Electron Transport Activities in the Periplasm

This chapter provides an overview of the main periplasmic oxidation and reduction reactions alongside some consideration of how the assembly of electron transfer proteins is achieved. NapC is normally one of the proteins that is coded for within the nap operon, which includes genes for the subunits, NapA and NapB, of a periplasmic nitrate reductase. There are a number of possibilities as to why many oxidation reduction reactions are catalyzed in their periplasm. First, periplasmic handling avoids the need to provide transport systems for import of a substrate and export of a product. This leads to a second consideration, which is the exclusion of potentially toxic chemical species from the cytoplasm. A third possible reason is that the energetics of the oxidation/reduction reaction is only compatible with electron delivery to or from the electron transfer chain at the level of the c-type cytochromes. This chapter generalizes the electron transport system of a bacterium in terms of a quinone and/or a c-type cytochrome entry/exit point to the electron transfer chain. The chapter has sought to provide some general principles concerning the nature of periplasmic electron transfer pathways and the assembly of the individual proteins. Globular proteins, with membrane anchors, do function on the external surface of the membrane in some gram-positive organisms and thus permit processes such as denitrification that are nevertheless more commonly associated with gram-negative organisms.