GLYCOLYSIS AND FATE OF PYRUVATE

CENTRAL ROLE OF GLUCOSE IN CARBOHYDRATE METABOLISM

• Extracellular Matrix and Cell Wall Polysaccharides
• Storage Forms: Glycogen, starch, sucrose
• Synthesis of Structural Polymers
• Glucose Oxidation: Via pentose phosphate pathway
  • Produces ribose 5-phosphate
• Storage and Oxidation via Glycolysis
  • End product: Pyruvate

LEARNING OBJECTIVES

By the end of the session, students should be able to:

1. Outline the Reactions in Glycolysis
   • Involves the breakdown of glucose and other monosaccharides such as fructose and galactose.
2. Describe the Formation and Function of 2,3 Bisphosphoglycerate
3. Explain the Functions and Regulation of Glycolysis
4. Describe Disease Processes from Enzyme Deficiencies
   • Metabolism of fructose, galactose, and glucose.
5. Discuss the Fate of Pyruvate Under Different Conditions
   • Anaerobic and aerobic conditions.

PART ONE: OVERVIEW OF GLYCOLYSIS

• Reactions in the Glycolytic Pathway
• Functions of Glycolysis
• Regulation of Glycolysis
• Diseases Involving Glycolysis

SOURCES OF GLUCOSE IN THE HUMAN BODY

1. Dietary Sources: Carbohydrates (e.g., starch, lactose, sucrose)
2. Glycogen: Excess glucose stored as glycogen in the liver and skeletal muscle.
   • Example: Consuming meat or liver provides glycogen.
3. Gluconeogenesis: Synthesis of glucose from non-carbohydrate compounds.

WHAT IS GLYCOLYSIS?

• Definition: Major pathway for carbohydrate metabolism, particularly glucose, fructose, and galactose.
• Characteristics:
  • Universal pathway found in living organisms.
  • Occurs in the cytosol (cellular fluid).
  • Anaerobic process (occurs without oxygen).
• Major Role: Producing energy in the form of ATP.
• Process: Six-carbon glucose is broken down into two three-carbon pyruvate molecules.
• Phases:
  • Preparatory Phase
  • Payoff Phase

PREPARATORY PHASE OF GLYCOLYSIS

• Overview: Consists of ten enzyme-catalyzed reactions, first five constitute the preparatory phase:
  • Glucose is phosphorylated and split into two molecules of glyceraldehyde 3-phosphate (G3P).

REACTIONS OF GLYCOLYSIS (Detailed)

1. First Reaction:

   • Reaction: Glucose + ATP → Glucose 6-phosphate + ADP
   • Enzyme: Hexokinase (also glucokinase in the liver)
   • Cofactor: Magnesium ions required
   • Type: Irreversible reaction; glucokinase has a lower affinity and higher Km compared to hexokinase.

2. Second Reaction:

   • Reaction: Glucose 6-phosphate ↔ Fructose 6-phosphate
   • Enzyme: Phosphohexose isomerase
   • Type: Reversible reaction.

3. Third Reaction:

   • Reaction: Fructose 6-phosphate + ATP → Fructose 1,6-bisphosphate + ADP
   • Enzyme: Phosphofructokinase 1 (PFK-1)
   • Importance: Rate-limiting step of glycolysis; allosterically regulated by ADP (positive effector) and ATP/citrate (negative effectors).
   • Type: Irreversible reaction.

4. Fourth Reaction:

   • Reaction: Fructose 1,6-bisphosphate ↔ Glyceraldehyde 3-phosphate + Dihydroxyacetone phosphate
   • Enzyme: Aldolase
   • Type: Reversible reaction.

5. Fifth Reaction:

   • Reaction: Dihydroxyacetone phosphate ↔ Glyceraldehyde 3-phosphate
   • Enzyme: Triose phosphate isomerase
   • Type: Reversible reaction; ends the preparatory phase turning glucose 6-phosphate into 2 G3P.

PAYOFF PHASE OF GLYCOLYSIS

• Process: The two molecules of glyceraldehyde 3-phosphate undergo ATP production.

  • Sixth Reaction:
  • Reaction: Glyceraldehyde 3-phosphate + NAD⁺ + Pi ↔ 1,3-bisphosphoglycerate + NADH + H⁺
  • Enzyme: Glyceraldehyde 3-phosphate dehydrogenase
  • Type: Reversible reaction; involves oxidation of aldehyde to a carboxylic acid.

• Seventh Reaction:**

  • Reaction: 1,3-bisphosphoglycerate + ADP ↔ 3-phosphoglycerate + ATP
  • Enzyme: Phosphoglycerate kinase
  • Importance: First substrate-level phosphorylation.
  • ΔG′: –49.3 kJ/mole.
  • Type: Reversible reaction.

• Eighth Reaction:

  • Reaction: 3-phosphoglycerate ↔ 2-phosphoglycerate
  • Enzyme: Phosphoglycerate mutase.
  • Type: Reversible reaction.

• Ninth Reaction:

  • Reaction: 2-phosphoglycerate ↔ Phosphoenolpyruvate + H₂O
  • Enzyme: Enolase.
  • Type: Reversible reaction.

• Tenth Reaction:

  • Reaction: Phosphoenolpyruvate + ADP → Pyruvate + ATP
  • Enzyme: Pyruvate kinase
  • Importance: Second substrate-level phosphorylation; ΔG′: –61.9 kJ/mol.
  • Type: Irreversible reaction.

NET EQUATION OF GLYCOLYSIS

• Overall Reaction: Glucose + 2NAD⁺ + 2ADP + 2Pi → 2 Pyruvate + 2NADH + 2H⁺ + 2ATP + 2H₂O

2,3 BISPHOSPHOGLYCERATE SHUNT

• Production: 2,3 bisphosphoglycerate (2,3-BPG) is produced from 1,3-bisphosphoglycerate in a shunt active in red blood cells (RBCs).

  • Reaction: 1,3-bisphosphoglycerate ↔ 2,3-bisphosphoglycerate via mutase.
  • 2,3-BPG can be converted back to 3-phosphoglycerate, which re-enters glycolysis.
  • Enzyme: Phosphatase.
  • Reaction: 2,3-bisphosphoglycerate + H₂O ↔ 3-phosphoglycerate + Pi.

• Function of 2,3-BPG:

  • It is a negative allosteric effector of hemoglobin, regulating oxygen binding and facilitating delivery to tissues.

REGULATION OF GLYCOLYSIS

• Key Enzymes:
  1. Hexokinase:
  • Inhibited by Glucose-6-Phosphate; entails product inhibition.
  1. Phosphofructokinase-1 (PFK-1):
  • Allosteric enzyme; positive effectors: AMP; negative effectors: ATP and citrate; rate-limiting step.
  1. Pyruvate Kinase:
  • Last reaction in glycolysis; activated by ADP; inhibited by ATP.

DISEASES INVOLVING GLYCOLYSIS

• Hexokinase Deficiency:

  • Results in reduced glycolytic intermediates; decreases ATP in mature RBCs, affecting their energetic function.
  • Consequence: Reduced 2,3-BPG, leading to increased oxygen binding to hemoglobin.

• Pyruvate Kinase Deficiency:

  • Accumulation of glycolytic intermediates, including 2,3-BPG; reduced ATP production; primarily impacts mature RBCs, leading to membrane integrity loss and hemolytic anemia.

PART B: METABOLISM OF OTHER SUGARS

• Metabolism of Galactose:

  1. Galactose Phosphorylation:
  • Reaction: Galactose + ATP → Galactose 1-phosphate + ADP
  • Enzyme: Galactokinase.
  1. Conversion to Glucose 1-Phosphate:
  • Reaction: Galactose 1-phosphate + UDP-glucose ↔ Glucose 1-phosphate + UDP-galactose (catalyzed by UDP-glucose uridyl transferase).
  1. Isomerization:
  • Reaction: Glucose 1-phosphate ↔ Glucose 6-phosphate (enzyme: Phosphoglucomutase).
  1. UDP-Galactose Conversion:
  • Reaction: UDP-galactose ↔ UDP-glucose (enzyme: epimerase).

• Metabolism of Fructose:

  • Phosphorylation by Hexokinase:
    • Reaction: Fructose + ATP → Fructose 6-phosphate + ADP.
  • Phosphorylation by Fructokinase:
    • Reaction: Fructose + ATP → Fructose 1-phosphate + ADP.
  • Fructose 1-Phosphate Breakdown:
  • Reaction: Fructose 1-phosphate ↔ Dihydroxyacetone phosphate + Glyceraldehyde (enzyme: Fructose 1-phosphate aldolase).
  • Conversion to Glyceraldehyde 3-Phosphate:
    • Dihydroxyacetone phosphate ↔ Glyceraldehyde 3-phosphate (enzyme: Triose phosphate isomerase).

DISEASES DUE TO DEFICIENCIES IN SUGAR METABOLISM

• Lactose Intolerance:

  • Due to lactase deficiency; causes abdominal pain and diarrhea upon milk consumption.

• Galactosemia:

  • Deficiency of galactose 1-phosphate uridyl transferase; leads to metabolite accumulation; symptoms include cataracts, growth failure, mental retardation, and hepatotoxicity. Dietary management involves avoiding galactose-containing foods.

• Fructose Intolerance:

  • Fructose 1-phosphate aldolase deficiency leads to liver enlargement and hypoglycemia; treatment involves avoiding fructose-rich foods.

FATE OF PYRUVATE UNDER ANAEROBIC CONDITIONS

• Lactic Acid Fermentation:

  • Occurs in anoxic conditions, mainly in RBCs or muscles during exertion.
  • Reaction: Pyruvate ↔ Lactate (reduction to regenerate NAD⁺ to sustain glycolysis).

• Alcohol Fermentation:

  • Occurs in yeast cells.
  • Process: Two-step reaction:
  1. Pyruvate + TPP → Acetaldehyde + CO₂
  2. Acetaldehyde + NADH + H⁺ ↔ Ethanol + NAD⁺

FATE OF PYRUVATE UNDER AEROBIC CONDITIONS

• Aerobic Respiration:
  • Pyruvate enters the mitochondria to undergo oxidative decarboxylation to Acetyl-CoA.
  • Reaction: Pyruvate + NAD⁺ → Acetyl-CoA + CO₂ + NADH + H⁺
  • Enzyme: Pyruvate dehydrogenase enzyme complex.

SUMMARY OF PYRUVATE FATE

• Anaerobic Conditions:
  • Fermentation: 2 Lactate or 2 Ethanol + 2CO₂.
• Aerobic Conditions:
  • Oxidative Decarboxylation: Converts pyruvate to 2 Acetyl-CoA, allowing entry into the citric acid cycle for further ATP production.