Interaction and compartmentalization of the components of bacterial enzyme systems involved in cell energetics.

Bacterial enzyme systems, especially those which are involved in cell energetics, often show a common characteristic feature: their constituents (either interacting enzymes or subunits of a given enzyme complex) are physically separated. They are located in different functional entities, such as cytoplasm or periplasmic space. This kind of cellular and macromolecular organization enables the cell to establish spatially separated but neighbouring zones in which distinct conditions are created or maintained. This intrinsic imbalance is one of the keys for the process of life. As the mediator between the two compartments, the cytoplasm and the periplasmic space, the cytoplasmic membrane--itself a functional entity--not only acts as a barrier, but carries a set of functional enzyme components, thus contributing to the interaction between compartments. Examples to illustrate this concept are enzyme systems involved in anaerobic glycine metabolism, aerobic utilization of carbon monoxide, proton or sodium translocation across the membrane, and intracellular hydrogen cycling used by the cell for the generation of a proton gradient.