Glycolysis: Key Steps and Enzymatic Reactions

Glycolysis

Introduction to Glycolysis

Glycolysis is a central metabolic pathway that converts glucose into pyruvate, generating ATP and NADH in the process. It occurs in the cytoplasm of cells and is fundamental to both aerobic and anaerobic metabolism.

• Location: Cytoplasm

• Main Purpose: Breakdown of glucose to extract energy for cellular metabolism

• Key Products: Pyruvate, ATP, NADH

Step 1: Hexokinase Reaction

Hexokinase catalyzes the phosphorylation of glucose to form glucose 6-phosphate, using ATP as the phosphate donor.

• Enzyme: Hexokinase

• Reaction: Glucose + ATP → Glucose 6-phosphate + ADP

• ΔG'°: -16.7 kJ/mol (exergonic, irreversible step)

Equation:

$\text{Glucose} + \text{ATP} \rightarrow \text{Glucose 6-phosphate} + \text{ADP}$

Example: This step traps glucose inside the cell and marks the first committed step of glycolysis.

Step 2: Phosphoglucose Isomerase Reaction

Phosphoglucose isomerase converts glucose 6-phosphate (an aldose) into fructose 6-phosphate (a ketose).

• Enzyme: Phosphoglucose isomerase

• Reaction: Glucose 6-phosphate ⇌ Fructose 6-phosphate

• ΔG'°: +1.7 kJ/mol (reversible)

Equation:

$\text{Glucose 6-phosphate} \rightleftharpoons \text{Fructose 6-phosphate}$

Example: This rearrangement prepares the molecule for subsequent phosphorylation.

Step 3: Phosphofructokinase-1 (PFK-1) Reaction

Phosphofructokinase-1 catalyzes the phosphorylation of fructose 6-phosphate to fructose 1,6-bisphosphate, using ATP. This is a key regulatory and irreversible step in glycolysis.

• Enzyme: Phosphofructokinase-1 (PFK-1)

• Reaction: Fructose 6-phosphate + ATP → Fructose 1,6-bisphosphate + ADP

• ΔG'°: -14.2 kJ/mol (irreversible, rate-limiting step)

Equation:

$\text{Fructose 6-phosphate} + \text{ATP} \rightarrow \text{Fructose 1,6-bisphosphate} + \text{ADP}$

Example: This step commits the substrate to glycolysis and is highly regulated by cellular energy status.

Step 4: Aldolase Reaction

Aldolase cleaves fructose 1,6-bisphosphate into two three-carbon sugars: glyceraldehyde 3-phosphate (G3P) and dihydroxyacetone phosphate (DHAP).

• Enzyme: Aldolase

• Reaction: Fructose 1,6-bisphosphate ⇌ Glyceraldehyde 3-phosphate (G3P) + Dihydroxyacetone phosphate (DHAP)

• ΔG'°: +23.8 kJ/mol (reversible under cellular conditions)

Equation:

$\text{Fructose 1,6-bisphosphate} \rightleftharpoons \text{Glyceraldehyde 3-phosphate} + \text{Dihydroxyacetone phosphate}$

Example: Both products are interconvertible, but only G3P continues directly in glycolysis.

Additional info: The steps above represent the preparatory phase of glycolysis, where ATP is consumed to phosphorylate glucose and its derivatives, priming them for subsequent energy-yielding steps.