The Respiratory Terminal Oxidases (RTOs) of Cyanobacteria

Abstract

Cyanobacteria are unique in performing oxygenic photosynthesis and aerobic respiration in the same compartment. Therefore the bioenergetic situation of cyanobacteria can be considered to be the most complex among all organisms known. While the components of oxygenic photosynthesis are largely known, the components of cyanobacterial respiration still present a number of mysteries for two reasons: (1) Many cyanobacteria contain several respiratory electron transport chains (up to ten) and (2) there are at least eight types of respiratory terminal oxidases (RTOs) that catalyse the reduction of O2 to water in cyanobacteria. Starting from a list of all RTOs in all currently sequenced cyanobacterial genomes, this review defines the eight different types of known cyanobacterial RTOs: namely, mitochondrial-type cytochrome c oxidase, four types of alternate respiratory terminal oxidases (ARTOs), cytochrome c oxidase of the cbb3type, cytochrome bd-type quinol oxidases (cyanide sensitive or cyanide insensitive), and plastidic type terminal oxidase (Ptox). The goal of this chapter is to review current understanding of the function of these RTOs, especially in cyanbacteria that contain more than one RTO. The author starts from the hypothesis that in strains with more than one RTO, each must have a specific function, otherwise the genes encoding “superfluous” RTOs would quickly be lost by spontaneous mutation, because most RTOs consist of several polypeptide subunits that must be synthesized during every cell division cycle. Two conclusions can be drawn from the available data: (1) Every known cyanobacterium contains at least one RTO, and (2) no one type of RTO is present in all cyanobacteria. Furthermore, the available data probably allow the conclusion that all cyanobacteria contain at least one RTO that catalyzes the transfer of electrons from a soluble cytochrome c to O2 (cytochrome c oxidase reaction).