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Chapter 16 : Transfer of Energy and Information across the Periplasm in Iron Transport and Regulation
Two periplasmic functions are discussed in this chapter: (i) the transfer of energy from the cytoplasmic membrane across the periplasm to the outer membrane, and (ii) the transfer of information from the outer membrane across the periplasm through the cytoplasmic membrane into the cytoplasm. To enter gram-negative bacteria, substrates must cross three compartments: the outer membrane, the periplasm, and the cytoplasmic membrane. Most substrates diffuse through outer membrane porins into the periplasm and then are actively transported, i.e., with energy coupling, across the cytoplasmic membrane. Early studies have demonstrated that TonB is involved in iron transport and that the proton motive force serves as energy source for TonB-dependent processes. It is currently impossible to decide whether the various results reflect different TonB states during the reaction cycle—energization, deenergization, interaction with ExbB and ExbD within the cytoplasmic membrane and the periplasm, and binding to and release from transporters—or whether some results are experimental artifacts caused in vitro by the use of truncated forms that lack the N-terminal membrane anchor, which is essential for TonB activity. The periplasm—once considered not more than a space between the outer membrane and the cytoplasmic membrane—is now known to be a compartment through which energy and information flow. Understanding of the underlying mechanisms will require much sophisticated work since the three compartments cannot be taken apart without loss of essential aspects of periplasmic functions.