluconeogenesis, and the pentose phosphate pathway highlights the regulatory mechanisms that maintain cellular energy homeostasis, adapt to metabolic needs, and respond energetically to fluctuations in nutrient availability.