Lactic acid bacteria are obligate fermentors, and cannot obtain energy by oxidative or respiratory processes (with the exception noted previously; Box 2-1). Technically, the precursor-product exchange systems, described below, provide an alternate way for these organisms to earn ATP "credits" by conserving the energy that would ordinarily be used to perform metabolic work. However, the substrate level phosphorylation reactions that occur during fermentation are by far the major means by which these cells make ATP. For ho-mofermentative lactic acid bacteria, hexoses are metabolized via the enzymes of the gly-colytic Embden-Meyerhoff pathway.
One of the key enzymes of this pathway is aldolase, which commits the sugar to the pathway by splitting fructose-1,6-diphosphate into the two triose phosphates that eventually serve as substrates for ATP-generating reactions. The Embden-Meyerhoff pathway yields two moles of pyruvate and two moles of ATP per mole of hexose (Figure 2-10). The pyru-vate is then reduced to L- or D-lactate by the enzyme, lactate dehydrogenase. More than 90% of the substrate is converted to lactic acid during homofermentative metabolism.
Importantly, the NADH formed during the glyceraldehyde-3-phosphate dehydrogenase reaction must be re-oxidized by lactate dehydro-genase, so that the [NADH]/[NAD+] balance is maintained. Homofermentative lactic acid bacteria include Lactococcus lactis, Streptococcus thermophilus, Lactobacillus helveticus, and L. delbrueckii subsp. bulgaricus (used as dairy starter organisms); Pediococcus sp. (used in sausage cultures); and Tetragenococcus (used in soy sauces).
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