Glycolysis: Preparatory phase

The first five steps are regarded as a preparatory phase since they actually consume energy as the glucose is converted to two three-carbon sugars phosphates (G3P). The bold abbreviations in the two tables correspond to the nomenclature used in the diagram.

Step 1 | Substrate: glucose Glc | Enzyme: hexokinase HK | Enzyme class: transferase | Comment: ATP used at this step. Glucose is usually from the hydrolysis of starch or glycogen. This reaction has a highly negative change in free energy, and is thus, irreversible.

Step 2 | Substrate: glucose-6-phosphate G6P | Enzyme: phosphoglucose isomerase PGI | Enzyme class: isomerase | Comment: The change in structure is observed through a redox reaction, in which the aldehyde has been reduced to an alcohol, and the adjacent carbon has been oxidized to form a ketone. While this reaction is not normally favorable, it is driven by a low concentration of F6P, which is constantly consumed during the next step of glycolysis. (This phenomenon can be explained through Le Chatelier's Principle.)

Step 3 | Substrate: fructose 6-phosphate F6P | Enzyme: phosphofructokinase PFK-1 | Enzyme class: transferase | Comment: The energy expenditure of another ATP in this step is justified in 2 ways: the glycolytic process (up to this step) is now irreversible, and the energy supplied destablises the molecule.

Step 4 | Substrate: fructose 1,6-bisphosphate F1,6BP | Enzyme: aldolase ALDO | Enzyme class: lyase | Comment: Destablising the molecule in the previous reaction allows the hexose ring to be split by ALDO into two triose sugars, DHAP and GADP.

Step 5 | Substrate: dihydroxyacetone phosphate DHAP | Enzyme: triose phosphate isomerase TPI | Enzyme class: isomerase | Comment: TPI rapidly interconverts DHAP with glyceraldehyde 3-phosphate (GADP) that proceeds further into glycolysis.


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