Unusual Plastoquinones in Non-Phototrophic Nitrifying Bacteria

Isoprenoid quinones are important compounds in most organisms. They are essential in electron and proton transport in respiratory and photosynthetic electron transport chains, and additional functions include oxidative stress defence. The biologically most relevant quinones are naphthoquinones including menaquinone and benzoquinones including ubiquinone and plastoquinone. They differ in their polar headgroup structures, physicochemical properties, and distribution among organisms. Menaquinone is the most widespread quinone in prokaryotes, ubiquinone occurs only in bacteria of the phylum Pseudomonadota and eukaryotes, and plastoquinone exists in phototrophic Cyanobacteria and plants. We found that chemolithoautotrophic nitrifying bacteria of the genus Nitrospira (phylum Nitrospirota) exclusively possess unusual methyl-plastoquinones with a standard redox potential below that of canonical plastoquinone and ubiquinone but above menaquinone, suggesting functional roles in reverse electron transport, ammonia oxidation, alternative energy metabolisms, and oxidative stress mitigation. This extends the known diversity of quinones and suggests that plastoquinone derivatives are essential in ecologically important, non-phototrophic bacteria.