FIGURE 4.

Schematic illustration of the glucose fermentation end products by Lactococcus lactis. After glucose is internalized in the cell, its breakdown results in pyruvate. The pyruvate molecules can be converted to several end products. Lactate is the main product of lactate dehydrogenase (ldh). Under aerobic conditions, pyruvate is decarboxylaxed by the pyruvate dehydrogenase (pdh) complex to produce acetyl-CoA. Acetaldehyde (indicated in a green box), ethanol and acetate are products of the activity of phosphotransacetylase (pta), aldehyde/alcohol dehydrogenases (adh) and acetate kinase (ack), respectively. Under aerobic and acidic conditions, a shift toward the 4-carbon compounds (indicated in blue boxes, diacetyl, acetoin and 2,3-butanediol) occurs. Diacetyl is produced by oxidative decarboxylation (ODC). Acetoin can be produced by activity of a 2-acetolactate dehydrogenase (aldB) or by diacetyl reduction by the diacetyl reductase (dar). 2,3-butanediol is produced by the acetoin dehydrogenase (butA), but this reaction is reversible and 2,3-butanediol can be converted into acetoin by the 2,3-butanediol dehydrogenase (butB).