A class C radical S-adenosylmethionine methyltransferase synthesizes 8-methylmenaquinone

The membranous quinone/quinol pool is essential to the majority of life forms and has been widely used as an important biomarker in microbial taxonomy. In the anaerobic world, the most important quinones are menaquinone (MK) and a methylated form of MK, designated methylmenaquinone (MMK), which is anticipated to serve specifically in low-potential electron transport chains involved in anaerobic respiration. However, it has remained unclear how MMK is generated. Here, we show that a novel enzyme homologous to class C radical SAM methyltransferases (RSMTs) synthesizes MMK using MK as substrate. Such enzymes, termed either MenK or MqnK, are present in MMK-producing bacteria (and some archaea) that possess either the classical MK biosynthesis pathway (Men) or the futalosine pathway (Mqn). An mqnK deletion mutant of the model Epsilonproteobacterium Wolinella succinogenes was unable to produce MMK₆ but its formation was restored upon genomic complementation using either the native mqnK gene or menK from the human gut bacterium Adlercreutzia equolifaciens or Shewanella oneidensis. Moreover, any of the menK genes enabled Escherichia coli cells to produce MMK₈ and a methylated form of 2-demethylmenaquinone₈ (DMK₈ ). The results expand the knowledge on quinone synthesis and demonstrate an unprecedented function for a class C RSMT-type enzyme in primary cell metabolism.