Why do some yeast cells produce ethanol? Scientists think yeast cells produce ethanol as a 'safety valve, ' to prevent overload when their metabolic operation reaches a critical level. The implications of this new theory could be far-reaching, as well as why cancer cells waste energy by producing lactate. The implications of this new theory were published in  Nature Metabolism .

Cells use nutrients like glucose to make new cells. But sometimes, some of these nutrients were wasted . For example, the yeast Saccharomyces cerevisiae , which is used to produce beer, breaks glucose down into ethanol rather than carbon dioxide.

'Metabolizing a six-carbon molecule to a two-carbon molecule, rather than carbon dioxide, means part of the energy and matter stored in glucose is lost . It makes no sense ', says Matthias Heinemann.

Evolution should have put an end to such a waste of resources, so biologists have tried to find a reason for its existence. 'And similar wastefulness can be seen in other cells', says Heinemann. A widely known example is cancer cells . These fast-growing cells excrete lactate, which is a similar waste of energy. And many bacteria waste energy as well.

Cells switch from respiration to fermentation

Heinemann was able to determine the Gibbs energy dissipation rate as a function of glucose uptake. At first, the Gibbs energy dissipation increases with increasing rates of glucose, but then the plateau is reached – and at that point, ethanol production starts. 'This is the point where the cells switch from respiration to fermentation ', explains Heinemann.

Yeast and E. coli live in completely different environments, yet have about the same dissipation limit that is even at about the same value. The exact reason for this limit is still unknown but there is a hypothesis. 'Cellular metabolism has a maximum rate at which it can still operate.' When this is reached, the cells open to 'safety valve' and glucose is broken down to ethanol, acetate or lactate, leaving part of the energy unused.

The mystery of the Warburg effect is now solved

Part of the energy is dissipated as heat, but this is too little to bother the cells. When enzymes catalyze a chemical reaction, they get a tiny push during the reaction, which makes them move. If they work very fast, this could mean that there is too much movement inside the cells, which could damage certain cellular structures.

Studies on the movement of enzymes inside the cell at different metabolic rates could confirm this and Heinemann believes that he has now solved the mystery of not just the ethanol production but also the Warburg effect in cancer cells.

 'There are some experiments going on with drugs that block lactation production as a way to treat cancer. The mechanism of these drugs could be close to the cells 'safety valve.' Heinemann believes. 'However, there is an upper rate limit at which cells can be free from this entropy, and this limit governs how cells operate their metabolism.'