PTS, Lac permease
that transport lactose via the PTS, galactose-6-phosphate is the actual hydrolysis product (resulting from hydrolysis of lactose-phosphate by phospho-P-galactosi-dase). Galactose-6-phosphate feeds directly into the tagatose pathway, as described earlier and in Figure 4. However, as noted earlier, free galactose will appear and accumulate in fermented dairy products made with thermophilic starter cultures containing S. thermophilus, Lb. bulgaricus, or other galactose-nonfermenting strains. Yogurt and mozzarella cheese, for example, can contain up to 2.5 and 0.8% galactose, respectively. Therefore, metabolism of free galactose may be of practical importance.
For the lactococci and some lactobacilli, free galactose appears to be transported by either a galactose-specific PTS or by a galactose permease. The intracel-lular product of the galactose PTS (galactose-6-phosphate) simply feeds into the tagatose pathway. When galactose accumulates via galactose permease, the intra-cellular product is free galactose. Subsequent metabolism occurs via the Leloir pathway, which phosphorylates galactose, and then converts galactose-1-phos-phate into glucose-6-phosphate (Fig. 7). The latter then feeds into the glycolytic pathway. Interestingly, in Lc. lactis, galactose permease may be the primary means for transporting galactose, since it has a much higher apparent affinity for galactose than the PTS transporter.
The Leloir pathway is used not only by lactococci, but it is also the pathway used by Lb. helveticus, Leuconostoc spp., and galactose-fermenting strains of S. thermophilus. During growth on lactose, these bacteria rely on a lactose galactose i ATP galactokinase r.
UDP glu k galactose-1-P uridyl transferase
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