Lactose Metabolism

The dominant feature of LAB metabolism is substrate-level phosphorylation-mediated ATP generation from carbohydrate fermentation. Rapid lactose fermentation in lactococci is associated with the involvement of a phosphoenolpyruvate-dependent sugar phospho-transferase system (PTS) in which phosphoenolpyruvate is the phosphoryl donor. Most dairy starter Lc. lactis strains possess a lactose PTS so that lactose enters the cell as lactose phosphate, which is hydrolyzed by phospho-h-galactosidase into glucose and galactose-6-phosphate. Glucose is phosphorylated by glucokinase and metabolized by the Embden-Meyerhof homofermentative glycolysis pathway via pyruvic acid to lactate (Fig. 2). Ga-lactose-6-phosphate enters glycolysis via the tagatose-6-phosphate pathway. An alternative mechanism of lactose uptake in LAB involves lactose permease as the carrier; the lactose is subsequently hydrolyzed to glucose and galactose by h-galactosidase (Fig. 2). Many LAB possess both a lactose PTS and lactose permease system for lactose metabolism (95).

There are two major routes for the intermediary metabolism of glucose in LAB (Fig. 2). The homofermentative pathway generates 2 moles of lactate and ATP for each mole of glucose utilized. The glycolysis (Embden-Meyerhof) pathway is used by all LAB in Cheddar cheese, with the exception of Leuconostoc spp. and heterofermentative lactobacilli. The alternative heterofermentative pathway utilized by these LAB yields one ATP and one mole each of lactate, ethanol, and CO2 for each mole of glucose utilized. In some LAB that transport lactose with a permease, galactose is phosphorylated and transformed to glucose-6-phosphate by the Leloir pathway (95).

Regeneration of the pyridine nucleotide (NAD+) occurs in the concomitant enzymemediated reduction of pyruvate to lactate by L- or D-lactate dehydrogenase. The proportion of the individual isomers formed by individual LAB strains is dependent on the possession of the appropriate lactate dehydrogenases and their respective activities, although some species possess an L-lactic acid racemase that converts L-lactic acid to the D-form. Lactococci produce only L-lactate. However, metabolites other than lactic acid may be formed from pyruvic acid in response to changes in environmental conditions or alternative electron acceptor availability (95,99).

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