Diversity of Fermentation Pathways in Bacteria
Recall that in aerobic organism, there are situations where oxygen is not available. For example, muscle cells during exercise and in red blood cells (which lack mitochondria). Thanks to metabolic adaption, the cells can continue to generate ATP (and recycle NAD+) through fermentation.
Bacteria typically live in low oxygen environments and thus rely on fermentation for energy. In fact, they support a wide and diverse set of fermentation pathways.
When oxygen is abundance, glucose travels the familiar aerobic pathways to generate ATP and recycle NAD+. That is, glycolysis -> Acetyl-CoA -> Citric Acid Cycle -> oxidative phosphorylation (all of which will described in great detail in later sections). This wasn’t covered in lecture, but it’s important to note that bacteria, which are prokaryotes, lack mitochondria, so these latter reactions occur in the bacteria cytoplasm. Indeed, it’s thought that the mitochondria in eukaryotes evolved from simple bacteria.
When oxygen is low, E.Coli activates several types of fermentation pathways, which lead to the formation of multiple fermentation products: lactate, ethanol, acetate and citrate.
They use two groups of fermentation pathways: linear and branched.
The goal is the regeneration of NAD+ with one fermentation product and a constant yield of ATP.
Lactate fermentation. ATP only produced during glycolysis. NAD+ regeneration occurs conversion of pyruvate to lactate.
Ethanol fermentation. Same step from glycolysis. During the last step, NAD+ is regenerated.
Here, a single precursor will be used by different fermentation pathways leading to several fermentation products and a variable ATP yield.
In E. Coli there are several pathways that get activated by enzymes based on oxygen sensing mechanisms.
Looking at just the branched pathway from Acetyl-CoA:
- Ethanol pathway regenerates NAD+
- The acetate pathway yields ATP. Specifically, Acetyl-CoA has a high hydrolysis potential, as does Acetyl-P. Consequently, the dephosphorylation of Acetyl-P into acetate is exergonic enough to drive the synthesis of ATP.
Thus, the modulation of activity of the different fermentation pathways in E. Coli are controlled by its needs for NAD+ and energy. This leads to a variable ATP yield.
Note that pyruvate is not the only fermentation precursor. In some cases these precursors are the products of other fermentation pathways.
Bacteria do not store large amounts of fuel molecules. Instead, they depend on available nutrients in their surroundings. To better support a wide range of carbon sources, bacteria evolved a diversity of pathways.
He concludes this lecture by showing two examples of citrate fermentation. These are quick, almost anecdotal examples. The first is soy sauce, in which soy bean and wheat are mixed together with specific bacteria and a large amount of salt, leading to fermentation. The citrate is converted into acetate and formate through the fermentation, which lead to the specific flavors found in Soy Sauce.
The second is “cultured butter,” or “european style butter,” in which a set of bacteria found in milk lead to a different set of fermentation products–diacetyl, acetoin, lactate, and acetate– and ultimately the butter.