Glycolysis Fates of Pyruvate

Main Ideas

• Glycolysis produces pyruvate that still contains substantial energy. Its net products include two pyruvate, two NADH, and two ATP. What happens to pyruvate next depends largely on oxygen availability.
• Pyruvate is a metabolic crossroads:
  • With sufficient oxygen, it can be converted to acetyl-CoA and enter the citric acid cycle.
  • Under anaerobic conditions, it can be converted to lactate or, in organisms such as yeast, to ethanol.
  • Pyruvate also has other metabolic routes, which will be covered later.

Lactic Acid Fermentation

During intense exercise, muscle cells may use oxygen faster than it can be supplied, so they temporarily rely on anaerobic metabolism. Glycolysis converts glucose to pyruvate while reducing NAD⁺ to NADH.

In lactic acid fermentation, pyruvate accepts electrons from NADH and is reduced to lactate. NADH is oxidized back to NAD⁺, allowing glycolysis to continue. Without regenerated NAD⁺, glycolysis would stop. Lactate can later be reoxidized.

Lactic acid fermentation is also used by bacteria to produce foods such as yogurt and sauerkraut; some probiotic bacteria belong to this group.

The lecture describes the fermentation step as spontaneous. Considered alongside glycolysis, it reverses the oxidation–reduction changes that produced pyruvate: pyruvate is reduced while NADH is oxidized.

Ethanol Fermentation

Ethanol fermentation also restores the NAD⁺ needed for glycolysis. It converts pyruvate to ethanol in two steps:

1. Decarboxylation: Pyruvate loses carbon dioxide and becomes acetaldehyde.
2. Reduction: NADH transfers electrons to acetaldehyde, producing ethanol and regenerating NAD⁺.

This pathway occurs in yeast and is relevant to bread baking and beer production. The carbon dioxide released during fermentation causes bubbling in beer.

In the first step, pyruvate decarboxylase uses thiamine pyrophosphate (TPP), a cofactor derived from vitamin B₁ (thiamine). The instructor distinguishes thiamine from thymine, a DNA base, and notes that TPP also plays a role in pyruvate dehydrogenase, which will be discussed later.

In the second step, a zinc ion in the enzyme’s active site helps polarize acetaldehyde’s carbonyl group, enabling it to accept hydrogen and electrons from NADH.

Key Lesson

Both fermentation pathways allow glycolysis to continue without relying on oxygen-dependent electron transport by oxidizing NADH back to NAD⁺. The lecture closes by previewing the next topic: how cells make glucose rather than break it down.

Speakers and Sources

• Speaker: One unnamed instructor or narrator.
• Sources or other speakers: No additional speakers or cited sources are identified.