Glycolysis is a central metabolic pathway that occurs in the cytoplasm of cells and is the first step in both aerobic and anaerobic respiration. It consists of a series of enzymatic reactions that convert one molecule of glucose (a six-carbon sugar) into two molecules of pyruvate (a three-carbon compound). Glycolysis is a critical process in cellular metabolism that also generates small amounts of ATP (adenosine triphosphate) and NADH that can be used in other metabolic pathways.
Characteristics of Glycolysis
- The term "glycolysis" originates from the Greek words "glycos," meaning sugar, and "lysis," meaning splitting.
- The glycolysis pathway was discovered by Gustav Embden, Otto Meyerhof, and J. Parnas and is also known as the EMP pathway.
- During glycolysis, one molecule of glucose, a six-carbon sugar, is enzymatically converted into two molecules of pyruvate, each with three carbons.
- This process involves a series of enzymatic reactions, including phosphorylation and oxidation, and results in the production of small amounts of ATP (adenosine triphosphate) and NADH (nicotinamide adenine dinucleotide).
- Glycolysis is a highly conserved and essential process found in nearly all living organisms. It provides a primary source of energy and serves as a precursor for other metabolic pathways, such as the citric acid cycle and fermentation, depending on oxygen availability.
Pathway of Glycolysis
Glycolysis is a multi-step metabolic pathway that involves a series of enzymatic reactions in the cytoplasm of cells. It breaks down one molecule of glucose into two molecules of pyruvate while generating small amounts of ATP and NADH. The pathway is as follows:
Step 1- Hexokinase
Hexokinase is an enzyme that phosphorylates or adds a phosphate group to glucose in the cytoplasm of a cell. A phosphate group from ATP is transferred to glucose, resulting in glucose 6-phosphate, or G6P. During this phase, one molecule of ATP is consumed.
Step 2- Phosphoglucoisomerase
Phosphoglucoisomerase is an enzyme that converts Glucose-6-Phosphate (G6P) to its isomer fructose-6-phosphate (F6P). Isomers have the same chemical formula but differ in their structural arrangement.
Step 3- Phosphofructokinase
The enzyme phosphofructokinase catalyses the transfer of a phosphate group to Fructose-6-Phosphate (F6P) to create fructose 1,6-bisphosphate (FBP). So far, two ATP molecules have been used.
Step 4- Aldolase
Aldolase splits fructose 1,6-bisphosphate into two three-carbon molecules: dihydroxyacetone phosphate (DHAP) and glyceraldehyde 3-phosphate (GAP).
Step 5- Triose-phosphate isomerase
The enzyme triose-phosphate isomerase transforms dihydroxyacetone phosphate (DHAP) to glyceraldehyde 3-phosphate (GAP) quickly (these isomers can interconvert). GAP is the substrate required for the next step.
Step 6- Glyceraldehyde 3-phosphate dehydrogenase
In this reaction, the enzyme glyceraldehyde 3-phosphate dehydrogenase (GAPDH) performs two steps:
- It dehydrogenates GAP first by transferring one of its hydrogen (H+) molecules to the oxidising agent nicotinamide adenine dinucleotide (NAD+), resulting in NADH + H+.
- The enzyme then adds an inorganic phosphate to oxidised GAP to form 1,3-bisphosphoglycerate (BPG). Both molecules of GAP generated in the previous step are dehydrogenated and phosphorylated.
Step 7- Phosphoglycerate kinase
To form ATP, the enzyme Phosphoglycerate kinase transfers a phosphate from BPG to an ADP molecule. This occurs for each BPG molecule. This process produces two molecules of 3-phosphoglycerate (3-PGA) and two molecules of ATP.
Step 8- Phosphoglyceromutase
To generate two 2-phosphoglycerate (2-PGA) molecules, the enzyme Phosphoglyceromutase moves the Phosphate group of the two 3-PGA molecules from the third to the second carbon.
Step 9- Enolase
Enolase is an enzyme that removes a molecule of water from 2-phosphoglycerate to produce phosphoenolpyruvate (PEP). This occurs for each of the two 2-PGA molecules from Step 8.
Step 10- Pyruvate Kinase
Pyruvate and ATP are formed when the enzyme pyruvate kinase transfers a Phosphate group from PEP to ADP. This occurs for each PEP molecule. This process produces two pyruvate molecules and two ATP molecules.
Key points of Glycolysis
The key points of glycolysis are as follows:
- Glycolysis is the process in which a glucose molecule is broken down into two molecules of pyruvate.
- The process takes place in the cytoplasm of plant and animal cells.
- Ten different enzymes are involved in glycolysis.
- The end products of the reaction include 2 pyruvate, 2 ATP, and 2 NADH molecules.
Four ATP molecules and two NADH molecules are produced during glycolysis. Each NADH can contribute to ATP production during oxidative phosphorylation under aerobic conditions. 2 ATPs are utilised in glycolysis. So, net 2 ATP is produced after one glycolysis cycle.