Carbohydrate Metabolism: Glycolysis and Gluconeogenesis

Anaerobic Glycolysis

• Overview
  • Central pathway of glucose metabolism in all cells.
  • Anaerobic: Does not require $\text{O}_2$.
  • Occurs in the cytoplasm.
  • Erythrocytes rely solely on glucose and glycolysis for energy.
  • Produces 2 moles of pyruvate per glucose molecule.
  • Aerobic glycolysis: Glucose metabolized to $\text{CO}<em>2$ and $\text{H}</em>2\text{O}$ due to the presence of mitochondria.
  • Anaerobic glycolysis: Occurs in cells lacking mitochondria (e.g., red blood cells) where pyruvate is reduced to lactate.

Glucose Phosphorylation

• Phosphorylation prevents glucose from leaving the cell.
• Glucose uptake:
  • Erythrocytes: Facilitated by GLUT-1 transporter.
  • Other cells: GLUT-4 transporter is used.
• Allosteric Inhibition: hexokinase is allosterically inhibited by glucose-6-P
• Activity of hexokinase is favored by insulin.

Coupled Reactions

• Example: Conversion of glucose to glucose-6-phosphate.

Phosphofructokinase (PFK-1)

• Key Enzyme: in glycolysis.
• Regulation
  • Allosteric activation by $\text{AMP}$ and fructose-2,6-P.
  • Allosteric inhibition by $\text{ATP}$ (raises $\text{K}_m$ for fructose-6-P) and citrate.
  • Activity is favored by insulin.

Sugar-Splitting

• Fructose-1,6-bisphosphate is split into:
  • Dihydroxyacetone phosphate
  • Glyceraldehyde-3-phosphate
• Catalyzed by aldolase and triose phosphate isomerase.

Energy Transfer

• Glyceraldehyde-3-phosphate is converted to 1,3-bisphosphoglycerate.
• Reaction involves:
  • Glyceraldehyde-3-phosphate dehydrogenase (GAPDH)
  • $NAD^+$ reduction to NADH

ATP Formation

• 1,3-bisphosphoglycerate is converted to 3-phosphoglycerate.
• Process: Substrate-level phosphorylation.
• Enzyme: Phosphoglycerate kinase (PGK).

Rearrangement

• 3-phosphoglycerate is rearranged to 2-phosphoglycerate.
• Enzyme: Phosphoglycerate mutase.

Dehydration

• 2-phosphoglycerate is dehydrated to phosphoenolpyruvate.
• Enzyme: Enolase.
• Requires $Mg^{2+}$.
• Fluoride acts as an irreversible inhibitor.

Phosphate Transfer

• Transfer of phosphate from phosphoenolpyruvate (PEP) to ADP is irreversible.
• Allosteric Activation: Fructose 1,6-bisphosphate.
• Allosteric Inactivation: ATP and alanine.
• Activity of pyruvate kinase (PK) is favored by insulin.

Glycolysis Regulation

• Insulin: Favors glycolysis by activating glucokinase, phosphofructokinase, and pyruvate kinase.
• Inhibition: Glycolysis is inhibited by glucagon and glucocorticoids.

Pyruvate Pathways

• With Oxygen: Pyruvate can be converted to Acetyl-CoA and enter the Krebs cycle.
• Without Oxygen: Pyruvate can be converted to Lactate or Ethanol.

Anaerobic Glycolysis (Fermentation)

• Definition: Anaerobic process.
• ATP Production: Formation of 2 ATP (net) in glycolysis.
• NAD+ Regeneration: Essential for continuous glycolytic activity.

Lactic Acid Fermentation

• Pyruvate is converted to lactic acid.
• Enzyme: Lactate dehydrogenase (LDH).
• Regenerates $NAD^+$ from NADH.

Alcoholic Fermentation

• Pyruvate is converted to ethanol.
• Involves the intermediate acetaldehyde.
• Releases $\text{CO}_2$.
• Regenerates $\text{NAD}^+$.

Oral Implications

• Dental Caries: Initiated by fermentation.
• Plaque pH: Decreases to approximately 4 within 5 minutes of fermentation.
• Aciduric Bacteria: Produce lactate, formate, and pyruvate that cause demineralization of enamel.

Cori Cycle

• Function: Reutilizes lactate.
• Process: Lactate produced in muscle is transported to the liver, converted to pyruvate, then to glucose via gluconeogenesis, and returned to the muscle.

2,3-Bisphosphoglycerate (2,3-BPG)

• Role: Negative allosteric inhibitor of $\text{O}_2$-affinity of hemoglobin.
• Promotes the release of $\text{O}_2$ in peripheral tissue.
• Fetal hemoglobin (HbF) is less sensitive to 2,3-BPG than adult hemoglobin (HbA).

2,3-BPG Biosynthesis and Degradation

• Bypassing phosphoglycerate kinase results in a decreased ATP yield by 2.

Gluconeogenesis

• Primary Source: Blood glucose 8 hours after eating.
• Location: Primarily in the liver (minor in kidneys).
• Requirements: Energy (fatty acid metabolism) and carbon atoms (pyruvate, lactate, amino acids, glycerol).
• Muscle Protein: Major source of blood glucose during fasting and starvation.
• Rate Limiting: Availability of substrate and rate of proteolysis in muscle.
• Note: Gluconeogenesis is not the reverse of glycolysis.

Regulation of Gluconeogenesis

• Increased Gluconeogenesis: Glucagon and glucocorticoids.
• Glucocorticoids: Induce synthesis of hepatic amino transferases.
• High Glucagon: Favors induction of gluconeogenic enzymes, such as phosphoenolpyruvate carboxykinase (PEPCK).
• Glycolytic Enzymes: Hexokinase, phosphofructokinase, and pyruvate kinase are depressed.
• Insulin: Inhibits gluconeogenesis.

Irreversible Steps

• Glycolysis has 3 irreversible steps that must be overcome in gluconeogenesis.
• 4 unique enzymes circumvent these reactions.

Counterregulation

• Involves phosphorylation and dephosphorylation of enzymes under the control of glucagon and insulin.
• Fructose-1,6-bisphosphatase:
  • Activated by ATP.
  • Inhibited by AMP and fructose-2,6-phosphate.
• Pyruvate carboxylase:
  • Contains biotin (vitamin B7). Carries $\text{CO}_2$.
• PEPCK Absent in muscle, active in liver.

Amino Acids

• Alanine and glutamine are the major amino acids exported from muscle for gluconeogenesis.

Glycerol

• Enters gluconeogenesis at triose phosphates.
• Glucose cannot be synthesized from fatty acids.

Role of Insulin

• After a meal, insulin mediates the dephosphorylation of PFK-2/fructose-1,6-bisphosphatase.
• Increases PFK-2 activity, activating PFK-1 and inhibiting fructose-1,6-bisphosphatase.
• Inhibits gluconeogenesis.
• Glucose is incorporated into glycogen or routed into glycolysis for lipogenesis.
• In the mitochondrion, gluconeogenesis is regulated by acetyl-CoA.