When your muscles can't get enough oxygen during a short burst of exercise, they start to make use of a pathway called lactic acid fermentation, which generates a small three-carbon compound called lactic acid or lactate as a byproduct of glucose breakdown. Lactic acid is not useful to your muscle cells, but your liver turns it back into glucose later on after exercise.
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As lactic acid accumulates inside your muscle cells, it enters your bloodstream. Your liver soaks up the circulating lactate. Later on while you are resting, your liver is busy oxidizing the lactic acid to pyruvate through a reaction catalyzed by an enzyme called lactate dehydrogenase. The enzyme uses the electrons removed from lactate to reduce a molecule of NAD to NADH. Pyruvate enters small capsule-shaped structures called mitochondria via a transporter, where it may meet with one of a couple different fates.
Citric Acid Cycle
Inside the mitochondria, pyruvate may be converted to acetyl-CoA and CO2 by an enzyme called the pyruvate dehydrogenase complex. In this case, the acetyl-CoA will feed into a biochemical pathway called the citric acid cycle, and your liver cell will use the energy it extracts by oxidizing these carbons to store energy in the form of adenosine triphosphate or ATP. By so doing, however, the liver merely satisfies its own requirements and not those of other cells. The liver also needs to turn the lactic acid into glucose. It does so through a process called gluconeogenesis.
When lactic acid is abundant in your liver cells after exercise, the gluconeogenesis pathway is a little bit different from the one your liver employs at other times. It begins in the mitochondria, where an enzyme called pyruvate carboxylase adds a molecule of bicarbonate to pyruvate and converts it to oxaloacetate. This reaction requires energy expenditure in the form of a molecule of ATP. Next, another enzyme called mitochondrial PEP carboxykinase converts oxaloacetate into phosphoenolpyruvate or PEP and free carbon dioxide. This step also requires energy investment in the form of a molecule of GTP. The PEP produced by PEP carboxykinase is exported from the mitochondria and converted back to glucose through a series of nine enzyme-catalyzed reactions inside the cell.
The series of events by which glucose is converted into lactate and back again is called the Cori cycle. Your muscles ultimately gain less energy from glucose breakdown and lactic acid fermentation than your liver must expend to make the lactate back into glucose. Consequently, the Cori cycle entails a net energy loss. Your body makes use of it during intense workouts, when your bloodstream can't furnish your muscles with all the oxygen they need. At times like these, lactic acid fermentation becomes the only way your muscles can keep metabolizing glucose for fuel.