The second half of glycolysis is known as the pay-off phase, characterised by a net gain of the energy-rich molecules ATP and NADH. Since glucose leads to two triose sugars in the preparatory phase, each reaction in the pay-off phase occurs twice per glucose molecule. This yields 2 NADH molecules and 4 ATP molecules, leading to a net gain of 2 NADH molecules and 2 ATP molecules from the gylcolytic pathway per glucose.
Step 6 | Substrate: glyceraldehyde 3-phosphate GADP | Enzyme: glyceraldehyde 3-phosphate dehydrogenase GAP | Enzyme class: oxidoreductase | Comment: Triose sugars are dehydrogenated and inorganic phosphate is added to them. The hydrogen is used to reduce two molecules of NAD, a hydrogen carrier, to give NADH+H+.
Step 7 | Substrate: 1,3-bisphosphoglycerate 1,3BPG | Enzyme: phosphoglycerate kinase PGK | Enzyme class: transferase | Comment: A reaction that converts ADP to ATP by an enzymatic transfer of a phosphate to ADP; is an example of substrate-level phosphorylation.
Step 8 | Substrate: 3-phosphoglycerate 3PG | Enzyme: phosphoglyceromutase PGAM | Enzyme class: mutase | Comment: Notice that this enzyme is a mutase and not an isomerase. While an isomerase changes the oxidation state of the carbons being reacted, a mutase does not.
Step 9 | Substrate:2-phosphoglycerate 2PG | Enzyme: enolase ENO | Enzyme class: lyase | Comment: none
Step 10 | Substrate: phosphoenolpyruvate PEP | Enzyme: pyruvate kinase PK | Enzyme class: transferase | Comment: Another example of substrate-level phosphorylation that converts ADP to ATP, forming pyruvate (Pyr).
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