| MadSci Network: Microbiology |
Zymase was initially identified in yeast as a biochemical activity that was able to convert sugar into ethanol and carbon dioxide. Glycolysis is a portion of these processes. The activity was demonstrated in a cell free extract, which was historically significant because it showed that the biochemical reactions that occur inside living cells are just regular chemical reactions that can occur outside a living organism. Previously, it was thought that at least some of the reactions taking place inside cells required some kind of vaguely described vital essence, and could therefore not be duplicated in a non-living system.
Zymase activity was discovered long before biologists understood much about chemical reactions in the cell, and since then,the biochemical pathways that convert sugar into carbon dioxide and ethanol have been completely solved and the corresponding enzymes identified.
The conversion of sugar to carbon dioxide or ethanol begins with glycolysis. This pathway converts glucose into pyruvate. The enzymes used are:
The enzymes of the citric acid cycle (aka Krebs cycle and TCA cycle) are:
Pyruvate can also be fermented into ethanol. In the first step, pyruvate is converted to acetaldehyde by pyruvate decarboxylase. This reaction releases carbon dioxide. Next, acetaldehyde is converted to ethanol by alcohol dehydrogenase.
Dr. Alex Brands
[Moderator's Note: A college-level Biochemistry textbook will provide much more detail about the biochemical processes that Alex has summarized. Biochemistry by Berg, Tymoczko and Stryer is a great resource for this. This text is available online from the NCBI Bookshelf and can be searched for terms like glycolysis. -- SM]
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