The antibiotic roseoflavin is a riboflavin (vitamin B2) analog. One step of the roseoflavin biosynthetic pathway is catalyzed by the phosphatase RosC, which dephosphorylates 8-demethyl-8-amino-riboflavin-5'-phosphate (AFP) to 8-demethyl-8-amino-riboflavin (AF). RosC also catalyzes the potentially cell-damaging dephosphorylation of the AFP analog riboflavin-5'-phosphate also called "flavin mononucleotide" (FMN), however, with a lower efficiency. We performed X-ray structural analyses and mutagenesis studies on RosC from Streptomyces davaonensis to understand binding of the flavin substrates, the distinction between AFP and FMN and the catalytic mechanism of this enzyme. This work is the first structural analysis of an AFP phosphatase. Each monomer of the RosC dimer consists of an α/β-fold core, which is extended by three specific elongated strand-to-helix sections and a specific N-terminal helix. Altogether these segments envelope the flavin thereby forming a novel flavin-binding site. We propose that distinction between AFP and FMN is provided by substrate-induced rigidification of the four RosC specific supplementary segments mentioned above and by an interaction between the amino group at C8 of AFP and the β -carboxylate of D166. This key amino acid is involved in binding the ring system of AFP and positioning its ribitol phosphate part. Accordingly, site-specific exchanges at D166 disturbed the active site geometry of the enzyme and drastically reduced the catalytic activity. Based on the structure of the catalytic core we constructed a whole series of RosC variants but a disturbing, FMN dephosphorylating "killer enzyme", was not generated.

NADP-dependent methylene-tetrahydromethanopterin (methylene-H4MPT) dehydrogenase (MtdA) catalyzes the reversible dehydrogenation of methylene-H4MPT to form methenyl-H4MPT+ by using NADP(+) as a hydride acceptor. This hydride transfer reaction is involved in the oxidative metabolism from formaldehyde to CO2 in methylotrophic and methanotrophic bacteria. Here, we report on the crystal structures of the ternary MtdA-substrate complexes from Methylorubrum extorquens AM1 obtained in open and closed forms. Their conversion is accomplished by opening/closing the active site cleft via a 15 degrees rotation of the NADP, relative to the pterin domain. The 1.08 angstrom structure of the closed and active enzyme-NADP-methylene-H4MPT complex allows a detailed geometric analysis of the bulky substrates and a precise prediction of the hydride trajectory. Upon domain closure, the bulky substrate rings become compressed resulting in a tilt of the imidazolidine group of methylene-H4MPT that optimizes the geometry for hydride transfer. An additional 1.5 A structure of MtdA in complex with the nonreactive NADP(+) and methenyl-H4MPT+ revealed an extremely short distance between nicotinamide-C4 and imidazoline-C14a of 2.5 angstrom, which demonstrates the strong pressure imposed. The pterin-imidazolidine-phenyl butterfly angle of methylene-H4MPT bound to MtdA is smaller than that in the enzyme-free state but is similar to that in H-2- and F-420-dependent methylene-H4MPT dehydrogenases. The concept of compression-driven hydride transfer including quantum mechanical hydrogen tunneling effects, which are established for flavin- and NADP-dependent enzymes, can be expanded to hydride-transferring H4MPT-dependent enzymes. (C) 2020 Elsevier Ltd. All rights reserved.

Peroxiredoxins (Prxs) are thiol peroxidases that scavenge various peroxide substrates such as hydrogen peroxide (H2O2), alkyl hydroperoxides and peroxinitrite. They also function as chaperones and are involved in signal transduction by H2O2 in eukaryotic cells. The genome of Aquifex aeolicus, a microaerophilic, hyperthermophilic eubacterium, encodes four Prxs, among them an alkyl hydroperoxide reductase AhpC2 which was found to be closely related to archaeal 1-Cys peroxiredoxins. We determined the crystal structure of AhpC2 at 1.8 Å resolution and investigated its oligomeric state in solution by electron microscopy. AhpC2 is arranged as a toroid-shaped dodecamer instead of the typically observed decamer. The basic folding topology and the active site structure are conserved and possess a high structural similarity to other 1-Cys Prxs. However, the C-terminal region adopts an opposite orientation. AhpC2 contains three cysteines, Cys49, Cys212, and Cys218. The peroxidatic cysteine CP49 was found to be hyperoxidized to the sulfonic acid (SO3H) form, while Cys212 forms an intra-monomer disulfide bond with Cys218. Mutagenesis experiments indicate that Cys212 and Cys218 play important roles in the oligomerization of AhpC2. Based on these structural characteristics, we proposed the catalytic mechanism of AhpC2. This study provides novel insights into the structure and reaction mechanism of 1-Cys peroxiredoxins.

Degradation of the aromatic ketone acetophenone is initiated by its carboxylation to benzoylacetate catalyzed by acetophenone carboxylase (Apc) in a reaction dependent on the hydrolysis of two ATP to ADP and Pi. Apc is a large protein complex which dissociates during purification into a heterooctameric Apc(αα′βγ)2 core complex of 482 kDa and Apcε of 34 kDa. In this report, we present the X-ray structure of the Apc(αα′βγ)2 core complex from Aromatoleum aromaticum at ca. 3 Å resolution which reveals a unique modular architecture and serves as model of a new enzyme family. Apcβ contains a novel domain fold composed of two β-sheets in a barrel-like arrangement running into a bundle of eight short polyproline (type II)-like helical segments. Apcα and Apcα′ possess ATP binding modules of the ASKHA superfamily integrated into their multidomain structures and presumably operate as ATP-dependent kinases for acetophenone and bicarbonate, respectively. Mechanistic aspects of the novel carboxylation reaction requiring massive structural rearrangements are discussed and criteria for specifically annotating the family members Apc, acetone carboxylase and hydantoinase are defined.

DM9 domain was first identified in Drosophila melanogaster, and it was subsequently found to integrate with or without other protein domains across a wide range of invertebrates and vertebrates. In the present study, a member of DM9 domain containing protein (DM9CP) family from marine invertebrate Crassostrea gigas (designated CgDM9CP-1), which was only composed of two DM9 domains, was taken as a protein model to study the biological functions of DM9 domain and its molecular determinants. CgDM9CP-1 was found to exhibit high binding specificity and avidity toward d-mannose residue. It served as a pattern recognition receptor (PRR) with a broad range of recognition spectrum to various pathogen-associated molecular patterns, including lipopolysaccharide, peptidylglycan, mannan, and β-1, 3-glucan in a d-mannose-dependent manner, as well as bacteria and fungi. In order to reveal the molecular mechanism underlying its pattern recognition activity, the crystal structures of wild-type and loss-of-function mutants were solved, and Asp22 and Lys43 were found to be the critical residues for ligand recognition. Moreover, CgDM9CP-1 protein was found to mainly distribute on the surface of C. gigas hemocytes, and it could be translocated into cytoplasm and colocalized with the engulfed microbes during hemocyte phagocytosis. The present result clearly indicated that CgDM9CP-1 was a PRR, and it provided an important clue for the better understanding of DM9CP function.

Common structural elements in proteins such as α-helices or β-sheets are characterized by uniformly repeating, energetically favorable main chain conformations which additionally exhibit a completely saturated hydrogen-bonding network of the main chain NH and CO groups. Although polyproline or polyglycine type II helices (PPII or PGII ) are frequently found in proteins, they are not considered as equivalent secondary structure elements because they do not form a similar self-contained hydrogen-bonding network of the main chain atoms. In this context our finding of an unusual motif of glycine-rich PGII -like helices in the structure of the acetophenone carboxylase core complex is of relevance. These PGII -like helices form hexagonal bundles which appear to fulfill the criterion of a (largely) saturated hydrogen-bonding network of the main-chain groups and therefore may be regarded in this sense as a new secondary structure element. It consists of a central PGII -like helix surrounded by six nearly parallel PGII -like helices in a hexagonal array, plus an additional PGII -like helix extending the array outwards. Very related structural elements have previously been found in synthetic polyglycine fibers. In both cases, all main chain NH and CO groups of the central PGII -helix are saturated by either intra- or intermolecular hydrogen-bonds, resulting in a self-contained hydrogen-bonding network. Similar, but incomplete PGII -helix patterns were also previously identified in a GTP-binding protein and an antifreeze protein.

ABSTRACT Cytochrome c oxidases (CcOs), members of the heme-copper containing oxidase (HCO) superfamily, are the terminal enzymes of aerobic respiratory chains. The cbb3-type cytochrome c oxidases (cbb3-CcO) form the C-family and have only the central catalytic subunit in common with the A- and B-family HCOs. In Pseudomonas stutzeri, two cbb3 operons are organized in a tandem repeat. The atomic structure of the first cbb3 isoform (Cbb3-1) was determined at 3.2 Å resolution in 2010 (S. Buschmann, E. Warkentin, H. Xie, J. D. Langer, U. Ermler, and H. Michel, Science 329:327–330, 2010, http://dx.doi.org/10.1126/science.1187303). Unexpectedly, the electron density map of Cbb3-1 revealed the presence of an additional transmembrane helix (TMH) which could not be assigned to any known protein. We now identified this TMH as the previously uncharacterized protein PstZoBell_05036, using a customized matrix-assisted laser desorption ionization (MALDI)–tandem mass spectrometry setup. The amino acid sequence matches the electron density of the unassigned TMH. Consequently, the protein was renamed CcoM. In order to identify the function of this new subunit in the cbb3 complex, we generated and analyzed a CcoM knockout strain. The results of the biochemical and biophysical characterization indicate that CcoM may be involved in CcO complex assembly or stabilization. In addition, we found that CcoM plays a role in anaerobic respiration, as the ΔCcoM strain displayed altered growth rates under anaerobic denitrifying conditions. IMPORTANCE The respiratory chain has recently moved into the focus for drug development against prokaryotic human pathogens, in particular, for multiresistant strains (P. Murima, J. D. McKinney, and K. Pethe, Chem Biol 21:1423–1432, 2014, http://dx.doi.org/10.1016/j.chembiol.2014.08.020). cbb3-CcO is an essential enzyme for many different pathogenic bacterial species, e.g., Helicobacter pylori, Vibrio cholerae, and Pseudomonas aeruginosa, and represents a promising drug target. In order to develop compounds targeting these proteins, a detailed understanding of the molecular architecture and function is required. Here we identified and characterized a novel subunit, CcoM, in the cbb3-CcO complex and thereby completed the crystal structure of the Cbb3 oxidase from Pseudomonas stutzeri, a bacterium closely related to the human pathogen Pseudomonas aeruginosa.

Isoprenoid biosynthesis in many bacteria, plant chloroplasts and parasitic protozoa but not in humans proceeds via the mevalonate independent 2-C-methyl-D-erythritol-4-phosphate (MEP) pathway. Its penultimate reaction step is catalyzed by (E)-1-hydroxy-2-methyl-but-2-enyl-4-diphosphate (HMBPP) synthase (GcpE/IspG) which transforms 2-C-methyl-D-erythritol-2, 4-cyclo-diphosphate (MEcPP) to HMBPP. In this report we present the structure of GcpE of Thermus thermophiles in complex with its product HMBPP at a resolution of 1.65 Å. The GcpE-HMBPP like the GcpE-MEcPP structure is found in a closed, the ligand-free GcpE structure in an open enzyme state. Imposed by the rigid protein scaffold inside the active site funnel, linear HMBPP and circular MEcPP adopt highly similar conformations. The confined space also determines the conformational freedom of transition state intermediates and the design of anti-infective drugs. The apical Fe of the [4Fe-4S] cluster is coordinated to MEcPP in the GcpE-MEcPP complex and to a hydroxyl/water ligand but not to HMBPP in the GcpE-HMBPP complex. The GcpE-HMBPP structure can be attributed to one step in the currently proposed GcpE reaction cycle.

The molybdenum storage protein (MoSto) can store more than 100 Mo or W atoms as discrete polyoxometalate (POM) clusters. Here, we describe the three POM cluster sites along the threefold axis of the protein complex based on four X-ray structures with slightly different polyoxomolybdate compositions between 1.35 and 2 Å resolution. In contrast to the Moα-out binding site occupied by an Mo3 cluster, the Moα-in and Moβ binding sites contain rather weak and non-uniform electron density for the Mo atoms (but clearly identifiable by anomalous data), suggesting the presence of POM cluster ensembles and/or degradation products of larger aggregates. The "Moα-in cluster ensemble" was interpreted as an antiprism-like Mo6 species superimposed with an Mo7 pyramide and the "Moβ cluster ensemble" as an Mo13 cluster (present mostly in a degraded form) composed of a pyramidal Mo7 and a Mo3 building block linked by three spatially separated MoOx units. Inside the ball-shaped Mo13 cluster sits an occluded central atom, perhaps a metal ion. POM cluster formation at the Moα-in and Moβ sites appears to be driven by filtering out and binding/protecting self-assembled transient species complementary to the protein template.

The reversible redox reaction between coenzyme F420 and H2 to F420H2 is catalyzed by an F420-reducing [NiFe]-hydrogenase (FrhABG), which is an enzyme of the energy metabolism of methanogenic archaea. FrhABG is a group 3 [NiFe]-hydrogenase with a dodecameric quaternary structure of 1.25MDa as recently revealed by high-resolution cryo-electron microscopy. We report on the crystal structure of FrhABG from Methanothermobacter marburgensis at 1.7Å resolution and compare it with the structures of group 1 [NiFe]-hydrogenases, the only group structurally characterized yet. FrhA is similar to the large subunit of group 1 [NiFe]-hydrogenases regarding its core structure and the embedded [NiFe]-center but is different because of the truncation of ca 160 residues that results in similar but modified H2 and proton transport pathways and in suitable interfaces for oligomerization. The small subunit FrhG is composed of an N-terminal domain related to group 1 enzymes and a new C-terminal ferredoxin-like domain carrying the distal and medial [4Fe-4S] clusters. FrhB adopts a novel fold, binds one [4Fe-4S] cluster as well as one FAD in a U-shaped conformation and provides the F420-binding site at the Si-face of the isoalloxazine ring. Similar electrochemical potentials of both catalytic reactions and the electron-transferring [4Fe-4S] clusters, determined to be E°′≈−400mV, are in full agreement with the equalized forward and backward rates of the FrhABG reaction. The protein might contribute to balanced redox potentials by the aspartate coordination of the proximal [4Fe-4S] cluster, the new ferredoxin module and a rather negatively charged FAD surrounding.

Autotrophic members of the Sulfolobales (crenarchaeota) use the 3-hydroxypropionate/4-hydroxybutyrate cycle to assimilate CO2 into cell material. The product of the initial acetyl-CoA carboxylation with CO2, malonyl-CoA, is further reduced to malonic semialdehyde by an NADPH-dependent malonyl-CoA reductase (MCR); the enzyme also catalyzes the reduction of succinyl-CoA to succinic semialdehyde onwards in the cycle. Here, we present the crystal structure of Sulfolobus tokodaii malonyl-CoA reductase in the substrate-free state and in complex with NADP(+) and CoA. Structural analysis revealed an unexpected reaction cycle in which NADP(+) and CoA successively occupy identical binding sites. Both coenzymes are pressed into an S-shaped, nearly superimposable structure imposed by a fixed and preformed binding site. The template-governed cofactor shaping implicates the same binding site for the 3'- and 2'-ribose phosphate group of CoA and NADP(+), respectively, but a different one for the common ADP part: the beta-phosphate of CoA aligns with the alpha-phosphate of NADP(+). Evolution from an NADP(+) to a bispecific NADP(+) and CoA binding site involves many amino acid exchanges within a complex process by which constraints of the CoA structure also influence NADP(+) binding. Based on the paralogous aspartate-beta-semialdehyde dehydrogenase structurally characterized with a covalent Cys-aspartyl adduct, a malonyl/succinyl group can be reliably modeled into MCR and discussed regarding its binding mode, the malonyl/succinyl specificity, and the catalyzed reaction. The modified polypeptide surrounding around the absent ammonium group in malonate/succinate compared with aspartate provides the structural basis for engineering a methylmalonyl-CoA reductase applied for biotechnical polyester building block synthesis.