Fermentation is the process by which living organisms recycle \(NADH \rightarrow NAD^+\). \(NAD^+\) is a required molecule necessary for the oxidation of Glyceraldehyde-3-phosphate to produce the high energy molecule 1,3-bisphosphoglycerate (Step 6 of Glycolysis). Fermentation occurs in the cytosol of cells.
Because \(NAD^+\) is used in Glycolysis it is important that living cells have a way of recycling \(NAD^+\) from \(NADH\). One way that a cell recycles \(NAD^+\) is through the process of respiration, a set of sequential electron transfers involving an electron transport chain to a terminal electron acceptor. In aerobic organisms, the terminal electron acceptor is oxygen. In anaerobic organisms, the terminal electron acceptor can vary from species to species and include but are not limited to various metals like Fe(III), Mn(IV) and Co(III), CO2, nitrate, sulfur This process reduces NADH back to \(NAD^+\) which can then be used again in step 6 of Glycolysis or other red/ox reactions in the cell. Another way that \(NAD^+\) is recycled from \(NADH\) is by a process called fermentation.
Example: Lactic acid fermentation in contracting muscle
Lactic acid fermentation occurs by converting pyruvate into lactate using the enzyme Lactate dehydrogenase and producing \(NAD^+\) in the process. This process takes place in oxygen depleted muscle and some bacteria. It is responsible for the sour taste of sauerkraut and yogurt. \(NAD^+\) is required for the oxidation of glyceraldehyde-3-P to produce 1,3-Bisphosphoglycerate (Step 6 of Gycolysis). If the supply of \(NAD^+\) is not replenished by the ETC or fermentation, glycolysis is unable to proceed. Fermentation is a necessary process for anaerobic organisms to produce energy. The yield of energy is much less than if the organism were to continue on through the TCA cycle and ETC, but energy is produce nonetheless.
Example: Alcoholic fermentation in yeast
The purpose of fermentation in yeast is the same as that in muscle and bacteria, to replenish the supply of NAD+ for glycolysis, but this process occurs in two steps:
- Alcoholic fermentation consists of pyruvate being first converted into acetaldehyde by the enzyme pyruvate decarboxylase and releasing \(CO_2\).
- In the second step acetaldehyde is converted into ethanol using alcohol dehydrogenase and producing \(NAD^+\) in the process. It is this recycled \(NAD^+\) that can be used to continue on with glycolysis.
- Garrett, H., Reginald and Charles Grisham. Biochemistry. Boston: Twayne Publishers, 2008.
- Raven, Peter. Biology. Boston: Twayne Publishers, 2005.
- Draw the chemical structures of pyruvate, ethanol and lactate (the reactant and products of fermentation)
- Why is fermentation necessary? (Hint: see step 6 of Glycolysis)
- What type of environment is necessary for fermentation to occur?
- Where does fermentation occur? What part of the cell?
- Explain the alternative to fermentation and why it is able to proceed. (Hint: Final electron acceptor)