Chapter 25 Phosphotransferase systems or PTSs as carbohydrate transport and as signal transduction systems

Abstract

Bacterial carbohydrate transport systems are classified primarily according to the energy source, which they utilize. Three types of active transport systems are: (1) primary transport systems, (2) secondary transport systems, and (3) phosphoenolpyruvate (PEP)-dependent carbohydrate: phosphotransferase systems PTS(s). Primary and secondary systems catalyze the translocation of their substrates in an energy dependent process through the membrane. This causes an accumulation of chemically unchanged substrate molecules at one side, usually the cytoplasmic side, of the inner cell membrane. As indicated by the name, the PTS is considered as an unusually complex system to phosphorylate carbohydrates. In contrast to the other active transport systems, PTSs couple the translocation of their substrates through the membrane with the phosphorylation of these substrates at the expense of PEP hydrolysis. Their activity thus causes intracellular accumulation of substrate-phosphate molecules as the first metabolic intermediates. The PTS represents a self-priming transport system in which an intermediate in the catabolism of its substrates is used to drive transport and to allow the phosphorylation of these substrates. The high phosphate transfer potential of PEP is retained in the phospho-His and phospho-Cys residues of phosphorylated PTS proteins. This property facilitates the reversible and cost-neutral phosphate transfer among PTS proteins and their communication with other proteins. The versatility of the PTS is enhanced furthermore by a modular construction at the gene and protein level, and the use of units that are often interchangeable and hence facilitate cross-regulation.