Biochemistry - Lesson 11: Glycolysis and Fermentation
Glycolysis is a central metabolic pathway that breaks down glucose (a six-carbon sugar) into two molecules of pyruvate, generating ATP and reducing equivalents (NADH). This process is evolutionarily conserved in nearly all organisms, serving as a key entry point for the oxidation of carbohydrates. Under anaerobic or oxygen-limited conditions, pyruvate can be reduced to lactate (lactic acid fermentation) or ethanol (alcoholic fermentation), ensuring NAD⁺ regeneration to sustain glycolysis.
Overview of Glycolysis
Glycolysis takes place in the cytoplasm and is typically divided into two phases:
- Preparatory Phase: Uses ATP to phosphorylate glucose and fructose-6-phosphate, priming them for cleavage into two triose phosphates.
- Payoff Phase: Generates ATP and NADH from each triose phosphate, leading to pyruvate formation. Net yield per glucose is 2 ATP, 2 NADH, and 2 pyruvate.
Below is a table summarizing the 10 enzyme-catalyzed steps:
| Step | Substrate → Product | Enzyme | Key Points |
|---|---|---|---|
| 1 | Glucose → Glucose-6-phosphate | Hexokinase (or Glucokinase in liver) | ATP used; irreversible |
| 2 | Glucose-6-phosphate → Fructose-6-phosphate | Phosphoglucose isomerase | Isomerization of aldose to ketose |
| 3 | Fructose-6-phosphate → Fructose-1,6-bisphosphate | Phosphofructokinase-1 (PFK-1) | ATP used; major regulatory step; irreversible |
| 4 | Fructose-1,6-bisphosphate → DHAP + Glyceraldehyde-3-phosphate | Aldolase | Cleavage into two 3-carbon sugars |
| 5 | DHAP → Glyceraldehyde-3-phosphate | Triose phosphate isomerase | Ensures both 3-carbon units proceed identically |
| 6 | Glyceraldehyde-3-phosphate → 1,3-Bisphosphoglycerate | Glyceraldehyde-3-phosphate dehydrogenase | NAD⁺ reduced to NADH; phosphate added |
| 7 | 1,3-Bisphosphoglycerate → 3-Phosphoglycerate | Phosphoglycerate kinase | Generates ATP (substrate-level phosphorylation) |
| 8 | 3-Phosphoglycerate → 2-Phosphoglycerate | Phosphoglycerate mutase | Rearrangement of phosphate group |
| 9 | 2-Phosphoglycerate → Phosphoenolpyruvate (PEP) | Enolase | Dehydration; high-energy PEP formed |
| 10 | PEP → Pyruvate | Pyruvate kinase | ATP generated; irreversible |
Steps 1, 3, and 10 are effectively irreversible under physiological conditions and represent key regulatory points: hexokinase, phosphofructokinase-1 (PFK-1), and pyruvate kinase coordinate glycolytic flux in response to cellular demands.
Regulation Highlights
- PFK-1 is allosterically inhibited by ATP and citrate, signaling high-energy status, and activated by AMP, ADP, or fructose-2,6-bisphosphate (a potent regulator in some organisms).
- Hexokinase is inhibited by its product (glucose-6-phosphate), preventing buildup of phosphorylated glucose if PFK-1 is slow.
- Pyruvate kinase is activated by fructose-1,6-bisphosphate (feedforward stimulation) and inhibited by ATP and alanine.
Fates of Pyruvate: Fermentations
Under aerobic conditions, pyruvate typically enters mitochondria and is converted to acetyl-CoA for the citric acid cycle. However, in anaerobic or hypoxic situations, cells rely on fermentation to recycle NADH back to NAD⁺, allowing glycolysis to continue producing ATP.
Lactic Acid Fermentation
Many microorganisms and animal muscle cells convert pyruvate to lactate using lactate dehydrogenase:
This ensures a rapid ATP supply in muscle during strenuous exercise, albeit generating lactic acid (causing short-term muscle fatigue). Lactate can later be transported to the liver (Cori cycle) for gluconeogenesis.
Alcoholic Fermentation
Yeasts and some bacteria convert pyruvate to ethanol and CO2, regenerating NAD⁺. This two-step process involves pyruvate decarboxylase (releasing CO2) and alcohol dehydrogenase:
This pathway underpins alcoholic beverage production and bread leavening (CO2 bubbles).
Summary
Glycolysis initiates carbohydrate catabolism by splitting glucose into two pyruvates, netting ATP and NADH. Irreversible steps (hexokinase, PFK-1, pyruvate kinase) orchestrate pathway flux and integrate signals on cellular energy demand. In oxygen-limited environments, pyruvate is reduced via fermentation (lactic acid or ethanol) to regenerate NAD⁺, thereby sustaining ATP production from glycolysis. The regulation and versatility of glycolysis underscore its central role in metabolism, connecting to aerobic respiration, gluconeogenesis, and numerous biosynthetic routes.
Suggested Reading:
Lehninger Principles of Biochemistry (chapters on glycolysis and fermentation)
Biochemistry by Berg, Tymoczko, and Stryer (sections detailing glycolytic reactions, regulation, and pyruvate metabolism)
