The Role of Respiration in Symbiotic Nitrogen Fixation

Abstract

A few introductory remarks on the constituent components of bacterial respiratory chains may be useful before aspects pertinent to the title of this article are discussed. Aerobic bacteria are generally able to make use of a wide range of oxygen concentrations for respiratory energy conservation (Haddock, Jones 1977). This flexibility is brought about by two or more terminal oxidases that have different affinities for O₂. Reducing equivalents are derived from carbon substrates by the enzymatic action of various dehydrogenases which channel the electrons into the quinone/quinol pool within the cytoplasmic membrane. From here, the electrons flow to membrane-integrated terminal oxidases, the sites of O₂ reduction. Two principal classes of oxidases are known to exist in aerobic bacteria, the quinol oxidases and the cytochrome c oxidases (Poole 1988). Quinol oxidases receive the electrons directly from the quinol pool, whereas cytochrome c oxidases receive the electrons from cytochrome c that has been reduced before by ubiquinol-cytochrome c oxidoreductase (also called cytochrome bc ₁ complex; Trumpower 1990). Hence, when bacteria employ two or more terminal oxidases, the respiratory chain is branched, and the branch-point is either at the quinol pool (e.g. when two quinol oxidases or a cytochrome c oxidase plus a quinol oxidase are present), or it is at the bc ₁ complex (e.g. when two cytochrome c oxidases are present). In contrast to mitochondria which have only one respiratory chain, the existence of a branched respiratory chain is the rule in aerobic bacteria.