Biosynthesis and function of microbial methylmenaquinones

The membranous quinone/quinol pool is essential for the majority of life forms and its composition has been widely used as a biomarker in microbial taxonomy. The most abundant quinone is menaquinone (MK), which serves as an essential redox mediator in various electron transport chains of aerobic and anaerobic respiration. Several methylated derivatives of MK, designated methylmenaquinones (MMKs), have been reported to be present in members of various microbial phyla possessing either the classical MK biosynthesis pathway (Men) or the futalosine pathway (Mqn). Due to their low redox midpoint potentials, MMKs have been proposed to be specifically involved in appropriate electron transport chains of anaerobic respiration. The class C radical SAM methyltransferases MqnK, MenK and MenK2 have recently been shown to catalyse specific MK methylation reactions at position C-8 (MqnK/MenK) or C-7 (MenK2) to synthesise 8-MMK, 7-MMK and 7,8-dimethylmenaquinone (DMMK). MqnK, MenK and MenK2 from organisms such as Wolinella succinogenes, Adlercreutzia equolifaciens, Collinsella tanakaei, Ferrimonas marina and Syntrophus aciditrophicus have been functionally produced in Escherichia coli, enabling extensive quinone/quinol pool engineering of the native MK and 2-demethylmenaquinone (DMK). Cluster and phylogenetic analyses of available MK and MMK methyltransferase sequences revealed signature motifs that allowed the discrimination of MenK/MqnK/MenK2 family enzymes from other radical SAM enzymes and the identification of C-7-specific menaquinone methyltransferases of the MenK2 subfamily. It is envisaged that this knowledge will help to predict the methylation status of the menaquinone/menaquinol pool of any microbial species (or even a microbial community) from its (meta)genome.

Bacterial nitrous oxide respiration: electron transport chains and copper transfer reactions

Biologically catalyzed nitrous oxide (N₂O, laughing gas) reduction to dinitrogen gas (N₂) is a desirable process in the light of ever-increasing atmospheric concentrations of this important greenhouse gas and ozone depleting substance. A diverse range of bacterial species produce the copper cluster-containing enzyme N₂O reductase (NosZ), which is the only known enzyme that converts N₂O to N₂. Based on phylogenetic analyses, NosZ enzymes have been classified into clade I or clade II and it has turned out that this differentiation is also applicable to nos gene clusters (NGCs) and some physiological traits of the corresponding microbial cells. The NosZ enzyme is the terminal reductase of anaerobic N₂O respiration, in which electrons derived from a donor substrate are transferred to NosZ by means of an electron transport chain (ETC) that conserves energy through proton motive force generation. This chapter presents models of the ETCs involved in clade I and clade II N₂O respiration as well as of the respective NosZ maturation and maintenance processes. Despite differences in NGCs and growth yields of N₂O-respiring microorganisms, the deduced bioenergetic framework in clade I and clade II N₂O respiration is assumed to be equivalent. In both cases proton motive quinol oxidation by N₂O is thought to be catalyzed by the Q cycle mechanism of a membrane-bound Rieske/cytochrome bc complex. However, clade I and clade II organisms are expected to differ significantly in terms of auxiliary electron transport processes as well as NosZ active site maintenance and repair.

Regulation of Respiratory Pathways in Campylobacterota: A Review

The Campylobacterota, previously known as Epsilonproteobacteria, are a large group of Gram-negative mainly, spiral-shaped motile bacteria. Some members like the Sulfurospirillum spp. are free-living, while others such as Helicobacter spp. can only persist in strict association with a host organism as commensal or as pathogen. Species of this phylum colonize diverse habitats ranging from deep-sea thermal vents to the human stomach wall. Despite their divergent environments, they share common energy conservation mechanisms. The Campylobacterota have a large and remarkable repertoire of electron transport chain enzymes, given their small genomes. Although members of recognized families of transcriptional regulators are found in these genomes, sofar no orthologs known to be important for energy or redox metabolism such as ArcA, FNR or NarP are encoded in the genomes of the Campylobacterota. In this review, we discuss the strategies that members of Campylobacterota utilize to conserve energy and the corresponding regulatory mechanisms that regulate the branched electron transport chains in these bacteria.

TsdC, a unique lipoprotein from Wolinella succinogenes that enhances tetrathionate reductase activity of TsdA

The diheme cytochromes c of the widespread TsdA family are bifunctional thiosulfate dehydrogenase/tetrathionate reductases. Here, biochemical information was collected about TsdA from the Epsilonproteobacterium Wolinella succinogenes (WsTsdA). The situation in W. succinogenes is unique since TsdA is closely associated with the unprecedented lipoprotein TsdC encoded immediately downstream of tsdA in the same direction of transcription. WsTsdA purified from Escherichia coli catalyzed both thiosulfate oxidation and tetrathionate reduction. After co-production of TsdC and WsTsdA in E. coli, TsdC was found to mediate membrane attachment of TsdA and to ensure its full catalytic activity. This effect was much stronger in the tetrathionate-reducing than in the thiosulfate-oxidizing direction. It is concluded that the TsdAC complex predominantly acts as a tetrathionate reductase in vivo.

The Modified Heparin-Binding L-Asparaginase of Wolinella succinogenes

The modified asparaginase Was79 was derived from the recombinant wild-type L-asparaginase of Wolinella succinogenes. The Was79 contains the amino acid substitutions V23Q and K24T responsible for the resistance to trypsinolysis and the N-terminal heparin-binding peptide KRKKKGKGLGKKR responsible for the binding to heparin and tumor K562 cells in vitro. When tested on a mouse model of Fischer lymphadenosis L5178Y, therapeutic efficacy of Was79 was significantly higher than that of reference enzymes at all single therapeutic doses used (125-8000 IU/kg). At Was79 single doses of 500-8000 IU/kg, the complete remission rate of 100 % was observed. The Was79 variant can be expressed intracellularly in E. coli as a less immunogenic formyl-methionine-free form at high per cell production levels.