ATP yield in the payoff phase of glycolysis
The payoff phase of glycolysis, its second half, makes four ATP per glucose, which is a net gain of two ATP once the two ATP spent earlier are subtracted.
Remember
Each glucose gives two three-carbon molecules, so the payoff reactions run twice and make four ATP, and once the two ATP spent earlier are subtracted, the net gain is two.
Glycolysis has two phases. In the investment phase, reactions 1 to 3 use up 2 ATP when they phosphorylate the glucose (add a phosphate group to it). Reaction 4 then splits the six-carbon sugar in two, giving two three-carbon molecules called triose phosphates (a triose is a three-carbon sugar). Because of this split, reactions 6 to 10 each happen twice for every glucose.
The payoff phase is the second half of glycolysis, and it is where ATP is collected. Two reactions in this phase make ATP: 1,3-bisphosphoglycerate to 3-phosphoglycerate, and phosphoenolpyruvate to pyruvate, which uses the enzyme pyruvate kinase and is reaction 10. In both, a phosphate group is passed straight to ADP. This is substrate-level phosphorylation: ATP is made without any electron transport chain.
Each of these two reactions happens once for each triose phosphate, so each makes 2 ATP per glucose and the phase makes 4 ATP in total. Take away the 2 ATP spent in the investment phase and the net gain is 2 ATP per glucose. Reaction 6 captures NADH, but it makes no ATP itself.
Reaction 10 has a large negative delta G, meaning it releases a lot of energy and runs in one direction, so it is irreversible. Reactions 1 and 3 are irreversible too.
Key facts
- Gross yield
- the payoff phase makes 4 ATP per glucose, 2 for each triose phosphate
- Cost
- the investment phase, reactions 1 to 3, spends 2 ATP
- Net gain
- 4 ATP made, then taking away the 2 ATP spent leaves 2 ATP per glucose
- ATP-making reactions
- 1,3-bisphosphoglycerate to 3-phosphoglycerate, and phosphoenolpyruvate to pyruvate
- Irreversible step
- reaction 10, the pyruvate kinase step, is one of the three irreversible steps of glycolysis (reactions 1, 3 and 10)
- NADH
- reaction 6 captures NADH, and 2 NADH are gained per glucose
- Red blood cells
- they have no mitochondria and depend on glycolysis for all of their ATP, and pyruvate kinase deficiency causes haemolytic anaemia (red cells breaking down early)
Exam traps
- Two ATP is the net gain for the whole pathway, not the amount made in the payoff phase, because both three-carbon products of one glucose go through that phase, so it makes four ATP.
- Four ATP is the gross yield (what is made), not the net gain, because the 2 ATP spent in the investment phase are subtracted, leaving a net gain of 2.
- Oxidative phosphorylation does not make the ATP of the payoff phase, because it takes place in mitochondria, along the electron transport chain, whereas glycolysis passes a phosphate group from the substrate to ADP.
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