- Nutrient transport Transport events and transporters • Three major classes of transport systems in prokaryotes + Simple transport + Group translocation + ABC system • All of these transport systems require energy of the proton motive force or ATP or some other energy-rich organic compound. - Figure 1: The three classes of transport systems 1. - Simple transporters - Lac Permease of E.coli • E.coli metabolize the disaccharide sugar lactose, and lactose is transported into cells of E.coli by the activity of a simple transporter called Lac Permease (Type of symporter. - Activity of Lac Permease is the energy-driven accumulation of lactose in the cytoplasm against the concentration gradient. - Lactose is broken down and used for ATP synthesis and to make new carbon skeletons 2. - Group translocation • Transported substance is chemically modified during the transport process and an energy-rich organic compound • Involve transport of the sugars glucose, mannose and fructose in E.coli • These compounds are phosphorylated during transport by the phosphotransferase system • Phosphotransferase system consists of a family of proteins (five proteins are necessary to transport a given sugar) Figure 3: Mechanism of the phosphotransferase system of E.coli: For glucose uptake, the system consists of five proteins: Enzyme (Enz) I, Enzymes IIa, IIb, and IIc, and HPr. - A phosphate cascade occurs from phosphoenolpyruvate (PE-P) to Enzyme IIc and the latter actually transports and phosphorylates the sugar. - Periplasmic binding proteins and ABC system • ABC transport systems (ATP-binding cassette): ATP-dependent active transport found in various gram-negative bacteria • Involve substrate-specific binding proteins located in the bacterial periplasm, the gel-like substance between the bacterial cell wall and cytoplasmic membrane • Often involved in uptake of organic compounds (sugars, amino acids. - inorganic nutrients (sulfate and phosphate ) and certain metal • Characteristic of periplasmic binding proteins: High substrate affinity • Gram-positive bacteria lack a periplasm (also have ABC transport systems. - Substrate-binding proteins are anchored to the external surface of the cytoplasmic membrane Peptidoglycan Periplasmic binding protein Periplasm Transported Out substance Membrane- spanning transporter ATP-hydrolyzing In protein Figure 4: Mechanism of an ABC transport: The periplasmic binding protein has high affinity for substrate, the membrane-spanning proteins form the transport channel, and the cytoplasmic ATP-hydrolyzing proteins supply the energy for the transport