Glycolysis

Glycolysis

is the metabolic pathway that converts one molecule of Glucose (a 6-carbon sugar) into two molecules of Pyruvate (a 3-carbon molecule).

• _{^{Location: The cytosol of all living cells.}}

• _{^{Oxygen Requirement: It does not require oxygen (it can occur under aerobic or anaerobic conditions).}}

Net Yield per molecule of Glucose

• 2 ATP (Produces 4, but consumes 2)

• 2 NADH (Carries high-energy electrons to the Electron Transport Chain)

• 2 Pyruvate

The pathway is divided into two distinct halves

The Energy Investment Phase (Steps 1-5)

The Energy Payoff Phase (Steps 6-10)

The Energy Investment Phase (Steps 1-5)

Consumes 2 ATP to phosphorylate and split the glucose molecule into two 3-carbon fragments.

The Energy Payoff Phase (Steps 6-10)

Produces 4 ATP and 2 NADH

The Three Irreversible (Kinase) Steps

_{^{The board loves the irreversible steps because these are the metabolic "checkpoints" that regulate the entire pathway. They are tightly controlled by hormones (Insulin/Glucagon) and cellular energy levels.}}

Step 1: Trapping the Glucose

Step 3: The Rate-Limiting Step (The Most Important Enzyme)

Step 10: The Final Payoff

Step 1: Trapping the Glucose

• Enzyme: Hexokinase (in most tissues) or Glucokinase (in the liver and pancreas).

• Reaction: Glucose → Glucose-6-Phosphate (G6P).

• Energy Cost: Uses 1 ATP.

Hexokinase

Found everywhere. Has a low Km (high affinity for glucose) so it works even when blood sugar is low. Inhibited by its product (G6P).

Glucokinase

Found only in the Liver and beta-cells of the Pancreas. Has a high Km (low affinity) and a high Vmax. It only acts when blood sugar is very high (after a meal), acting as a glucose sensor to trigger insulin release or store excess glucose as glycogen.

The Rate-Limiting Step (The Most Important Enzyme)

_{^{This is the committed step of glycolysis. Once the cell performs this reaction, the molecule is destined to finish glycolysis.}}

• Enzyme: Phosphofructokinase-1 (PFK-1).

• Reaction: Fructose-6-Phosphate $\rightarrow$ Fructose-1,6-bisphosphate.

• Energy Cost: Uses 1 ATP.

• Regulation: activators, inhibitors

Activators

AMP (signals low energy), Fructose-2,6-bisphosphate (triggered by insulin).

Inhibitors

ATP (signals abundant energy), Citrate.

Step 10: The Final Payoff

• Enzyme: Pyruvate Kinase (PK).

• Reaction: Phosphoenolpyruvate (PEP) $\rightarrow$ Pyruvate.

• Energy Yield: Produces 2 ATP (one for each 3-carbon chain). This is called substrate-level phosphorylation.

• Regulation: Activated by Fructose-1,6-bisphosphate (feed-forward activation). Inhibited by ATP and Alanine.

Step 6 (Glyceraldehyde-3-Phosphate Dehydrogenase)

This is the only step in glycolysis that produces NADH. It converts Glyceraldehyde-3-Phosphate into 1,3-Bisphosphoglycerate.

Step 7 (Phosphoglycerate Kinase)

This is the first step that produces ATP via substrate-level phosphorylation (converting 1,3-Bisphosphoglycerate to 3-Phosphoglycerate).

The Fate of Pyruvate (Aerobic vs. Anaerobic)

Once glycolysis yields Pyruvate, the cell must decide what to do with it based on oxygen availability.

Aerobic Conditions (Oxygen present)

Pyruvate enters the mitochondria and is converted by Pyruvate Dehydrogenase into Acetyl-CoA, which enters the Krebs Cycle. The NADH goes to the Electron Transport Chain to make massive amounts of ATP.

Anaerobic Conditions (No Oxygen / RBCs)

In actively exercising muscle or in Red Blood Cells (which lack mitochondria), the Electron Transport Chain shuts down. To keep glycolysis running, the cell must regenerate NAD^+

Enzyme: Lactate Dehydrogenase (LDH)

converts Pyruvate into Lactic Acid (Lactate), oxidizing NADH back into NAD^+ in the process.