This section examines the by-products of Glycolysis beyond Pyruvate. Namely, ATP and NADH.

ATP Hydrolysis

ATP + H2O -> ADP + P :  ΔG = -30.5 kJ/mol
ADP + P -> ATP + H2O :  ΔG = +30.5 kJ/mol

ATP hydrolysis can not be the most exergonic reaction in a cell. If it were, there wouldn’t be a reaction exergonic enough to regenerate the ATP and the cell would run out of energy. The free energy of hydrolysis of PEP, for example is -62 kJ/mol. For G6P, it’s -14 kJ/mol, putting ATP somewhere in the middle.

Many endergonic reactions can be coupled with the hydrolysis of ATP to give them the energy needed to proceed.

Example:
PEP + H2O  -> Pyruvate + P : ΔG = -62 kJ/mol
ADP + P -> ATP + H2O :  ΔG = +30.5 kJ/mol

Since they share a common reactant, they can be combined:
PEP + ADP -> Pyruvate + ATP  ΔG = -32 kJ/mol

Of related importance: both reactions must be catalyzed a common enzyme. Here, that’s pyruvate kinase.

Example:
PEP + H2O  -> Pyruvate + P : ΔG = -62 kJ/mol
G + P -> G6P + H2O :  ΔG = +15 kJ/mol

Though they share common reactants and the summed ΔG is negative, the reaction will not proceed because they do not share a common enzyme.

Importance of NADH Recycling

Since Glycolysis requires NAD+ electron acceptors, it’s important that the reduced NADH get recycled back into NAD+ (i.e., it needs to get oxidized). Otherwise, Glycolysis will stop. This is where pyruvate comes in…

Fate of Pyruvate

Aerobic Case:

Pyruvate enters the mitochondria where it is decarboxylated into an acetyl group. It then bonds to Coenzyme A (CoA) to form Acetyl-CoA. This resulting molecule enters the Citric Acid Cycle producing even more NADH, the sum of which is converted back into NAD+ during the Electron Transport Chain. But more on this later….

Anaerobic Case:

In the absence of oxygen, pyruvate will enter fermentation. In humans, this leads to the production of lactate, which occurs in one reaction via lactate dehydrogenase. This reaction is also coupled with the oxidation of NADH to NAD+. It often occurs, for example, in muscles during intense activity. This will also be covered in greater detail later.

The other type of fermentation produces Ethanol. This is a two-step process and requires bacteria or yeast. It does not occur in humans (obviously!). Pyruvate is first reduced and decarboxylated into acetaldehyde, catalyzed by pyruvate decarboxylase–this step also produces CO2. Then, the acetaldehyde is converted/reduced into ethanol. During this last step, the NADH is converted back into NAD+.

This is how wine and beer are produced, of course.  The bubbles in Champagne come from the released CO2.

Note: a reverse version of ethanol fermentation occurs in the liver when it degrades alcohol.