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Methylene‐tetrahydropterin reductases catalyze the reduction of a methylene to a methyl group bound to a reduced pterin as C1 carrier in various one‐carbon (C1) metabolisms. F420‐dependent methylene‐tetrahydromethanopterin (methylene‐H4MPT) reductase (Mer) and the flavin‐independent methylene‐tetrahydrofolate (methylene‐H4F) reductase (Mfr) use a ternary complex mechanism for the direct transfer of a hydride from F420H2 and NAD(P)H to the respective methylene group, whereas FAD‐dependent methylene‐H4F reductase (MTHFR) uses FAD as prosthetic group and a ping–pong mechanism to catalyze the reduction of methylene‐H4F. A ternary complex structure and a thereof derived catalytic mechanism of MTHFR is available, while no ternary complex structures of Mfr or Mer are reported. Here, Mer from Methanocaldococcus jannaschii (jMer) was heterologously produced and the crystal structures of the enzyme with and without F420 were determined. A ternary complex of jMer was modeled on the basis of the jMer‐F420 structure and the ternary complex structure of MTHFR by superimposing the polypeptide after fixing hydride‐transferring atoms of the flavins on each other, and by the subsequent transfer of the methyl‐tetrahydropterin from MTHFR to jMer. Mutational analysis of four functional amino acids, which are similarly positioned in the three reductase structures, indicated despite the insignificant sequence identity, a common catalytic mechanism with a 5‐iminium cation of methylene‐tetrahydropterin as intermediate protonated by a shared glutamate. According to structural, mutational and phylogenetic analysis, the evolution of the three reductases most likely proceeds via a convergent development although a divergent scenario requiring drastic structural changes of the common ancestor cannot be completely ruled out. [ABSTRACT FROM AUTHOR]

Methanogenic archaea inhabiting anaerobic environments play a crucial role in the global biogeochemical material cycle. The most universal electrogenic reaction of their methane-producing energy metabolism is catalyzed by N 5 -methyl-tetrahydromethanopterin: coenzyme M methyltransferase (MtrABCDEFGH), which couples the vectorial Na+ transport with a methyl transfer between the one-carbon carriers tetrahydromethanopterin and coenzyme M via a vitamin B12 derivative (cobamide) as prosthetic group. We present the 2.08 Å cryo-EM structure of Mtr(ABCDEFG)3 composed of the central Mtr(ABFG)3 stalk symmetrically flanked by three membrane-spanning MtrCDE globes. Tetraether glycolipids visible in the map fill gaps inside the multisubunit complex. Putative coenzyme M and Na+ were identified inside or in a side-pocket of a cytoplasmic cavity formed within MtrCDE. Its bottom marks the gate of the transmembrane pore occluded in the cryo-EM map. By integrating Alphafold2 information, functionally competent MtrA-MtrH and MtrA-MtrCDE subcomplexes could be modeled and thus the methyl-tetrahydromethanopterin demethylation and coenzyme M methylation half-reactions structurally described. Methyl-transfer-driven Na+ transport is proposed to be based on a strong and weak complex between MtrCDE and MtrA carrying vitamin B12, the latter being placed at the entrance of the cytoplasmic MtrCDE cavity. Hypothetically, strongly attached methyl-cob(III)amide (His-on) carrying MtrA induces an inward-facing conformation, Na+ flux into the membrane protein center and finally coenzyme M methylation while the generated loosely attached (or detached) MtrA carrying cob(I)amide (His-off) induces an outward-facing conformation and an extracellular Na+ outflux. Methyl-cob(III)amide (His-on) is regenerated in the distant active site of the methyl-tetrahydromethanopterin binding MtrH implicating a large-scale shuttling movement of the vitamin B12-carrying domain. [ABSTRACT FROM AUTHOR]

Crystallization is the limiting step in X-ray structure determination of biological macromolecules. As crystallization experiments can be largely automatized, the diversity of precipitant solutions is often the determinant factor to obtain crystals of high quality. Here, we introduce a 96-well screening kit of crystallization conditions, centered on three ethoxylate-based organic polymers as precipitants and various additional compounds to promote crystal formation. This crystallization screen was tested on various non-standard proteins from bacteria and archaea. Structure determination succeeded for seven out of thirteen targets based on crystals that frequently diffracted to a higher resolution than those obtained with commercially available screening kits. Crystallization hits were rarely similar among the three ethoxylate-based organic polymers and, in comparison, with already available crystallization screens. Hence, the presented crystallization screen is an efficient tool to complement other screens and increase the likelihood of growing crystals suitable for X-ray structure determination. [ABSTRACT FROM AUTHOR]

FAD‐independent methylene‐tetrahydrofolate (methylene‐H4F) reductase (Mfr), recently identified in mycobacteria, catalyzes the reduction of methylene‐H4F to methyl‐H4F with NADH as hydride donor by a ternary complex mechanism. This biochemical reaction corresponds to that of the ubiquitous FAD‐dependent methylene‐H4F reductase (MTHFR), although the latter uses a ping‐pong mechanism with the prosthetic group as intermediate hydride carrier. Comparative genomics and genetic analyses indicated that Mfr is indispensable for the growth of Mycobacterium tuberculosis, which lacks the MTHFR encoding gene. Therefore, Mfr appears to be an excellent target for the design of antimycobacterial drugs. Here, we report the heterologous production, enzymological characterization, and the crystal structure of Mfr from the thermophilic mycobacterium Mycobacterium hassiacum (hMfr), which shows 78% sequence identity to Mfr from M. tuberculosis. Although hMfr and MTHFR have minor sequence identity and different catalytic mechanisms, their structures are highly similar, thus suggesting a divergent evolution of Mfr and MTHFR from a common ancestor. Most of the important active site residues of MTHFR are conserved and equivalently positioned in the tertiary structure of hMfr. The Glu9Gln variant of hMfr exhibits a drastic reduction of the catalytic activity, which supports the predicted function of the glutamate residue as proton donor in both hMfr and MTHFR. Thus, highly similar binding modes for the C1‐carriers and the reducing agents in hMfr and MTHFR are assumed. [ABSTRACT FROM AUTHOR]

Large amounts (up to 300 Tg per year) of the potent greenhouse gas, methane, are oxidized to C[O.sub.2] (reaction 1) in marine sediments by consortia of methanotrophic archaea (ANME-1, ANME-2 [...], The anaerobic oxidation of methane (AOM) with sulphate, an area currently generating great interest in microbiology, is accomplished by consortia of methanotrophic archaea (ANME) and sulphate-reducing bacteria (1,2). The enzyme activating methane in methanotrophic archaea has tentatively been identified as a homologue of methyl-coenzyme M reductase (MCR) that catalyses the methane-forming step in methanogenic archaea (3,4).Here we report an X-ray structure of the 280 kDa heterohexameric ANME-1MCR complex. It was crystallized uniquely from a protein ensemble purified from consortia of microorganisms collected with a submersible from a Black Sea mat catalysing AOM with sulphate (4). Crystals grown from the heterogeneous sample diffract to 2.1 A resolution and consist of a single ANME-1 MCR population, demonstrating the strong selective power of crystallization. The structure revealed ANME-1 MCR in complex with coenzyme M and coenzyme B, indicating the same substrates for MCR from methanotrophic and methanogenic archaea. Differences between the highly similar structures of ANME-1 MCR and methanogenic MCR include a [F.sub.430] modification, a cysteine-rich patch and an altered post-translational amino acid modification pattern, which may tune the enzymes for their functions in different biological contexts.

The heme-copper oxidases (HCOs) accomplish the key event of aerobic respiration; they couple [O.sub.2] reduction and transmembrane proton pumping. To gain new insights into the still enigmatic process, we structurally characterized a C-family HCO--essential for the pathogenicity of many bacteria--that differs from the two other HCO families, A and B, that have been structurally analyzed. The x-ray structure of the C-family [cbb.sub.3] oxidase from Pseudomonas stutzeri at 3.2 angstrom resolution shows an electron supply system different from families A and B. Like family-B HCOs, C HCOs have only one pathway, which conducts protons via an alternative tyrosine-histidine cross-link. Structural differences around heroes b and b3 suggest a different redox-driven proton-pumping mechanism and provide clues to explain the higher activity of family-C HCOs at tow oxygen concentrations. 10.1126/science.1187303

Sulfide:quinone oxidoreductase (SQR) is a flavoprotein with homologues in all domains of life except plants. It plays a physiological role both in sulfide detoxification and in energy transduction. We isolated the protein from native membranes of the hyperthermophilic bacterium Aquifex aeolicus, and we determined its X-ray structure in the 'as-purified,' substrate-bound, and inhibitor-bound forms at resolutions of 2.3, 2.0, and 2.9 [Angstrom], respectively. The structure is composed of 2 Rossmann domains and 1 attachment domain, with an overall monomeric architecture typical of disulfide oxidoreductase flavoproteins. A. aeolicus SQR is a surprisingly trimeric, periplasmic integral monotopic membrane protein that inserts about 12 [Angstrom] into the lipidic bilayer through an amphipathic helix-turn-helix tripodal motif. The quinone is located in a channel that extends from the si side of the FAD to the membrane. The quinone ring is sandwiched between the conserved amino acids Phe-385 and Ile-346, and it is possibly protonated upon reduction via Glu-318 and/or neighboring water molecules. Sulfide polymerization occurs on the re side of FAD, where the invariant Cys-156 and Cys-347 appear to be covalently bound to polysulfur fragments. The structure suggests that FAD is covalently linked to the polypeptide in an unusual way, via a disulfide bridge between the 8-methyl group and Cys-124. The applicability of this disulfide bridge for transferring electrons from sulfide to FAD, 2 mechanisms for sulfide polymerization and channeling of the substrate, [S.sup.2-], and of the product, [S.sub.n], in and out of the active site are discussed. flavoprotein | monotopic membrane protein | sulfide metabolism | X-ray crystallography | extremophilic organism

Various microbial metabolisms use H+/Na+-translocating ferredoxin:NAD+ reductase (Rnf) either to exergonically oxidize reduced ferredoxin by NAD+ for generating a transmembrane electrochemical potential or reversely to exploit the latter for producing reduced ferredoxin. For cryo-EM structural analysis, we elaborated a quick four-step purification protocol for the Rnf complex from Clostridium tetanomorphum and integrated the homogeneous and active enzyme into a nanodisc. The obtained 4.27 Å density map largely allows chain tracing and redox cofactor identification complemented by biochemical data from entire Rnf and single subunits RnfB, RnfC and RnfG. On this basis, we postulated an electron transfer route between ferredoxin and NAD via eight [4Fe-4S] clusters, one Fe ion and four flavins crossing the cell membrane twice related to the pathway of NADH:ubiquinone reductase. Redox-coupled Na+ translocation is provided by orchestrating Na+ uptake/release, electrostatic effects of the assumed membrane-integrated FMN semiquinone anion and accompanied polypeptide rearrangements mediated by different redox steps. Rnf reversibly reduces NAD+ by ferredoxin coupled with Na+/H+ pumping for microbial ion gradient or reduced ferredoxin formation. Here, a cryo-EM structure provides information about the participating iron and flavin cofactors, the electron transfer route and the putative Na+ passage. [ABSTRACT FROM AUTHOR]

Anaerobic toluene degradation involves β‐oxidation of the first intermediate (R)‐2‐benzylsuccinate to succinyl‐CoA and benzoyl‐CoA. Here, we characterize the last enzyme of this pathway, (S)‐2‐benzoylsuccinyl‐CoA thiolase (BbsAB). Although benzoylsuccinyl‐CoA is not available for enzyme assays, the recombinantly produced enzymes from two different species showed the reverse activity, benzoylsuccinyl‐CoA formation from benzoyl‐CoA and succinyl‐CoA. Activity depended on the presence of both subunits, the thiolase family member BbsB and the Zn‐finger protein BbsA, which is affiliated to the DUF35 family of unknown function. We determined the structure of BbsAB from Geobacter metallireducens with and without bound CoA at 1.7 and 2.0 Å resolution, respectively. CoA binding into the well‐known thiolase cavity triggers an induced‐fit movement of the highly disordered covering loop, resulting in its rigidification by forming multiple interactions to the outstretched CoA moiety. This event is coupled with an 8 Å movement of an adjacent hairpin loop of BbsB and the C‐terminal domain of BbsA. Thereby, CoA is placed into a catalytically productive conformation, and a putative second CoA binding site involving BbsA and the partner BbsB′ subunit is simultaneously formed that also reaches the active center. Therefore, while maintaining the standard thioester‐dependent Claisen‐type mechanism, BbsAB represents a new type of thiolase. [ABSTRACT FROM AUTHOR]