Lactose-6-phosphate

Figure 2-12. The phosphotransferase system (PTS) in Gram positive bacteria. As shown in panel A, the PTS cascade is initiated by the cytoplasmic proteins Enzyme I (EI) and HPr. Phosphorylated HPr (HPr~P) then transfers the phosphoryl group (obtained originally from PEP) to the substrate-specific Enzyme II complex. The latter consists of several proteins or domains, shown here as EIIA, EIIB, and EIIC. However, depending on the organism and the substrate, EII complexes may be organized differently, for example, as EIIA and EIIBC or as EIIABC. Regulation of the PTS is mediated, in part, via the phosphorylation state of HPr (panel B). Phosphorylation by HPr kinase results in formation of HPr[Ser~P], which, along with CcpA and fructose diphosphate (FDP), form a dimeric complex that recognizes and binds to CRE sites and prevents transcription of catabolic genes.

phosphotransferase systems present in the cell. The other PTS components, however, are substrate-specific. The latter proteins form an Enzyme II complex, which most often consists of three protein domains: EIIA, EIIB, and EIIC. These EII domains may be present as individual proteins or are combined or fused as one or two proteins. For example, the lactose PTS Enzyme II complex in Lactococcus lactis contains three lactose-specific protein domains that exist in the form of two proteins, i.e.,EIIAlac and EIIBClac (where "lac" denotes lactose-specificity). As for all of the EII proteins,the EIIB and EIIC domains are membrane-associated.

In the case of hexoses, the product of the PTS is hexose-6-phosphate, which can then feed directly into the glycolytic pathway. The PTS has the advantage, therefore, of sparing the cell of the ATP that ordinarily would be required to phosphorylate the free sugar. When lactose is the substrate, the product is lactose-phosphate (or more specifically, glu-cose-p-1,4-galactosyl-6-phosphate). Lactose-phosphate is not hydrolyzed by the enzyme p-galactosidase, which is widespread in the microbial world, but rather by phospho-p-galactosidase. The products of this reaction are glucose and galactose-6-phosphate. The glucose is subsequently phosphorylated by hexokinase, and the glucose-6-phosphate that forms feeds into the glycolytic pathway, as described earlier. The galactose-6-phosphate, in contrast, is metabolized by the tagatose pathway (Figure 2-13), eventually leading to the formation of the same triose phosphates, glyceraldehyde-3-phosphate and dihydroxy-acetone phosphate, that form during glycolysis. It is perhaps not unexpected that the structural (and regulatory) genes coding for lactose transport and hydrolysis are located on the same operon as the galactose/tagatose genes (Figure 2-14).

Symport and ABC Transport Systems in Lactic Acid Bacteria

Although the PTS is widely distributed among lactic acid bacteria, several species rely on other active transport systems to transport sug-ars.The latter include symport systems, driven lactose-6-P

phospho-b -galactosidase lactose-6-P

galactose-6-P

galactose-6-P isomerase tagatose-6-P kinase

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