10.1 Glycolysis and pyruvate oxidation

Glycolysis is the first step in cellular respiration, breaking down glucose into pyruvate. This process occurs in the cytosol and doesn't require oxygen, making it crucial for energy production under both aerobic and anaerobic conditions.

Pyruvate's fate depends on oxygen availability. With oxygen present, pyruvate is oxidized to acetyl-CoA and enters the citric acid cycle. Without oxygen, it undergoes fermentation instead. Understanding both pathways is key to grasping how cells manage energy production.

Glycolysis and Pyruvate Oxidation

Steps and enzymes of glycolysis

Glycolysis is a 10-step pathway in the cytosol that splits one 6-carbon glucose into two 3-carbon pyruvate molecules. It doesn't require oxygen. The pathway has two phases: an energy investment phase (steps 1–5, which consume 2 ATP) and an energy payoff phase (steps 6–10, which produce 4 ATP and 2 NADH).

1. Hexokinase phosphorylates glucose to glucose-6-phosphate (G6P). Costs 1 ATP.
2. Phosphoglucose isomerase converts G6P to fructose-6-phosphate (F6P).
3. Phosphofructokinase-1 (PFK-1) phosphorylates F6P to fructose-1,6-bisphosphate (F1,6BP). Costs 1 ATP. This is the committed step of glycolysis: it's irreversible and rate-limiting, making PFK-1 the most important regulatory enzyme in the pathway.
4. Aldolase cleaves F1,6BP into glyceraldehyde-3-phosphate (G3P) and dihydroxyacetone phosphate (DHAP).
5. Triose phosphate isomerase converts DHAP to G3P. From here on, every step happens twice per glucose (two G3P molecules proceed).
6. Glyceraldehyde-3-phosphate dehydrogenase oxidizes G3P to 1,3-bisphosphoglycerate (1,3BPG), reducing $NAD^+$ to $NADH$. This is the key redox step.
7. Phosphoglycerate kinase transfers a phosphate from 1,3BPG to ADP, producing ATP and 3-phosphoglycerate (3PG). This is substrate-level phosphorylation.
8. Phosphoglycerate mutase rearranges 3PG to 2-phosphoglycerate (2PG).
9. Enolase dehydrates 2PG to phosphoenolpyruvate (PEP), a high-energy compound.
10. Pyruvate kinase transfers a phosphate from PEP to ADP, producing ATP and pyruvate. This is the second substrate-level phosphorylation.

Glucose to pyruvate conversion

The net yield per glucose molecule is straightforward to calculate once you remember that steps 6–10 each happen twice:

• ATP: 4 produced (steps 7 and 10, ×2 each) minus 2 consumed (steps 1 and 3) = net 2 ATP
• NADH: 2 produced (step 6, ×2) = net 2 NADH
• Pyruvate: 2 molecules produced

The ATP here comes from substrate-level phosphorylation, where a phosphate group is transferred directly from a substrate to ADP. This is different from oxidative phosphorylation, which uses the electron transport chain. The 2 NADH carry high-energy electrons that will later feed into the electron transport chain (under aerobic conditions) to generate additional ATP.

Pyruvate fate in aerobic vs anaerobic conditions

Aerobic conditions (oxygen available):

• Pyruvate is transported into the mitochondrial matrix, where the pyruvate dehydrogenase complex converts it to acetyl-CoA (see next section).
• NADH from glycolysis shuttles its electrons into the mitochondria to be used by the electron transport chain for ATP generation.

Anaerobic conditions (no oxygen):

• The electron transport chain can't run without $O_2$ as the final electron acceptor, so NADH accumulates and $NAD^+$ gets depleted. Glycolysis would stall without $NAD^+$ for step 6.
• Fermentation solves this by regenerating $NAD^+$, allowing glycolysis to keep running. Fermentation does not produce additional ATP; its sole purpose is recycling $NAD^+$.

Two main types of fermentation:

• Lactic acid fermentation (in muscle cells, some bacteria): Lactate dehydrogenase reduces pyruvate to lactate, oxidizing NADH back to $NAD^+$. This is what causes the burn during intense exercise.
• Alcoholic fermentation (in yeast, some bacteria): A two-step process. First, pyruvate decarboxylase removes $CO_2$ from pyruvate to form acetaldehyde. Then alcohol dehydrogenase reduces acetaldehyde to ethanol, oxidizing NADH to $NAD^+$. This is the basis of brewing and bread-making (the $CO_2$ makes dough rise).

Pyruvate dehydrogenase complex role

The pyruvate dehydrogenase complex (PDC) is a large multi-enzyme complex located in the mitochondrial matrix. It serves as the bridge between glycolysis and the citric acid cycle by catalyzing the irreversible conversion of pyruvate to acetyl-CoA.

The overall reaction:

$Pyruvate + CoA + NAD^+ \rightarrow Acetyl\text{-}CoA + CO_2 + NADH$

This is an oxidative decarboxylation: pyruvate loses a carbon as $CO_2$ (decarboxylation) while being oxidized (electrons transferred to $NAD^+$). Because the reaction is irreversible, it commits carbon to the citric acid cycle. Per glucose, this step produces 2 NADH and releases 2 $CO_2$ (since two pyruvates enter).

PDC regulation is tightly controlled because it's an irreversible gateway:

• Allosteric inhibition: High levels of acetyl-CoA and NADH (products of the reaction) inhibit PDC. This is classic product inhibition: when the cell has plenty of these molecules, there's no need to make more.
• Covalent modification: Pyruvate dehydrogenase kinase phosphorylates PDC, which inactivates it. Pyruvate dehydrogenase phosphatase removes the phosphate, reactivating it. Think of it this way: phosphorylation = off, dephosphorylation = on.

The acetyl-CoA produced by PDC then enters the citric acid cycle, where it's fully oxidized to $CO_2$, generating NADH and $FADH_2$ that drive ATP production through oxidative phosphorylation.