Fermentation: Synthesis of Ethanol

 
Monday's Molecule #26 was actually three molecules: pyruvate, acetaldehyde, and ethanol. They're part of the pathway from sugar to ethanol.

In the first part of the pathway glucose is converted to pyruvate by the standard reactions of glycolysis. In the presence of oxygen the end product, pyruvate, will be oxidized to acetyl-CoA and CO₂ by pyruvate dehydrogenase. Acetyl-CoA will enter the citric acid cycle to complete the oxidation of glucose. There are several other fates of pyruvate including the conversion to lactate or ethanol. Both of these pathways take place in the absence of oxygen. They are called fermentation pathways.

The ability of yeast to ferment grapes and other fruits has been known for several millenia. Yeast cells can take the sugar from fruit (or grain) and convert it to pyruvate. If you mix yeast and fruit in a container that doesn't have much oxygen then the yeast cells will obligingly produce ethanol, a compound that has proven to be useful in the human diet. Yeast also produces CO₂ under these conditions and shown in the pathway above.

Today we know all about the enzymes that carry out these reactions but one hundred years ago things were less clear. It wasn't certain that fermentation could occur outside of living cells.

In one of the major conceptual advances in biology, Eduard Buchner (in 1897) was able to ferment sugar using a cell free extract of yeast. The reason why this was such an important discovery is that it removed all doubt about vitalism and the possibility that life was some special property outside of chemistry. Buchner showed that the production of ethanol from sugar was just a series of chemical reactions that did not need a living cell. This led directly to the discovery of enzymes and the elucidation of their properties. In a very real sense Buchner is the father of biochemistry [Nobel Laureate: Eduard Buchner].