Chapter 3: Respiratory Electron Transport in Helicobacter and campylobacter

Summary

The microaerophilic, human gastro-intestinal pathogens, Campylobacter jejuni and Helicobacter pylori are closely related phylogentically, yet distinct in some major aspects of their physiology, especially with regard to electron transport. C. jejuni is a more versatile and metabolically active pathogen, with a complete citri-acid cycle, and a complex and highly branched respiratory chain allowing the use of a variety of electron donors such as formate, hydrogen, D-lactate, succinate, malate, NAD(P)H and alternative electron acceptors to oxygen, including fumarate, nitrate, nitrite, N- or S-oxides and hydrogen peroxide. H. pylori is a more specialized pathogen, largely restricted to the human stomach, with an incomplete citric-acid cycle and a simpler respiratory chain. Alternative electron acceptors to oxygen include fumarate, hydrogen peroxide and possibly N- or S-oxides. Both organisms contain a novel type of complex I, which lacks the two subunits NQO1 (NuoE) and NQO2 (NuoF), responsible for NADH binding and initial electron transfer reactions in other bacteria. The nature of the electron donor to complex I has yet to be identified. In both bacteria, reducing equivalents are transferred to the sole quinone, menaquinone-6. Menoquinaol reduces the cytochrome bc ₁ complex, which in turn reduces periplasmic cytochrome c. Unusually for a bacterium, H. pylori contains only a single terminal oxidase (a cb-type cytochrome c oxidase). However, C. jejuni also possesses a bd-like quinol oxidase in addition ot cb-type oxidase, allowing an additional but less coupled pathway of electron transfer to oxygen.