Probably the oldest biochemical reaction known to humans is brewing. It may even predate agriculture—the earliest humans didn't have to cultivate grapes to notice that certain fruits were intoxicating after a period of fermentation. The remains of beers and recipes for brewing have been found in the Egyptian pyramids and in Mesopotamia. Brewing predates the second‐oldest reaction, soap‐making, by centuries: Draw your own conclusions!
Therefore, it's somewhat fitting that biochemistry began as a science 100 years ago with the demonstration by the Büchner brothers that sucrose could be fermented into ethanol using a yeast‐cell extract. The study of glycolysis led to many of the concepts discussed in this book, including the roles of ATP, cofactors, and enzyme regulation.
Glycolysis describes the breakdown of a 6‐carbon carbohydrate to two molecules of a 3‐compound carboxylic acid:
The 3‐carbon acid, pyruvic acid, is then further metabolized, either aerobically or anaerobically:
The ∼ refers to the binding of the 2‐carbon compound with a special coenzyme, Coenzyme A. The 1‐carbon product is carbon dioxide. Pyruvate can also be metabolized anaerobically, in which case it receives electrons that were initially removed during glycolysis:
In animals, lactic acid is transported out of the muscles to the liver, where it is reconverted into glucose or amino acids. Lactic acid buildup is a source of the muscle stiffness that occurs after vigorous exercise. In yeast, pyruvate is converted to acetaldehyde and CO 2; then the acetaldehyde is reduced to ethanol.
Glycolysis can be divided into two parts, depending on whether the reactions consume or generate ATP. Enzyme nomenclature for the glycolytic pathway can be confusing. The names are historical rather than systematic and usually reflect the way the enzyme can be assayed. For example, the reduction of pyruvate to lactate is catalyzed by the enzyme lactate dehydrogenase, even though the relevant physiological reaction in glycolysis is the reduction of pyruvate, not the oxidation of lactate.