Pyruvate

lactate ethanol acetate formate lactate

Figure 8 Alternative routes of pyruvate metabolism.

kinase. If the environment is aerobic, pyruvate-formate lyase is inactive, and instead pyruvate is decarboxylated by pyruvate dehydrogenase to form acetate and CO2. Finally, excess pyruvate can be diverted to a-acetolactate via a-acetolactate synthase. This reaction has other important implications, since a-acetolactate is the precursor for diacetyl formation.

Although these alternative pathways for pyruvate metabolism are influenced largely by environmental conditions, mutants unable to produce lactate dehydrogenase also must deal with excess pyruvate and, therefore, produce other endproducts. Under certain conditions, cells may divert excess pyruvate to highly desirable products, specifically the aroma compound diacetyl. Ordinarily diacetyl is made from citrate (see below), but even citrate-nonfermenting cells will make diacetyl from lactose if appropriate conditions are established or if cells are genetically modified. For example, overexpression of NADH oxidase in Lc. lactis decreases lactate formation from pyruvate, and instead a-acetolactate, the precursor for diacetyl, is formed (de Felipe et al., 1998). Enhancing diacetyl production by metabolic engineering will be discussed later.

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