Overview of Glucose Catabolism via Glycolysis
- Glucose is an important source of energy, used by many organisms for their metabolic process.
- It has several advantages, such as its high potential energy, which acts as a good fuel.
- Complete oxidation of glucose, forms CO2 and water, with a change in standard free energy of -2840kJ/mol.
- Usually, cells store glucose in a form of glycogen and starch, which are high molecular weight polymers of glucose, without any cytosolic osmolarity fluctuations.
- Stored glucose is released for the production of ATP when energy is required by the cell, which can be processed by aerobic or anaerobic respiration.
- Glucose have several metabolic fates.
- It can act as a precursor, for intermediates of metabolic biosynthetic pathways. E.g., In E.Coli, the backbone of the carbon skeleton is derived from glucose for other biological molecules such as fatty acid, nucleotide, and amino acid required for the growth of E.Coli.
- In-plant & animal cells, four major fate of glucose is determined:
- Formation of complex polysaccharides, which is present in extracellular space i.e., glycocalyx.
- Stored in a form of starch or polysaccharide, and released when the cell needs energy.
- Universal Fate of Glucose- via Glycolysis which oxidize glucose into pyruvate i.e., a 3-C compound that releases ATP and metabolic intermediates.
- Glucose oxidation is also carried out by pentose phosphate pathways also termed the phosphogluconate pathway.
- Some organisms form glucose in order to carry out their metabolism, this happens when there is devoid of glucose molecules in the surrounding.
- For e.g., Photosynthetic organisms form glucose by reducing CO2 into triose, other than that non-photosynthetic cells, make glucose from 3- 4C precursors by the process of gluconeogenesis. [that actually represents reverse glycolysis].
Possible Metabolic Fates of Pyruvate
Introduction of Glycolysis
- Degradation of glucose, in a series of reactions catalyzed by glycolytic enzymes which yield two molecules of pyruvate [3-C compound].
- The standard free energy released from the glycolytic breakdown of glucose is conserved in the form of NADH and ATP.
- The elucidation of glycolysis was the first metabolic pathway, which was studied in past.
History of research in Glycolysis
- In 1897, Eduard Buchner discovered the process of fermentation carried out in yeast cell extracts.
- The pathway of fermentation was explained by Otto Warburg & Hans Von Euler– Chelpin.
- In 193O, Gustav Embden & Otto Meyerhoff highlighted the glycolysis reactions in muscle cells and yeast cells. [therefore glycolysis is also termed as Embden- Meyerhoff Pathway]
- Later onwards, this area of research was further progressed.
In which cells, glycolysis occurs?
- Glucose catabolism by glycolysis is a primary source of energy in some mammal tissues or specific cell types which includes sperm, brain, erythrocytes, and renal medulla.
- In some plants, it has been observed that tissues that store starch for e.g., potato tubers and aquatic plants such as watercress, energy is released by the process of glycolysis when it’s required.
- Most of the anaerobic microorganisms completely rely upon glycolysis.
- The site of glycolysis in the cytosol of the cell.
Overview of Glycolysis
- There are series of ten steps catalyzed by enzymes that break down glucose into pyruvate.
- The first five steps are known as a preparatory phase.
- The last five steps of the glycolytic pathway are the Payoff phase.
The major phases of the glycolytic breakdown of glucose:
- Each molecule of glucose enter through the preparatory phase, 2 molecules of glyceraldehyde 3–phosphate which passes to the payoff phase.
- Two molecules of pyruvate are yielded as the end product of glycolysis, in the preparatory phase, 2 ATP molecules are consumed, in the payoff phase, four molecules of ATP are produced.
- Net yield is 2 molecules of ATP per glucose molecule is converted into pyruvate.
The complete equation for glycolysis:
Glucose + 2NAD + 2ADP + 2Pi= 2Pyruvate + 2NADH + 2H + 2ATP + 2H2O
References and Sources