Introduction to the Steps of glycolysis

Glycolysis is the process of breaking down glucose to produce energy. In this note, we will learn what glycolysis is, why it is important, and how it works in living cells. We will learn the steps of glycolysis, also see some examples of organisms that use glycolysis in different ways.

What is glycolysis?

• Glycolysis is a series of chemical reactions that convert glucose, a six-carbon sugar, into two molecules of pyruvate, a three-carbon compound. Glycolysis also produces two molecules of ATP, the energy currency of the cell, and two molecules of NADH, an electron carrier that can be used for further energy production.
• Glycolysis is an ancient metabolic pathway that evolved long ago and is found in almost all living organisms. It is the first step of cellular respiration, which is the process of extracting energy from organic molecules. However, glycolysis does not require oxygen, and can also occur in anaerobic conditions, where oxygen is absent or limited.

How does glycolysis work?

• Glycolysis takes place in the cytoplasm of the cell and consists of two main phases: the energy-requiring phase and the energy-releasing phase.
• In the energy-requiring phase, glucose is phosphorylated by two molecules of ATP, meaning that two phosphate groups are attached to it. This makes glucose more unstable and ready to split in half. The phosphorylated glucose is then rearranged into fructose and phosphorylated again by another molecule of ATP. The resulting compound, fructose-1,6-bisphosphate, is then cleaved into two three-carbon sugars: glyceraldehyde-3-phosphate and dihydroxyacetone phosphate. These two sugars are interconvertible and can both enter the next phase of glycolysis.
• In the energy-releasing phase, each three-carbon sugar undergoes a series of reactions that involve oxidation, meaning that electrons are removed from them. These electrons are transferred to NAD+, a coenzyme that accepts electrons and becomes NADH. The oxidized sugars are also phosphorylated by inorganic phosphate groups, forming high-energy compounds that can donate their phosphate groups to ADP, forming ATP. This process is called substrate-level phosphorylation and produces four molecules of ATP in total. The final products of this phase are two molecules of pyruvate, which can be further processed depending on the availability of oxygen.

Steps of glycolysis

It has 10 steps. Let’s know the steps of glycolysis and see what enzymes are involved and what changes occur in the molecules.

Step 1: Phosphorylation of glucose

In this step, a phosphate group is transferred from ATP to glucose in the cell cytoplasm, by the action of enzyme hexokinase. In this reaction, one molecule of ATP is consumed and one molecule of ADP is produced. The product of this step is glucose-6-phosphate or G6P, which has a phosphate group attached to its sixth carbon atom.

Step 2: Isomerization of Glucose-6-phosphate

In this step, glucose-6-phosphate is isomerized into fructose-6-phosphate by the enzyme phosphoglucomutase. Isomers have the same molecular formula as each other but different atomic arrangements. In this case, the phosphate group is moved from the sixth carbon atom to the first carbon atom of the sugar ring.

Step 3: Phosphorylation of fructose-6-phosphate

In this step, another molecule of ATP transfers a phosphate group to fructose-6-phosphate and converts it into fructose-1,6-bisphosphate by the action of the enzyme phosphofructokinase. This enzyme is a key regulator of glycolysis and can be inhibited or activated by various factors such as ATP levels or pH. The product of this step is fructose-1,6-bisphosphate or FBP, which has two phosphate groups attached to its first and sixth carbon atoms.

Step 4: Cleavage of fructose 1, 6-diphosphate

In this step, the enzyme aldolase splits fructose 1,6-bisphosphate into a ketone and an aldehyde molecule. These sugars are dihydroxyacetone phosphate (DHAP) and glyceraldehyde 3-phosphate (GAP), which are isomers of each other. DHAP has a double bond between its second and third carbon atoms (a ketone group), while GAP has a double bond between its first and second carbon atoms (an aldehyde group).

Step 5: Isomerization of dihydroxyacetone phosphate

In this step, the enzyme triose-phosphate isomerase converts dihydroxyacetone phosphate into glyceraldehyde 3-phosphate which is the substrate needed for the next step of glycolysis. This enzyme is very efficient and can rapidly interconvert the two isomers. As a result, both molecules of DHAP produced in the previous step are converted into GAP, which means that there are now two molecules of GAP for each molecule of glucose that entered glycolysis.

Step 6: Oxidative Phosphorylation of Glyceraldehyde 3-phosphate

In this step, each molecule of GAP undergoes two reactions. First, it is dehydrogenated by the enzyme glyceraldehyde 3-phosphate dehydrogenase (GAPDH), which transfers one of its hydrogen (H⁺) molecules to the oxidizing agent nicotinamide adenine dinucleotide (NAD⁺) to form NADH + H⁺. This reaction also releases a high-energy electron that can be used for further energy production. Second, GAPDH adds a phosphate from the cytosol to the oxidized GAP to form 1,3-bisphosphoglycerate (BPG). The product of this step is BPG, which has two phosphate groups attached to its first and third carbon atoms.

Step 7: Transfer of phosphate from 1, 3-diphosphoglycerate to ADP

In this step, the enzyme phosphoglycerokinase transfers a phosphate from BPG to a molecule of ADP to form ATP. This reaction is an example of substrate-level phosphorylation, where a phosphate group is directly transferred from a substrate to ADP without involving an electron transport chain. The product of this step is 3-phosphoglycerate (3PG), which has one phosphate group attached to its third carbon atom.

Step 8: Isomerization of 3-phosphoglycerate

In this step, the enzyme phosphoglyceromutase relocates the phosphate group from the third carbon atom to the second carbon atom of 3PG, forming 2-phosphoglycerate (2PG). This reaction prepares the molecule for the next step of dehydration.

Step 9: Dehydration

In this step, the enzyme enolase removes a water molecule from 2PG, forming a double bond between the second and third carbon atoms. This reaction produces phosphoenolpyruvate (PEP), which has a high-energy phosphate bond that can be used to generate ATP in the next step.

Step 10: Substrate-level phosphorylation

In this step, the enzyme pyruvate kinase transfers a phosphate from PEP to ADP, forming ATP and pyruvate. This reaction is another example of substrate-level phosphorylation and completes the energy-releasing phase of glycolysis. The product of this step is pyruvate, which has three carbon atoms and can be further processed depending on the availability of oxygen.

Why is glycolysis important?

• Glycolysis is important because it provides a quick and universal way of generating energy from glucose, which is the most abundant and accessible sugar in nature. Glucose can be obtained from various sources, such as carbohydrates in food, glycogen stored in muscles and liver, or gluconeogenesis from non-carbohydrate precursors.
• Glycolysis is also important because it allows cells to survive in anaerobic conditions, where oxygen is scarce or absent. This is especially useful for some microorganisms that live in extreme environments, such as deep-sea vents or hot springs. Some of these organisms use glycolysis as their only source of energy and produce various end products from pyruvate, such as ethanol, lactate, or hydrogen gas. These end products are called fermentation products and allow the cells to regenerate NAD+ from NADH so that glycolysis can continue.
• Glycolysis is also important for some multicellular organisms that have high energy demands or low oxygen supply. For example, human muscles use glycolysis during intense exercise when oxygen cannot keep up with the rate of ATP consumption. In this case, pyruvate is converted into lactate, which accumulates in the muscles and causes fatigue and soreness. However, lactate can also be transported to the liver and converted back into glucose when oxygen becomes available again.

Conclusion

To summarize, glycolysis is a metabolic pathway that converts glucose into pyruvate, ATP, and NADH. It is an ancient and universal process that occurs in almost all living organisms. It is the first step of cellular respiration in aerobic conditions and can also occur in anaerobic conditions with different fermentation products. Glycolysis is important for providing a quick and adaptable source of energy for cells.