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10 Steps of Glycolysis: Enzymes, Diagram & ATP Yield

Glycolysis is a fundamental metabolic pathway that involves the breakdown of glucose to produce energy. This process can occur through two main pathways: aerobic and anaerobic glycolysis. Both pathways play crucial roles in cellular metabolism, but they operate under different conditions and result in distinct end products.

Key Takeaways:

  1. Glycolysis is a cellular process converting glucose to energy.
  2. It includes key steps like substrate-level phosphorylation and glucose-phosphate conversion.
  3. Enzymes like hexokinase and aldolase catalyze glycolysis stages.
  4. Glycolysis products include pyruvate, NADH, and ATP, vital for cell functions.
  5. Disorders related to glycolysis include diabetes, cancer, and heart disease.

What is Glycolysis?

Glycolysis is the first step in the cellular respiration process and occurs in the cytoplasm of the cell. It involves a series of ten enzymatic reactions that convert one molecule of glucose into two molecules of pyruvate, generating ATP and NADH in the process. This pathway is essential for energy production in both aerobic and anaerobic organisms.

Main Steps Involved In The Glycolysis Pathway

There are three main steps in glycolysis: substrate level phosphorylation, conversion of glucose-phosphate to fructose-phosphate, and the formation of two molecules of phosphate. In addition, there are several intermediate steps that occur between these three main steps such as the conversion of fructose-bisphosphate to glyceraldehyde-phosphate and the conversion of dihydroxyacetone phosphate to glycerol-phosphate.

Steps involved in the glycolysis pathway (click to enlarge). To download click here!

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Enzymes Involved In Glycolysis

There are several enzymes involved in glycolysis which help to catalyze the various steps of the glycolytic pathway. The enzymes involved in glycolysis are typically found in the cytoplasm of cells.

Enzyme Function

Hexokinase/glucokinase

Catalyzes the conversion of glucose to glucose-phosphate

Phosphofructokinase

Catalyzes the conversion of fructose-phosphate to fructose-bisphosphate

Aldolase

Cleaves fructose-bisphosphate into two molecules of glyceraldehyde-phosphate

Triose phosphate isomerase

Converts glyceraldehyde-phosphate to dihydroxyacetone phosphate

Enolase

Converts dihydroxyacetone phosphate to phosphoenolpyruvate

Pyruvate kinase

Catalyzes the conversion of phosphoenolpyruvate to pyruvate.

Products of Glycolysis and Downstream Effects

The products of glycolysis include pyruvate, NADH, and ATP.

  • Pyruvate is an important molecule that is used in several different pathways in the body. Pyruvate is produced as a result of the conversion of glucose to two molecules of acetyl-CoA. After formation of pyruvate, it is further converted to acetyl-coenzyme A (acetyl-CoA) in the citric acid cycle. The citric acid cycle is also known as the Krebs cycle, and it is responsible for the production of ATP from acetyl-CoA. In the absence of oxygen, pyruvate is converted into lactate. Lactic acid fermentation is an important process that helps to produce ATP in muscles.

  • NADH is a coenzyme that helps to transfer electrons in the electron transport chain. NADH is produced as a result of the reduction of nicotinamide adenine dinucleotide (NAD+) to NADH.

  • ATP is the main source of energy for cells. ATP is produced as a result of the transfer of phosphate groups from glucose-phosphate to ADP. In glycolysis, two molecules of ATP are used to convert glucose into energy. However, four molecules of ATP are produced as a result of this pathway. This means that there is a net production of two ATP in glycolysis.

Types of Glycolysis

Aerobic Glycolysis

Aerobic glycolysis occurs in the presence of oxygen. The pyruvate produced during glycolysis is transported into the mitochondria, where it undergoes further oxidation in the citric acid cycle (Krebs cycle). This is followed by oxidative phosphorylation in the electron transport chain, which produces a significant amount of ATP.

  1. Glucose Activation: Glucose is phosphorylated to glucose-6-phosphate by hexokinase, consuming one ATP.
  2. Fructose Formation: Glucose-6-phosphate is converted to fructose-6-phosphate, which is then phosphorylated to fructose-1,6-bisphosphate.
  3. Cleavage: Fructose-1,6-bisphosphate is split into two three-carbon molecules: glyceraldehyde-3-phosphate and dihydroxyacetone phosphate.
  4. Energy Extraction: Glyceraldehyde-3-phosphate is oxidized, producing NADH and ATP. The end product, pyruvate, enters the mitochondria for further energy production.

Anaerobic Glycolysis

Anaerobic glycolysis occurs in the absence of oxygen. Under these conditions, the pyruvate generated from glycolysis is converted into lactate in the cytoplasm. This process regenerates NAD+ from NADH, allowing glycolysis to continue producing ATP in the absence of oxidative phosphorylation.

  1. Glucose Activation: Similar to aerobic glycolysis, glucose is phosphorylated to glucose-6-phosphate, consuming one ATP.
  2. Fructose Formation: The pathway proceeds similarly until the formation of pyruvate.
  3. Lactate Formation: Pyruvate is reduced to lactate by lactate dehydrogenase, regenerating NAD+ from NADH. This allows glycolysis to continue producing ATP under anaerobic conditions.

Comparison of Aerobic and Anaerobic Glycolysis

Efficiency and Yield

Aerobic glycolysis, followed by oxidative phosphorylation, yields a high amount of ATP—approximately 36-38 ATP molecules per glucose molecule. In contrast, anaerobic glycolysis yields only 2 ATP molecules per glucose molecule, making it significantly less efficient.

Conditions and Applications

  • Aerobic Conditions: Cells rely on aerobic glycolysis during normal oxygen conditions, such as in most tissues of the body. It is the primary energy production pathway in muscles during prolonged, low-intensity exercise.
  • Anaerobic Conditions: Anaerobic glycolysis is critical during oxygen-deprived conditions, such as in muscle cells during intense exercise, or in certain microorganisms that thrive in anaerobic environments.

Metabolic Implications

The accumulation of lactate during anaerobic glycolysis can lead to muscle fatigue and cramps. In contrast, aerobic glycolysis and subsequent oxidative phosphorylation do not produce lactate, avoiding these negative effects.

Diseases Associated with the Glycolytic Pathway

Dysfunctional glycolysis can lead to problems with the production of energy. ATP is essential for cellular function, and problems with its production can lead to symptoms such as weakness, fatigue, and muscle pain. Diseases that can be caused by problems with glycolysis include diabetes, cancer, and heart disease.

Deficiency in glycolytic enzymes such as hexokinase can lead to diabetes. Defects in pyruvate kinase can also lead to heart disease. The glycolysis pathway is important for the survival of tumor cells. Cancer cells often rely on glycolysis for energy because the Warburg effect allows them to bypass oxidative phosphorylation.

Diseases caused due to overactive glycolytic pathway are less common, but they can be very serious. Overactive glycolysis can lead to lactic acidosis, which is a build-up of lactate in the blood. Lactic acidosis can be caused by problems with the enzymes that are involved in glycolysis or by a lack of oxygen. Lactic acidosis can lead to serious health problems such as coma and death.

Written by Colm Ryan

Colm Ryan PhD is a co-founder of Assay Genie. Colm carried out his undergraduate degree in Genetics in Trinity College Dublin, followed by a PhD at the University of Leicester. Following this Colm carried out a post-doc in the IGBMC in Strasbourg, France. Colm is now Chief Executive Officer at Assay Genie.

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What Is Glycolysis? — Definition and Overview

Glycolysis is the metabolic pathway that converts one glucose molecule into two pyruvate molecules, generating 2 ATP and 2 NADH in the process. This ten-step enzymatic process occurs in the cytoplasm of nearly all living cells and serves as the primary source of ATP for anaerobic respiration. Despite its simplicity relative to other metabolic pathways, glycolysis is one of the most important biochemical processes in biology, providing both energy and biosynthetic precursors for cellular functions.

The pathway's efficiency and speed make it essential during high energy demand or low oxygen conditions. Each glucose molecule is broken down progressively, with energy captured in high-energy phosphate bonds and reducing power stored in NADH molecules.

What Is the Overall Equation for Glycolysis?

C6H12O6 + 2 NAD+ + 2 ADP + 2 Pi → 2 Pyruvate + 2 NADH + 2 H+ + 2 ATP

This equation represents the net result of the ten glycolytic reactions. One glucose molecule (6-carbon sugar) is converted into two pyruvate molecules (3-carbon organic acid). The process requires 2 NAD+ as an electron acceptor and consumes 2 ATP in the preparatory phase, but generates 4 ATP in the payoff phase, yielding a net gain of 2 ATP per glucose. Additionally, the pathway produces 2 NADH, which can be reoxidized to NAD+ during the electron transport chain in aerobic respiration.

What Are the 10 Steps of Glycolysis?

Glycolysis is divided into two phases: the energy investment phase (steps 1-3) and the payoff phase (steps 6-10). Step 4-5 prepare the intermediates. Below is a comprehensive summary:

Step Enzyme Substrate → Product Energy Change
1 Hexokinase Glucose → Glucose-6-phosphate −ATP consumed
2 Phosphoglucose isomerase Glucose-6-phosphate → Fructose-6-phosphate No energy change
3 Phosphofructokinase-1 (PFK-1) Fructose-6-phosphate → Fructose-1,6-bisphosphate −ATP consumed (rate-limiting)
4 Aldolase Fructose-1,6-bisphosphate → DHAP + G3P No energy change
5 Triose phosphate isomerase DHAP → Glyceraldehyde-3-phosphate No energy change
6 G3P dehydrogenase G3P + NAD+ → 1,3-BPG + NADH +NADH produced
7 Phosphoglycerate kinase 1,3-BPG → 3-PG + ATP +ATP generated
8 Phosphoglycerate mutase 3-PG → 2-PG No energy change
9 Enolase 2-PG → Phosphoenolpyruvate No energy change
10 Pyruvate kinase PEP → Pyruvate + ATP +ATP generated

Steps 1–3 consume 2 ATP (investment phase), while steps 6–7 and 9–10 generate 4 ATP total (payoff phase), yielding a net of 2 ATP. The doubling effect occurs because step 4 creates two 3-carbon molecules, and all subsequent reactions occur twice per glucose.

How Is Glycolysis Regulated?

Phosphofructokinase-1 (PFK-1) is the primary rate-limiting enzyme of glycolysis, catalyzing step 3. This enzyme is the main control point because it catalyzes the first committed step of the pathway.

PFK-1 is subject to allosteric regulation by several metabolites:

  • Inhibitors: ATP and citrate (signals of sufficient energy), decrease PFK-1 activity
  • Activators: AMP and ADP (signals of low energy), increase PFK-1 activity
  • Activator: Fructose-2,6-bisphosphate (F-2,6-BP), a potent activator produced by PFK-2

Hormonal control also regulates glycolysis. Insulin (fed state) promotes glycolysis by increasing PFK-2 activity and glucose uptake. Glucagon (fasting state) inhibits glycolysis and activates gluconeogenesis. These mechanisms ensure glycolysis operates at appropriate rates based on cellular energy needs and metabolic state.

What Is the Difference Between Aerobic and Anaerobic Glycolysis?

While the glycolytic pathway itself is identical in both conditions, the fates of pyruvate and the availability of NAD+ differ significantly:

Feature Aerobic Glycolysis Anaerobic Glycolysis
Oxygen Requirement Needed for NAD+ regeneration via ETC Not required; NAD+ regenerated via lactate formation
End Product Pyruvate enters TCA cycle, fully oxidized Pyruvate converted to lactate (Cori cycle)
ATP Yield 30–32 ATP from complete glucose oxidation 2 ATP from glycolysis only
Location Cytoplasm + mitochondria Cytoplasm only
Clinical Relevance Primary ATP source in resting cells Intense exercise, hypoxia, cancer cells (Warburg effect)

Lactate formation (via lactate dehydrogenase) regenerates NAD+ in anaerobic conditions, allowing glycolysis to continue. Lactate is then transported to the liver for gluconeogenesis in the Cori cycle.

How Does Glycolysis Relate to Other Metabolic Pathways?

Glycolysis is a central metabolic hub connecting multiple pathways:

  • TCA Cycle (Citric Acid Cycle): Pyruvate enters the mitochondria and is converted to acetyl-CoA for complete oxidation and maximum ATP yield.
  • Gluconeogenesis: Seven of the ten glycolytic enzymes are reversible; the pathway can run backwards when glucose is needed from non-carbohydrate sources.
  • Pentose Phosphate Pathway: Glucose-6-phosphate can be diverted to generate NADPH and ribose-5-phosphate for nucleotide synthesis.
  • Glycogenesis/Glycogenolysis: Glucose-6-phosphate connects to glycogen synthesis and breakdown.
  • Fatty Acid Synthesis: Pyruvate and acetyl-CoA provide carbon skeletons for lipogenesis.
  • Amino Acid Synthesis: Pyruvate and glycolytic intermediates serve as precursors for non-essential amino acid biosynthesis.

This metabolic integration allows cells to coordinate energy production, storage, and biosynthesis in response to cellular needs and nutrient availability.

Frequently Asked Questions About Glycolysis

What is the net ATP yield of glycolysis?
The net ATP yield is 2 ATP per glucose molecule. Glycolysis consumes 2 ATP in the preparatory phase (steps 1 and 3) and produces 4 ATP in the payoff phase (steps 7 and 10, occurring twice per glucose), resulting in a net gain of 2 ATP.
Where does glycolysis occur in the cell?
Glycolysis occurs in the cytoplasm of all cells. Unlike the TCA cycle and electron transport chain, which are localized in the mitochondrial matrix and cristae respectively, glycolytic enzymes are distributed throughout the cytoplasm.
What is the rate-limiting enzyme of glycolysis?
Phosphofructokinase-1 (PFK-1) catalyzing step 3 is the primary rate-limiting enzyme. It is inhibited by ATP and citrate and activated by AMP, ADP, and fructose-2,6-bisphosphate.
What is the difference between glycolysis and gluconeogenesis?
Glycolysis breaks down glucose into pyruvate and produces ATP; gluconeogenesis synthesizes glucose from pyruvate and other non-carbohydrate precursors, consuming ATP. Seven of the ten glycolytic reactions are reversible, but three steps must be bypassed using different enzymes.
Why is glycolysis important in cancer cells?
Cancer cells exhibit the Warburg effect, preferentially using anaerobic glycolysis even in the presence of oxygen. This provides rapid ATP and biosynthetic precursors to support uncontrolled proliferation. Glycolytic intermediates serve as building blocks for nucleotides, amino acids, and lipids.

Written by Colm Ryan

Colm Ryan PhD is a co-founder of Assay Genie. Colm carried out his undergraduate degree in Genetics in Trinity College Dublin, followed by a PhD at the University of Leicester. Following this Colm carried out a post-doc in the IGBMC in Strasbourg, France. Colm is now Chief Executive Officer at Assay Genie.

10th Mar 2022 Jahnavi Konduru Naga, Msc

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