Table of Contents
Overview of Glycolysis
Glycolysis is the central pathway by which cells extract usable energy from glucose without directly using oxygen. It takes place in the cytosol of virtually all cells (prokaryotic and eukaryotic) and forms the first stage of cellular respiration. In glycolysis, one molecule of glucose ($C_6H_{12}O_6$) is converted into two molecules of pyruvate, with a net gain of ATP and NADH.
In simplified form, the overall reaction is:
$$
\text{Glucose} + 2 \, \text{ADP} + 2 \, P_i + 2 \, \text{NAD}^+ \\
\longrightarrow 2 \, \text{Pyruvate} + 2 \, \text{ATP} + 2 \, \text{NADH} + 2 \, H^+ + 2 \, H_2O
$$
Here $P_i$ stands for inorganic phosphate.
Glycolysis can be divided into two main phases:
- An energy investment phase (preparatory phase)
- An energy payoff phase (yield phase)
Each phase consists of characteristic steps controlled by specific enzymes.
The Energy Investment (Preparatory) Phase
In the first half of glycolysis, the cell invests ATP to activate glucose and split it into two 3‑carbon molecules. Although energy is consumed here, these steps are necessary to make the later extraction of energy possible.
Step 1: Phosphorylation of Glucose
- Enzyme: hexokinase (or in liver: glucokinase)
- Reaction:
$$
\text{Glucose} + \text{ATP} \rightarrow \text{Glucose-6-phosphate} + \text{ADP}
$$ - Purpose:
- Traps glucose in the cell (phosphorylated sugars do not easily cross the membrane).
- Begins to “activate” glucose chemically.
This step is essentially irreversible under physiological conditions and is one of the control points of glycolysis.
Step 2: Isomerization to Fructose-6-Phosphate
- Enzyme: phosphoglucose isomerase
- Reaction:
$$
\text{Glucose-6-phosphate} \rightleftharpoons \text{Fructose-6-phosphate}
$$ - Purpose:
- Converts an aldose (glucose) into a ketose (fructose).
- Prepares the molecule for a later step where it will be symmetrically split into two 3‑carbon units.
This step is reversible and near equilibrium.
Step 3: Second Phosphorylation – Key Regulatory Step
- Enzyme: phosphofructokinase-1 (PFK-1)
- Reaction:
$$
\text{Fructose-6-phosphate} + \text{ATP} \rightarrow \text{Fructose-1,6-bisphosphate} + \text{ADP}
$$ - Purpose:
- Adds a second phosphate group, further activating the sugar.
- Commits the molecule to glycolysis; fructose-1,6-bisphosphate is not used in other major pathways.
PFK-1 is a major “pacemaker” enzyme of glycolysis and is heavily regulated by cellular energy status (details of regulation belong to enzyme and metabolism regulation chapters).
Step 4: Cleavage into Two 3‑Carbon Molecules
- Enzyme: aldolase
- Reaction:
$$
\text{Fructose-1,6-bisphosphate} \rightleftharpoons \text{Dihydroxyacetone phosphate (DHAP)} + \text{Glyceraldehyde-3-phosphate (GAP)}
$$ - Purpose:
- Splits the 6‑carbon sugar into two 3‑carbon phosphorylated intermediates.
Both products are 3‑carbon compounds; however, only GAP continues directly in the next step of glycolysis.
Step 5: Conversion of DHAP to GAP
- Enzyme: triose phosphate isomerase
- Reaction:
$$
\text{DHAP} \rightleftharpoons \text{GAP}
$$ - Purpose:
- Converts DHAP into GAP so that both 3‑carbon molecules can be processed in the same way.
- After this step, there are two molecules of GAP per original glucose.
From this point on, all subsequent reactions occur twice for each glucose molecule, because there are two molecules of GAP.
Overall result of the investment phase:
- ATP consumed: 2
- ATP produced: 0
- Products: 2 GAP (glyceraldehyde-3-phosphate)
The Energy Payoff (Yield) Phase
In the second half of glycolysis, the two molecules of GAP are oxidized and further modified. This generates ATP (substrate-level phosphorylation) and NADH.
Step 6: Oxidation and Phosphorylation of GAP
- Enzyme: glyceraldehyde-3-phosphate dehydrogenase
- Reaction:
$$
\text{GAP} + P_i + \text{NAD}^+ \rightleftharpoons \text{1,3-Bisphosphoglycerate (1,3-BPG)} + \text{NADH} + H^+
$$ - Purpose:
- Oxidizes GAP, transferring electrons to $\text{NAD}^+$ and forming NADH (a reduced coenzyme).
- Attaches an inorganic phosphate, generating a high‑energy acyl phosphate (1,3-BPG).
Because this occurs twice per glucose:
- 2 NADH are produced in total.
Step 7: First Substrate-Level Phosphorylation
- Enzyme: phosphoglycerate kinase
- Reaction:
$$
\text{1,3-BPG} + \text{ADP} \rightleftharpoons \text{3-Phosphoglycerate} + \text{ATP}
$$ - Purpose:
- Transfers a high‑energy phosphate from 1,3-BPG to ADP to form ATP.
- This is an example of substrate-level phosphorylation (formation of ATP directly from a high-energy substrate).
Per glucose:
- 2 ATP are produced here (one from each 1,3-BPG).
Step 8: Rearrangement Within the Molecule
- Enzyme: phosphoglycerate mutase
- Reaction:
$$
\text{3-Phosphoglycerate} \rightleftharpoons \text{2-Phosphoglycerate}
$$ - Purpose:
- Shifts the phosphate group from the 3‑position to the 2‑position.
- Prepares for the formation of another high‑energy compound in the next step.
This is a reversible isomerization.
Step 9: Dehydration to Form Phosphoenolpyruvate (PEP)
- Enzyme: enolase
- Reaction:
$$
\text{2-Phosphoglycerate} \rightleftharpoons \text{Phosphoenolpyruvate (PEP)} + H_2O
$$ - Purpose:
- Removes water (dehydration).
- Creates PEP, a molecule with a very high‑energy phosphate bond.
Step 10: Second Substrate-Level Phosphorylation – Formation of Pyruvate
- Enzyme: pyruvate kinase
- Reaction:
$$
\text{PEP} + \text{ADP} \rightarrow \text{Pyruvate} + \text{ATP}
$$ - Purpose:
- Transfers the phosphate from PEP to ADP, forming ATP.
- Produces pyruvate, the end product of glycolysis.
This reaction is essentially irreversible under cellular conditions and is another key regulatory point of glycolysis.
Per glucose:
- 2 ATP are produced in this step.
Energy Yield and Balance of Glycolysis
Per molecule of glucose, the net energy and redox balance are:
- ATP:
- Investment: 2 ATP consumed (steps 1 and 3)
- Payoff: 4 ATP produced (2 in step 7, 2 in step 10)
- Net gain:
$$
4 - 2 = 2 \text{ ATP}
$$ - NADH:
- 2 NADH produced (step 6, once for each GAP)
- End products:
- 2 pyruvate
- 2 $H_2O$
- 2 $H^+$ (associated with NADH formation)
Under aerobic conditions, the NADH and pyruvate can be further oxidized in later stages of cellular respiration. Under anaerobic conditions, pyruvate is reduced (for example, to lactate or ethanol), which regenerates $\text{NAD}^+$ so that glycolysis can continue.
Regulation and Significance of Glycolysis
Key Regulatory Steps
Three glycolytic steps are effectively irreversible and play major roles in controlling the rate of glycolysis:
- Hexokinase (step 1)
- Phosphofructokinase-1, PFK-1 (step 3)
- Pyruvate kinase (step 10)
Their activity is influenced mainly by:
- Cellular ATP/ADP/AMP levels (energy status)
- Availability of substrates (e.g., glucose)
- Intermediates and end products (e.g., citrate, pyruvate)
- In some organisms and tissues, hormones (discussed in hormone chapters)
Fine details of enzyme regulation are treated elsewhere; here it is important to note that glycolysis is carefully adjusted to the cell’s current energy needs.
Biological Roles of Glycolysis
- Universal pathway: Occurs in nearly all organisms and in almost all cells.
- Anaerobic capability: Does not require oxygen directly; therefore essential for energy production in anaerobic environments or in cells with limited oxygen supply.
- Provides intermediates: Several glycolytic intermediates serve as starting materials for other pathways, such as:
- Amino acid synthesis
- Lipid synthesis
- Production of other carbohydrates
- Central hub of metabolism: Links to:
- Fermentation pathways (anaerobic)
- Pyruvate oxidation and citric acid cycle (aerobic cellular respiration)
Fates of Pyruvate After Glycolysis (Overview Only)
Glycolysis ends with the formation of pyruvate. What happens next depends on oxygen availability and organism type:
- In the presence of oxygen (aerobic conditions), pyruvate is typically transported into mitochondria (in eukaryotes) and converted into acetyl-CoA, which enters the citric acid cycle.
- In the absence of oxygen (anaerobic conditions) or in specialized cells, pyruvate can be reduced to:
- Lactate (lactic acid fermentation)
- Ethanol and $CO_2$ (alcoholic fermentation in yeast and some microorganisms)
The detailed mechanisms and consequences of these pathways are addressed in the chapters on pyruvate processing, the citric acid cycle, and fermentation.
Summary
- Glycolysis is a cytosolic pathway that converts one glucose to two pyruvate molecules.
- It consists of ten enzyme-catalyzed steps, grouped into an energy investment phase and an energy payoff phase.
- The net yield per glucose is 2 ATP and 2 NADH.
- Key irreversible steps (hexokinase, PFK-1, pyruvate kinase) regulate the pathway.
- Glycolysis operates with or without oxygen and supplies both energy and metabolic intermediates essential for many other cellular processes.