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ATP ADP

solute

ATP ADP

solute

Figure 2-15. Solute transport in lactic acid bacteria. In Panel A, a proton symport system, fueled by the proton gradient, drives solute uptake. The proton gradient is formed via the proton translocating ATPase. In panel B, ATP hydrolysis drives transport directly via nucleotide binding domains (NBD). The solute is first captured by a binding protein (BP) which delivers it to the integral membrane proteins (IMP).

dence), including lactose (in Lactobacillus bre-vis, L. delbrueckii, L. acidophilus), galactose (L. lactis), raffinose (P pentosaceus), melibiose (L. lactis), and xylose (Lactobacillus pentosus). Although generation of the PMF is mediated via the proton-translocating ATPase (also called the F0FrATPase), which requires ATP lactic acid bacteria may also make a PMF by alternative routes that spare the cell of ATP (Box 2-4).

For those organisms that transport lactose by a symport system, the intracellular product is lactose (chemically unaltered, in contrast to the lactose PTS, where the product is lactose-phosphate). Intracellular hydrolysis of the free lactose then occurs via p-galactosidase, yielding glucose and galactose. The former is phos-phorylated by hexokinase to form glucose-6-phosphate, which feeds directly into the Embden-Meyerhoff pathway.

To convert galactose into glucose-6-phos-phate requires the presence of the Leloir pathway (Figure 2-16), a three enzyme pathway whose expression is subject to strong negative regulation in some lactic acid bacteria. In particular, S. thermophilus and Lb. delbruecki subsp. bulgaricus transport and hydrolyze lactose by a PMF and a p-galactosidase, respectively, but ferment only the glucose and not the galactose moiety. Although these bacteria have the genes encoding for the Leloir pathway enzymes, the genes are poorly transcribed or expressed, due to mutations within the promoter region of the operon.As a result, the intracellular galactose is largely unfermented and is excreted via the lactose transport system (LacS in S. thermophilus), a seemingly wasteful process.What makes this process especially interesting, however, is that LacS can catalyze a lactose:galactose exchange reaction, such that galactose efflux drives lactose uptake (Box 2-4).Thus, what might initially appear to be an inefficient and wasteful act (secreting a perfectly good energy source into the medium), is, instead, an efficient, energy-saving means of exchanging a readily fermentable sugar for one that is slowly metabolized, if at all.

The other major group of transport systems used by lactic acid bacteria for accumulating sugars are the ABC transport systems. These systems consist of several proteins or protein domains (Figure 2-15). Two of these proteins or domains are membrane-spanning (i.e., integrated within the membrane) and serve as porters. Two intracellular proteins/domains bind and hydrolyze ATP. An additional protein is extracellular, but is tethered to the cell galactose

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