Glycolysis Overview

Glycolysis Overview

• Definition: Glycolysis is the process of breaking down glucose to extract energy, primarily producing ATP and NADH.
• Key Steps in Glycolysis:
• Glucose Activation:
  • Hexokinase catalyzes the first step by phosphorylating glucose to glucose-6-phosphate (G6P).
  • This reaction has a high affinity for glucose (Km ~ 1 mM).
• Isomerization:
  • G6P is converted into fructose-6-phosphate (F6P) by glucose-6-phosphate isomerase.
• Fructose Phosphate Phosphorylation:
  • Phosphofructokinase (PFK) catalyzes the conversion of F6P to fructose-1,6-bisphosphate (F1,6BP).
  • This is a rate-limiting and irreversible step, critical for regulation.
• Cleavage:
  • Aldolase splits F1,6BP into two 3-carbon sugars: dihydroxyacetone phosphate (DHAP) and glyceraldehyde-3-phosphate (GAP).
• Interconversion:
  • Triose phosphate isomerase converts DHAP into GAP, leading to the production of two GAP molecules per glucose.
• Oxidation and Phosphate Transfer:
  • Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) oxidizes GAP to 1,3-bisphosphoglycerate, producing NADH.
  • Phosphoglycerate kinase (PGK) converts 1,3-bisphosphoglycerate to 3-phosphoglycerate (3PG), producing ATP (first ATP generation).
• Further Conversion:
  • Mutase and enolase convert 3PG into phosphoenolpyruvate (PEP).
• Final ATP Generation:
  • Pyruvate kinase catalyzes the last step converting PEP to pyruvate and generates ATP (second ATP generation).
• Overall Reaction:
  • Glucose + 2 ADP + 2 Pi + 2 NAD+ → 2 Pyruvate + 2 ATP + 2 NADH

Enzyme Functions and Regulation

• Hexokinase: Activates glucose, prevents hydrolysis of intermediates, and exists in several isozymes.
• Phosphofructokinase (PFK): Considered the most crucial control point for glycolysis, regulated by:
• Citrate (inhibitor, indicating energy sufficiency).
• Fructose-2,6-bisphosphate (activator, indicating low-energy state).
• Pyruvate Kinase: Another key regulatory enzyme, active in the presence of fructose-1,6-bisphosphate, and subjected to hormonal regulation in liver (inactivated by phosphorylation).

Energy Production During Glycolysis

• ATP Yield: Net ATP production is 2 ATP per glucose molecule.
• Anaerobic Conditions: Under low oxygen levels, cells convert pyruvate to lactate (lactate fermentation) to regenerate NAD+ to maintain glycolysis.
• Alternatively, pyruvate can be converted to ethanol in yeast (fermentation).

Metabolic Connections

• Glycolysis interacts with other metabolic pathways:
• Pentose Phosphate Pathway: Provides NADPH and ribose-5-phosphate for nucleotide synthesis.
• 2,3-BPG Shunt: Regulates oxygen release from hemoglobin, particularly in red blood cells.
• Alternative Entry Points:
• Fructose and galactose can enter glycolysis via different pathways, bypassing key regulatory steps, which can lead to uncontrolled glucose utilization.

Cellular Transport Mechanisms

• Glucose Transporters (GLUTs):
• GLUT1: Baseline glucose uptake in many cells, not insulin sensitive.
• GLUT2: Found in liver/pancreas, functions based on glucose concentration, not insulin regulated.
• GLUT4: Insulin-sensitive; found in muscle and fat tissues, where glucose uptake increases in response to insulin.

Metabolic Effects and Predictions

• Enzyme Inhibitors: Understanding the effects of inhibitors (e.g., arsenate mimicking phosphate in reactions) on glycolysis and potential metabolic consequences (e.g., reduction in ATP production).