Comprehensive Study Guide to Glycolysis and the Embden-Meyerhoff Pathway

Overview of Glycolysis

Glycolysis is a fundamental biochemical reaction that serves as a cornerstone of cellular respiration. At its core, it involves the breakdown of a single glucose molecule, which is a six-carbon sugar (C6H12O6C_6H_{12}O_6), into two separate molecules of pyruvate, which is a three-carbon compound.

Historically, this metabolic pathway is also referred to as the Embden-Meyerhoff pathway, established around 1930.

Site of Occurrence and Biological Context

Glycolysis take place within the cytoplasm of the cell and occurs in all living cells. It is uniquely categorized as an anaerobic pathway because the process does not utilize oxygen to proceed.

This anaerobic nature is critical in specific physiological conditions, such as:

  • Skeletal Muscle Function: Muscles depend heavily on glycolysis for energy during periods of excessive activity. During such times, the supply of oxygen may become insufficient to sustain aerobic reactions, forcing the cells to rely on the glycolytic pathway for ATP production.

Detailed Breakdown of Glycolytic Steps

The transformation of glucose into pyruvate occurs through a series of ten distinct enzymatic reactions, subdivided into several phases:

1. Phosphorylation of Glucose
  • Enzyme: Hexokinase (often requires Mg++Mg^{++} as a cofactor).
  • Action: This step involves the transfer of a phosphate group from one molecule of ATP to the glucose molecule.
  • Result: This reaction forms glucose-6-phosphate and releases ADP.
2. Isomerisation
  • Enzyme: Phosphoglucose isomerase (notably referred to in the text as phosphoglucomutase).
  • Action: The enzyme converts glucose-6-phosphate into its isomer.
  • Result: Fructose-6-phosphate is created.
3. Second Phosphorylation
  • Enzyme: Phosphofructokinase (PFK).
  • Action: Another phosphate group is transferred from a second molecule of ATP to the fructose-6-phosphate.
  • Result: This creates fructose-1,6-bisphosphate.
4. Cleavage
  • Enzyme: Aldolase.
  • Action: The six-carbon fructose-1,6-bisphosphate is split into two distinct three-carbon molecules.
  • Result: The products of this split are glyceraldehyde-3-phosphate (G3P) and dihydroxyacetone phosphate (DHAP).
5. Isomerisation of DHAP
  • Enzyme: Triose phosphate isomerase.
  • Action: Because only glyceraldehyde-3-phosphate proceeds directly through the pathway, DHAP is converted into another G3P molecule.
  • Result: At the end of this step, there are now two G3P molecules for every original glucose molecule.
6. Oxidation and Phosphate Addition
  • Enzyme: Glyceraldehyde-3-phosphate dehydrogenase.
  • Action: This step involves the oxidation of G3P and the addition of an inorganic phosphate (PiP_i).
  • Result: This produces 1,3-bisphosphoglycerate. (Note: The transcript mentions the formation of NADH during this dehydrogenation/phosphorylation phase).
7. First ATP Formation (Dephosphorylation)
  • Enzyme: Phosphoglycerate kinase (utilizes Mg++Mg^{++}).
  • Action: A phosphate group is transferred from 1,3-bisphosphoglycerate to ADP.
  • Result: This generates one molecule of ATP and produces 3-phosphoglycerate.
8. Relocation of Phosphate (Rearrangement)
  • Enzyme: Phosphoglyceromutase.
  • Action: The phosphate group is moved from the third carbon to the second carbon of the molecule.
  • Result: 3-phosphoglycerate is converted into 2-phosphoglycerate.
9. Dehydration
  • Enzyme: Enolase.
  • Action: A water molecule (H2OH_2O) is removed from 2-phosphoglycerate.
  • Result: This reaction results in the formation of phosphoenolpyruvate (PEP).
10. Second ATP Formation (Dephosphorylation)
  • Enzyme: Pyruvate kinase.
  • Action: The phosphate group from PEP is transferred to ADP.
  • Result: This generates another ATP molecule and results in the final product: Pyruvate.

Summary of Final Results

For every single molecule of glucose that enters the glycolytic pathway, the net result includes the production of the following molecules:

  • Pyruvate: 2 molecules
  • ATP: 2 molecules (net gain, calculated as 4 produced minus 2 consumed in the early steps)
  • NADH: 2 molecules