A new mode of B12 binding and the direct participation of a potassium ion in enzyme catalysis: X-ray structure of diol dehydratase

Spatial transcriptomics identifies novel Pseudomonas aeruginosa virulence factors

To examine host-pathogen interactions, we leveraged a dual spatial transcriptomics approach that simultaneously captures the expression of Pseudomonas aeruginosa genes alongside the entire host transcriptome using a murine model of ocular infection. This method revealed differential pathogen- and host-specific gene expression patterns in infected corneas, which generated a unified transcriptional map of infection. By integrating these data, we developed a predictive ridge regression model trained on images from infected tissues. The model achieved an R2 score of 0.923 in predicting bacterial burden distributions and identifying novel biomarkers associated with disease severity. Among iron acquisition pathogen-specific gene transcripts that showed significant enrichment at the host-pathogen interface, we discovered the novel virulence mediator PA2590, which was required for bacterial virulence. This study therefore highlights the power of combining bacterial and host spatial transcriptomics to uncover complex host-pathogen interactions and identify potentially druggable targets.

The action of coenzyme B12-dependent diol dehydratase on 3,3,3-trifluoro-1,2-propanediol results in elimination of all the fluorides with formation of acetaldehyde

3,3,3-Trifluoro-1,2-propanediol undergoes complete defluorination in two distinct steps: first, the conversion into 3,3,3-trifluoropropionaldehyde catalyzed by adenosylcobalamin (coenzyme B12)-dependent diol dehydratase; second, non-enzymatic elimination of all three fluorides from this aldehyde to afford malonic semialdehyde (3-oxopropanoic acid), which is decarboxylated to acetaldehyde. Diol dehydratase accepts 3,3,3-trifluoro-1,2-propanediol as a relatively poor substrate, albeit without significant mechanism-based inactivation of the enzyme during catalysis. Optical and electron paramagnetic resonance (EPR) spectra revealed the steady-state formation of cob(II)alamin and a substrate-derived intermediate organic radical (3,3,3-trifluoro-1,2-dihydroxyprop-1-yl). The coenzyme undergoes Co-C bond homolysis initiating a sequence of reaction by the generally accepted pathway via intermediate radicals. However, the greater steric size of trifluoromethyl and especially its negative impact on the stability of an adjacent radical centre compared to a methyl group has implications for the mechanism of the diol dehydratase reaction. Nevertheless, 3,3,3-trifluoropropionaldehyde is formed by the normal diol dehydratase pathway, but then undergoes non-enzymatic conversion into acetaldehyde, probably via 3,3-difluoropropenal and malonic semialdehyde.

Phosphomethylpyrimidine Synthase (ThiC): Trapping of Five Intermediates Provides Mechanistic Insights on a Complex Radical Cascade Reaction in Thiamin Biosynthesis

Phosphomethylpyrimidine synthase (ThiC) catalyzes the conversion of AIR to the thiamin pyrimidine HMP-P. This reaction is the most complex enzyme-catalyzed radical cascade identified to date, and the detailed mechanism has remained elusive. In this paper, we describe the trapping of five new intermediates that provide snapshots of the ThiC reaction coordinate and enable the formulation of a revised mechanism for the ThiC-catalyzed reaction.

A perspective on the discovery of enzyme activators

Enzyme activation remains a largely under-represented and poorly exploited area of drug discovery despite some key literature examples of the successful application of enzyme activators by various mechanisms and their importance in a wide range of therapeutic interventions. Here we describe the background nomenclature, present the current position of this field of drug discovery and discuss the challenges of hit identification for enzyme activation, as well as our perspectives on the approaches needed to overcome these challenges in early drug discovery.

Structural Insights into the Very Low Activity of the Homocoenzyme B12 Adenosylmethylcobalamin in Coenzyme B12 -Dependent Diol Dehydratase and Ethanolamine Ammonia-Lyase

The X-ray structures of coenzyme B12 (AdoCbl)-dependent eliminating isomerases complexed with adenosylmethylcobalamin (AdoMeCbl) have been determined. As judged from geometries, the Co-C bond in diol dehydratase (DD) is not activated even in the presence of substrate. In ethanolamine ammonia-lyase (EAL), the bond is elongated in the absence of substrate; in the presence of substrate, the complex likely exists in both pre- and post-homolysis states. The impacts of incorporating an extra CH2 group are different in the two enzymes: the DD active site is flexible, and AdoMeCbl binding causes large conformational changes that make DD unable to adopt the catalytic state, whereas the EAL active site is rigid, and AdoMeCbl binding does not induce significant conformational changes. Such flexibility and rigidity of the active sites might reflect the tightness of adenine binding. The structures provide good insights into the basis of the very low activity of AdoMeCbl in these enzymes.

Reactivating chaperones for coenzyme B12-dependent diol and glycerol dehydratases and ethanolamine ammonia-lyase

Adenosylcobalamin (AdoCbl) or coenzyme B₁₂-dependent enzymes tend to undergo mechanism-based inactivation during catalysis or inactivation in the absence of substrate. Such inactivation may be inevitable because they use a highly reactive radical for catalysis, and side reactions of radical intermediates result in the damage of the coenzyme. How do living organisms address such inactivation when enzymes are inactivated by undesirable side reactions? We discovered reactivating factors for radical B₁₂ eliminases. They function as releasing factors for damaged cofactor(s) from enzymes and thus mediate their exchange for intact AdoCbl. Since multiple turnovers and chaperone functions were demonstrated, they were renamed "reactivases" or "reactivating chaperones." They play an essential role in coenzyme recycling as part of the activity-maintaining systems for B₁₂ enzymes. In this chapter, we describe our investigations on reactivating chaperones, including their discovery, gene cloning, preparation, characterization, activity assays, and mechanistic studies, that have been conducted using a wide range of biochemical and structural methods that we have developed.

Coenzyme B12-dependent eliminases: Diol and glycerol dehydratases and ethanolamine ammonia-lyase

Adenosylcobalamin (AdoCbl) or coenzyme B₁₂-dependent enzymes catalyze intramolecular group-transfer reactions and ribonucleotide reduction in a wide variety of organisms from bacteria to animals. They use a super-reactive primary-carbon radical formed by the homolysis of the coenzyme's Co-C bond for catalysis and thus belong to the larger class of "radical enzymes." For understanding the general mechanisms of radical enzymes, it is of great importance to establish the general mechanism of AdoCbl-dependent catalysis using enzymes that catalyze the simplest reactions-such as diol dehydratase, glycerol dehydratase and ethanolamine ammonia-lyase. These enzymes are often called "eliminases." We have studied AdoCbl and eliminases for more than a half century. Progress has always been driven by the development of new experimental methodologies. In this chapter, we describe our investigations on these enzymes, including their metabolic roles, gene cloning, preparation, characterization, activity assays, and mechanistic studies, that have been conducted using a wide range of biochemical and structural methodologies we have developed.

Spectroscopic and Computational Investigation of the Epoxyqueuosine Reductase QueG Reveals Intriguing Similarities with the Reductive Dehalogenase PceA

Queuosine (Q) is a highly modified nucleoside of transfer RNA that is formed from guanosine triphosphate over the course of eight steps. The final step in this process, involving the conversion of epoxyqueuosine (oQ) to Q, is catalyzed by the enzyme QueG. A recent X-ray crystallographic study revealed that QueG possesses the same cofactors as reductive dehalogenases, including a base-off Co(II)cobalamin (Co(II)Cbl) species and two [4Fe-4S] clusters. While the initial step in the catalytic cycle of QueG likely involves the formation of a reduced Co(I)Cbl species, the mechanisms employed by this enzyme to accomplish the thermodynamically challenging reduction of base-off Co(II)Cbl to Co(I)Cbl and to convert oQ to Q remain unknown. In this study, we have used electron paramagnetic resonance (EPR) and magnetic circular dichroism (MCD) spectroscopies in conjunction with whole-protein quantum mechanics/molecular mechanics (QM/MM) computations to further characterize wild-type QueG and select variants. Our data indicate that the Co(II)Cbl cofactor remains five-coordinate upon substrate binding to QueG. Notably, during a QM/MM optimization of a putative QueG reaction intermediate featuring an alkyl-Co(III) species, the distance between the Co ion and coordinating C atom of oQ increased to >3.3 Å and the C-O bond of the epoxide reformed to regenerate the oQ-bound Co(I)Cbl reactant state of QueG. Thus, our computations indicate that the QueG mechanism likely involves single-electron transfer from the transient Co(I)Cbl species to oQ rather than direct Co-C bond formation, similar to the mechanism that has recently been proposed for the tetrachloroethylene reductive dehalogenase PceA.

Bioinspired Electrolysis for Green Molecular Transformations of Organic Halides Catalyzed by B12 Complex

Naturally-occurring B₁₂ -dependent enzymes catalyze various molecular transformations that are of particular interest from the viewpoint of biological chemistry as well as synthetic organic chemistry. Inspired by the unique property of the B₁₂ -dependent enzymes, various catalytic reactions have been developed using its model complex. Among the B₁₂ model complexes, heptamethyl cobyrinate, synthesized from natural vitamin B₁₂ , is highly soluble in various organic solvents and a redox active cobalt complex with an excellent catalysis in electroorganic synthesis. The electrochemical dechlorination of pollutant organic chlorides, such as DDT, was effectively catalyzed by the B₁₂ complex. Modification of the electrode surface by the sol-gel method to immobilize the B₁₂ complex was also developed. The B₁₂ modified electrodes were effective for the dehalogenation of organic halides with high turnover numbers based on the immobilized B₁₂ complex. Electrolysis of an organic halide catalyzed by the B₁₂ complex provided dechlorinated products under anaerobic conditions, while the electrolysis under aerobic conditions afforded oxygen incorporated products, such as an ester and amide along with dechlorination. Benzotrichloride was transformed into ethylbenzoate or N,N-diethylbenzamide in the presence of ethanol or diethylamine, respectively. This amide formation was further expanded to a unique paired electrolysis. Electrochemical reductions of an alkene and alkyne were also catalyzed by the B₁₂ complex. A cobalt-hydrogen complex should be formed as a bioinspired intermediate. Using the B₁₂ complex, light-assisted electrosynthesis was also developed to save the applied energy.

Identification of a Novel Cobamide Remodeling Enzyme in the Beneficial Human Gut Bacterium Akkermansia muciniphila

Cobamides, comprising the vitamin B₁₂ family of cobalt-containing cofactors, are required for metabolism in all domains of life, including most bacteria. Cobamides have structural variability in the lower ligand, and selectivity for particular cobamides has been observed in most organisms studied to date.

The beneficial human gut bacterium Akkermansia muciniphila provides metabolites to other members of the gut microbiota by breaking down host mucin, but most of its other metabolic functions have not been investigated. A. muciniphila strain Muc^T is known to use cobamides, the vitamin B₁₂ family of cofactors with structural diversity in the lower ligand. However, A. muciniphila Muc^T is unable to synthesize cobamides de novo, and the specific forms that can be used by A. muciniphila have not been examined. We found that the levels of growth of A. muciniphila Muc^T were nearly identical with each of seven cobamides tested, in contrast to nearly all bacteria that had been studied previously. Unexpectedly, this promiscuity is due to cobamide remodeling—the removal and replacement of the lower ligand—despite the absence of the canonical remodeling enzyme CbiZ in A. muciniphila. We identified a novel enzyme, CbiR, that is capable of initiating the remodeling process by hydrolyzing the phosphoribosyl bond in the nucleotide loop of cobamides. CbiR does not share similarity with other cobamide remodeling enzymes or B₁₂-binding domains and is instead a member of the apurinic/apyrimidinic (AP) endonuclease 2 enzyme superfamily. We speculate that CbiR enables bacteria to repurpose cobamides that they cannot otherwise use in order to grow under cobamide-requiring conditions; this function was confirmed by heterologous expression of cbiR in Escherichia coli. Homologs of CbiR are found in over 200 microbial taxa across 22 phyla, suggesting that many bacteria may use CbiR to gain access to the diverse cobamides present in their environment.

Recent Progress in the Understanding and Engineering of Coenzyme B12-Dependent Glycerol Dehydratase

Coenzyme B₁₂-dependent glycerol dehydratase (GDHt) catalyzes the dehydration reaction of glycerol in the presence of adenosylcobalamin to yield 3-hydroxypropanal (3-HPA), which can be converted biologically to versatile platform chemicals such as 1,3-propanediol and 3-hydroxypropionic acid. Owing to the increased demand for biofuels, developing biological processes based on glycerol, which is a byproduct of biodiesel production, has attracted considerable attention recently. In this review, we will provide updates on the current understanding of the catalytic mechanism and structure of coenzyme B₁₂-dependent GDHt, and then summarize the results of engineering attempts, with perspectives on future directions in its engineering.

Prokaryotic Organelles: Bacterial Microcompartments in E. coli and Salmonella

Bacterial microcompartments (MCPs) are proteinaceous organelles consisting of a metabolic pathway encapsulated within a selectively permeable protein shell. Hundreds of species of bacteria produce MCPs of at least nine different types, and MCP metabolism is associated with enteric pathogenesis, cancer, and heart disease. This review focuses chiefly on the four types of catabolic MCPs (metabolosomes) found in Escherichia coli and Salmonella: the propanediol utilization (pdu), ethanolamine utilization (eut), choline utilization (cut), and glycyl radical propanediol (grp) MCPs. Although the great majority of work done on catabolic MCPs has been carried out with Salmonella and E. coli, research outside the group is mentioned where necessary for a comprehensive understanding. Salient characteristics found across MCPs are discussed, including enzymatic reactions and shell composition, with particular attention paid to key differences between classes of MCPs. We also highlight relevant research on the dynamic processes of MCP assembly, protein targeting, and the mechanisms that underlie selective permeability. Lastly, we discuss emerging biotechnology applications based on MCP principles and point out challenges, unanswered questions, and future directions.

Decoding the stoichiometric composition and organisation of bacterial metabolosomes

Some enteric bacteria including Salmonella have evolved the propanediol-utilising microcompartment (Pdu MCP), a specialised proteinaceous organelle that is essential for 1,2-propanediol degradation and enteric pathogenesis. Pdu MCPs are a family of bacterial microcompartments that are self-assembled from hundreds of proteins within the bacterial cytosol. Here, we seek a comprehensive understanding of the stoichiometric composition and organisation of Pdu MCPs. We obtain accurate stoichiometry of shell proteins and internal enzymes of the natural Pdu MCP by QconCAT-driven quantitative mass spectrometry. Genetic deletion of the major shell protein and absolute quantification reveal the stoichiometric and structural remodelling of metabolically functional Pdu MCPs. Decoding the precise protein stoichiometry allows us to develop an organisational model of the Pdu metabolosome. The structural insights into the Pdu MCP are critical for both delineating the general principles underlying bacterial organelle formation, structural robustness and function, and repurposing natural microcompartments using synthetic biology for biotechnological applications.

Enteric pathogens such as Salmonella depend on propanediol-utilising microcompartments (Pdu MCP), which self-assemble from cytosolic proteins. Using mass spectrometry-based absolute quantification, the authors here define the protein stoichiometry and propose an organizational model of a Salmonella Pdu MCP.

Production of 3-Hydroxypropanoic Acid From Glycerol by Metabolically Engineered Bacteria

3-hydroxypropanoic acid (3-HP) is a valuable platform chemical with a high demand in the global market. 3-HP can be produced from various renewable resources. It is used as a precursor in industrial production of a number of chemicals, such as acrylic acid and its many derivatives. In its polymerized form, 3-HP can be used in bioplastic production. Several microbes naturally possess the biosynthetic pathways for production of 3-HP, and a number of these pathways have been introduced in some widely used cell factories, such as Escherichia coli and Saccharomyces cerevisiae. Latest advances in the field of metabolic engineering and synthetic biology have led to more efficient methods for bio-production of 3-HP. These include new approaches for introducing heterologous pathways, precise control of gene expression, rational enzyme engineering, redirecting the carbon flux based on in silico predictions using genome scale metabolic models, as well as optimizing fermentation conditions. Despite the fact that the production of 3-HP has been extensively explored in established industrially relevant cell factories, the current production processes have not yet reached the levels required for industrial exploitation. In this review, we explore the state of the art in 3-HP bio-production, comparing the yields and titers achieved in different microbial cell factories and we discuss possible methodologies that could make the final step toward industrially relevant cell factories.

Live-Cell Imaging of Physiologically Relevant Metal Ions Using Genetically Encoded FRET-Based Probes

Essential biochemical reactions and processes within living organisms are coupled to subcellular fluctuations of metal ions. Disturbances in cellular metal ion homeostasis are frequently associated with pathological alterations, including neurotoxicity causing neurodegeneration, as well as metabolic disorders or cancer. Considering these important aspects of the cellular metal ion homeostasis in health and disease, measurements of subcellular ion signals are of broad scientific interest. The investigation of the cellular ion homeostasis using classical biochemical methods is quite difficult, often even not feasible or requires large cell numbers. Here, we report of genetically encoded fluorescent probes that enable the visualization of metal ion dynamics within individual living cells and their organelles with high temporal and spatial resolution. Generally, these probes consist of specific ion binding domains fused to fluorescent protein(s), altering their fluorescent properties upon ion binding. This review focuses on the functionality and potential of these genetically encoded fluorescent tools which enable monitoring (sub)cellular concentrations of alkali metals such as K+, alkaline earth metals including Mg2+ and Ca2+, and transition metals including Cu+/Cu2+ and Zn2+. Moreover, we discuss possible approaches for the development and application of novel metal ion biosensors for Fe2+/Fe3+, Mn2+ and Na+.

Metabolic functions of the human gut microbiota: the role of metalloenzymes

Covering: up to the end of 2017 The human body is composed of an equal number of human and microbial cells. While the microbial community inhabiting the human gastrointestinal tract plays an essential role in host health, these organisms have also been connected to various diseases. Yet, the gut microbial functions that modulate host biology are not well established. In this review, we describe metabolic functions of the human gut microbiota that involve metalloenzymes. These activities enable gut microbial colonization, mediate interactions with the host, and impact human health and disease. We highlight cases in which enzyme characterization has advanced our understanding of the gut microbiota and examples that illustrate the diverse ways in which metalloenzymes facilitate both essential and unique functions of this community. Finally, we analyze Human Microbiome Project sequencing datasets to assess the distribution of a prominent family of metalloenzymes in human-associated microbial communities, guiding future enzyme characterization efforts.

Exploring Lactobacillus reuteri DSM20016 as a biocatalyst for transformation of longer chain 1,2-diols: Limits with microcompartment

Lactobacillus reuteri metabolises glycerol efficiently to form 3-hydroxypropionic acid (3-HP) and 1,3-propanediol (1,3PDO) by the same mechanism as that for 1,2-propanediol (1,2PDO) conversion to propionic acid and propanol via its propanediol utilization (pdu) pathway. Pdu enzymes are encoded by the pdu-operon, which also contain genes encoding the shell proteins of the microcompartment housing the metabolic pathway. In this work the selectivity and kinetics of the reactions catalysed by L. reuteri DSM20016 Pdu enzymes glycerol dehydratase (GDH), 1,3-propanediol oxidoreductase (PduQ) and coenzyme-A acylating propionaldehyde dehydrogenase (PduP), produced recombinantly, was investigated against corresponding substrates of different chain lengths. Glycerol dehydratase exhibited activity against C2-C4 polyols, with the highest activity against glycerol and 1,2-propanediol (1,2-PDO). A double mutant of the pduC gene of GDH (PduC-S302A/Q337A) was constructed that displayed lowered activity against glycerol and 1,2PDO but extended the substrate range upto C6-diol. The best substrate for both PduQ and PduP was 3-hydroxypropanal (3HPA), although PduP exhibited nearly 10-fold higher specific activity. The enzymes also showed some activity against C3-C10 aliphatic aldehydes, with PduP having higher relative activity. Subsequently, transformation of polyols using whole cells of L. reuteri containing the wild type- and mutated GDH, respectively, confirmed the reduced activity of the mutant against glycerol and 1,2PDO, but its activity against longer substrates was negligible. In contrast, recombinant Escherichia coli BL21(DE3) cells harboring the GDH variant converted diols with up to C6 carbon chain length to their respective aldehydes, suggesting that the protein shell of the microcompartment in L. reuteri posed a barrier to the passage of longer chain substrate.

Monovalent Cation Activation of the Radical SAM Enzyme Pyruvate Formate-Lyase Activating Enzyme

Pyruvate formate-lyase activating enzyme (PFL-AE) is a radical S-adenosyl-l-methionine (SAM) enzyme that installs a catalytically essential glycyl radical on pyruvate formate-lyase. We show that PFL-AE binds a catalytically essential monovalent cation at its active site, yet another parallel with B₁₂ enzymes, and we characterize this cation site by a combination of structural, biochemical, and spectroscopic approaches. Refinement of the PFL-AE crystal structure reveals Na⁺ as the most likely ion present in the solved structures, and pulsed electron nuclear double resonance (ENDOR) demonstrates that the same cation site is occupied by ²³Na in the solution state of the as-isolated enzyme. A SAM carboxylate-oxygen is an M⁺ ligand, and EPR and circular dichroism spectroscopies reveal that both the site occupancy and the identity of the cation perturb the electronic properties of the SAM-chelated iron–sulfur cluster. ENDOR studies of the PFL-AE/[¹³C-methyl]-SAM complex show that the target sulfonium positioning varies with the cation, while the observation of an isotropic hyperfine coupling to the cation by ENDOR measurements establishes its intimate, SAM-mediated interaction with the cluster. This monovalent cation site controls enzyme activity: (i) PFL-AE in the absence of any simple monovalent cations has little–no activity; and (ii) among monocations, going down Group 1 of the periodic table from Li⁺ to Cs⁺, PFL-AE activity sharply maximizes at K⁺, with NH₄⁺ closely matching the efficacy of K⁺. PFL-AE is thus a type I M⁺-activated enzyme whose M⁺ controls reactivity by interactions with the cosubstrate, SAM, which is bound to the catalytic iron–sulfur cluster.

Directionality and the Role of Polarization in Electric Field Effects on Radical Stability

Accurate quantum-chemical calculations are used to analyze the effects of charges on the kinetics and thermodynamics of radical reactions, with specific attention given to the origin and directionality of the effects. Conventionally, large effects of the charges are expected to occur in systems with pronounced charge-separated resonance contributors. The nature (stabilization or destabilization) and magnitude of these effects thus depend on the orientation of the interacting multipoles. However, we show that a significant component of the stabilizing effects of the external electric field is largely independent of the orientation of external electric field (e.g. a charged functional group, a point charge, or an electrode) and occurs even in the absence of any pre-existing charge separation. This effect arises from polarization of the electron density of the molecule induced by the electric field. This polarization effect is greater for highly delocalized species such as resonance-stabilized radicals and transition states of radical reactions. We show that this effect on the stability of such species is preserved in chemical reaction energies, leading to lower bond-dissociation energies and barrier heights. Finally, our simplified modelling of the diol dehydratase-catalyzed 1,2-hydroxyl shift indicates that such stabilizing polarization is likely to contribute to the catalytic activity of enzymes.

Molecular Mechanisms of Enzyme Activation by Monovalent Cations

Regulation of enzymes through metal ion complexation is widespread in biology and underscores a physiological need for stability and high catalytic activity that likely predated proteins in the RNA world. In addition to divalent metals such as Ca²⁺, Mg²⁺, and Zn²⁺, monovalent cations often function as efficient and selective promoters of catalysis. Advances in structural biology unravel a rich repertoire of molecular mechanisms for enzyme activation by Na⁺ and K⁺ Strategies range from short-range effects mediated by direct participation in substrate binding, to more distributed effects that propagate long-range to catalytic residues. This review addresses general considerations and examples.

MtrA of the sodium ion pumping methyltransferase binds cobalamin in a unique mode

In the three domains of life, vitamin B₁₂ (cobalamin) is primarily used in methyltransferase and isomerase reactions. The methyltransferase complex MtrA–H of methanogenic archaea has a key function in energy conservation by catalysing the methyl transfer from methyl-tetrahydromethanopterin to coenzyme M and its coupling with sodium-ion translocation. The cobalamin-binding subunit MtrA is not homologous to any known B₁₂-binding proteins and is proposed as the motor of the sodium-ion pump. Here, we present crystal structures of the soluble domain of the membrane-associated MtrA from Methanocaldococcus jannaschii and the cytoplasmic MtrA homologue/cobalamin complex from Methanothermus fervidus. The MtrA fold corresponds to the Rossmann-type α/β fold, which is also found in many cobalamin-containing proteins. Surprisingly, the cobalamin-binding site of MtrA differed greatly from all the other cobalamin-binding sites. Nevertheless, the hydrogen-bond linkage at the lower axial-ligand site of cobalt was equivalently constructed to that found in other methyltransferases and mutases. A distinct polypeptide segment fixed through the hydrogen-bond linkage in the relaxed Co(III) state might be involved in propagating the energy released upon corrinoid demethylation to the sodium-translocation site by a conformational change.

Sodium/Potassium homeostasis in the cell

All animals are characterized by steep gradients of Na(+) and K(+) across the plasma membrane, and in spite of their highly similar chemical properties, the ions can be distinguished by numerous channels and transporters. The gradients are generated by the Na(+),K(+)-ATPase, or sodium pump, which pumps out Na(+) and takes up K(+) at the expense of the chemical energy from ATP. Because the membrane is more permeable to K(+) than to Na(+), the uneven ion distribution causes a transmembrane voltage difference, and this membrane potential forms the basis for the action potential and for much of the neuronal signaling in general. The potential energy stored in the concentration gradients is also used to drive a large number of the secondary transporters responsible for transmembrane carriage of solutes ranging from sugars, amino acids, and neurotransmitters to inorganic ions such as chloride, inorganic phosphate, and bicarbonate. Furthermore, Na(+) and K(+) themselves are important enzymatic cofactors that typically lower the energy barrier of substrate binding.In this chapter, we describe the roles of Na(+) and K(+) in the animal cell with emphasis on the creation and usage of the steep gradients across the membrane. More than 50 years of Na(+),K(+)-ATPase research has revealed many details of the molecular machinery and offered insights into how the pump is regulated by post-translational modifications and specific drugs.

Key enzymes catalyzing glycerol to 1,3-propanediol

Biodiesel can replace petroleum diesel as it is produced from animal fats and vegetable oils, and it produces about 10 % (w/w) glycerol, which is a promising new industrial microbial carbon, as a major by-product. One of the most potential applications of glycerol is its biotransformation to high value chemicals such as 1,3-propanediol (1,3-PD), dihydroxyacetone (DHA), succinic acid, etc., through microbial fermentation. Glycerol dehydratase, 1,3-propanediol dehydrogenase (1,3-propanediol-oxydoreductase), and glycerol dehydrogenase, which were encoded, respectively, by dhaB, dhaT, and dhaD and with DHA kinase are encompassed by the dha regulon, are the three key enzymes in glycerol bioconversion into 1,3-PD and DHA, and these are discussed in this review article. The summary of the main research direction of these three key enzyme and methods of glycerol bioconversion into 1,3-PD and DHA indicates their potential application in future enzymatic research and industrial production, especially in biodiesel industry.

Molecular architectures and functions of radical enzymes and their (re)activating proteins

Certain proteins utilize the high reactivity of radicals for catalysing chemically challenging reactions. These proteins contain or form a radical and therefore named 'radical enzymes'. Radicals are introduced by enzymes themselves or by (re)activating proteins called (re)activases. The X-ray structures of radical enzymes and their (re)activases revealed some structural features of these molecular apparatuses which solved common enigmas of radical enzymes—i.e. how the enzymes form or introduce radicals at the active sites, how they use the high reactivity of radicals for catalysis, how they suppress undesired side reactions of highly reactive radicals and how they are (re)activated when inactivated by extinction of radicals. This review highlights molecular architectures of radical B12 enzymes, radical SAM enzymes, tyrosyl radical enzymes, glycyl radical enzymes and their (re)activating proteins that support their functions. For generalization, comparisons of the recently reported structures of radical enzymes with those of canonical radical enzymes are summarized here.

Diverse bacterial microcompartment organelles

Bacterial microcompartments (MCPs) are sophisticated protein-based organelles used to optimize metabolic pathways. They consist of metabolic enzymes encapsulated within a protein shell, which creates an ideal environment for catalysis and facilitates the channeling of toxic/volatile intermediates to downstream enzymes. The metabolic processes that require MCPs are diverse and widely distributed and play important roles in global carbon fixation and bacterial pathogenesis. The protein shells of MCPs are thought to selectively control the movement of enzyme cofactors, substrates, and products (including toxic or volatile intermediates) between the MCP interior and the cytoplasm of the cell using both passive electrostatic/steric and dynamic gated mechanisms. Evidence suggests that specialized shell proteins conduct electrons between the cytoplasm and the lumen of the MCP and/or help rebuild damaged iron-sulfur centers in the encapsulated enzymes. The MCP shell is elaborated through a family of small proteins whose structural core is known as a bacterial microcompartment (BMC) domain. BMC domain proteins oligomerize into flat, hexagonally shaped tiles, which assemble into extended protein sheets that form the facets of the shell. Shape complementarity along the edges allows different types of BMC domain proteins to form mixed sheets, while sequence variation provides functional diversification. Recent studies have also revealed targeting sequences that mediate protein encapsulation within MCPs, scaffolding proteins that organize lumen enzymes and the use of private cofactor pools (NAD/H and coenzyme A [HS-CoA]) to facilitate cofactor homeostasis. Although much remains to be learned, our growing understanding of MCPs is providing a basis for bioengineering of protein-based containers for the production of chemicals/pharmaceuticals and for use as molecular delivery vehicles.

Non-enzymatic ribonucleotide reduction in the prebiotic context

Model studies of prebiotic chemistry have revealed compelling routes for the formation of the building blocks of proteins and RNA, but not DNA. Today, deoxynucleotides required for the construction of DNA are produced by reduction of nucleotides catalysed by ribonucleotide reductases, which are radical enzymes. This study considers potential non-enzymatic routes via intermediate radicals for the ancient formation of deoxynucleotides. In this context, several mechanisms for ribonucleotide reduction, in a putative H2 S/HS(.) environment, are characterized using computational chemistry. A bio-inspired mechanistic cycle involving a keto intermediate and HSSH production is found to be potentially viable. An alternative pathway, proceeding through an enol intermediate is found to exhibit similar energetic requirements. Non-cyclical pathways, in which HSS(.) is generated in the final step instead of HS(.) , show a markedly increased thermodynamic driving force (ca. 70 kJ mol(-1) ) and thus warrant serious consideration in the context of the prebiotic ribonucleotide reduction.

Exploring bacterial organelle interactomes: a model of the protein-protein interaction network in the Pdu microcompartment

Bacterial microcompartments (MCPs) are protein-bound organelles that carry out diverse metabolic pathways in a wide range of bacteria. These supramolecular assemblies consist of a thin outer protein shell, reminiscent of a viral capsid, which encapsulates sequentially acting enzymes. The most complex MCP elucidated so far is the propanediol utilizing (Pdu) microcompartment. It contains the reactions for degrading 1,2-propanediol. While several experimental studies on the Pdu system have provided hints about its organization, a clear picture of how all the individual components interact has not emerged yet. Here we use co-evolution-based methods, involving pairwise comparisons of protein phylogenetic trees, to predict the protein-protein interaction (PPI) network governing the assembly of the Pdu MCP. We propose a model of the Pdu interactome, from which selected PPIs are further inspected via computational docking simulations. We find that shell protein PduA is able to serve as a “universal hub” for targeting an array of enzymes presenting special N-terminal extensions, namely PduC, D, E, L and P. The varied N-terminal peptides are predicted to bind in the same cleft on the presumptive luminal face of the PduA hexamer. We also propose that PduV, a protein of unknown function with remote homology to the Ras-like GTPase superfamily, is likely to localize outside the MCP, interacting with the protruding β-barrel of the hexameric PduU shell protein. Preliminary experiments involving a bacterial two-hybrid assay are presented that corroborate the existence of a PduU-PduV interaction. This first systematic computational study aimed at characterizing the interactome of a bacterial microcompartment provides fresh insight into the organization of the Pdu MCP.

Many bacteria produce giant proteinaceous structures within their cells, which they use to carry out special metabolic reactions in their interior. Much has been learned recently about the individual components—shell proteins and encapsulated enzymes—that assemble together, thousands of subunits in all, to make these bacterial microcompartments or MCPs. However, in order to carry out their biological functions, these systems must be highly organized through specific protein-protein interactions, and such a higher level understanding of organization in MCP systems is lacking. In this study, we use genomic data and phylogenetic analysis to predict the network of interactions between the approximately 20 different kinds of proteins and enzymes present in the Pdu MCP. Then, we use computational docking to examine a subset of those that are predicted to involve enzymes bound to the interior surface of the shell proteins, and show that the results are consistent with recent experimental data. We further provide new experimental evidence for one of the predicted protein-protein interactions. This study expands our understanding of a complex system of proteins serving as a metabolic organelle in bacterial cells, and provides a foundation for further experimental investigations.

Spectroscopic studies of the Salmonella enterica adenosyltransferase enzyme SeCobA: molecular-level insight into the mechanism of substrate Cob(II)alamin activation

CobA from Salmonella enterica (SeCobA) is a member of the family of ATP:Co(I)rrinoid adenosyltransferase (ACAT) enzymes that participate in the biosynthesis of adenosylcobalamin by catalyzing the transfer of the adenosyl group from an ATP molecule to a reactive Co(I)rrinoid species transiently generated in the enzyme active site. This reaction is thermodynamically challenging, as the reduction potential of the Co(II)rrinoid precursor in solution is far more negative than that of available reducing agents in the cell (e.g., flavodoxin), precluding nonenzymic reduction to the Co(I) oxidation state. However, in the active sites of ACATs, the Co(II)/Co(I) redox potential is increased by >250 mV via the formation of a unique four-coordinate (4c) Co(II)rrinoid species. In the case of the SeCobA ACAT, crystallographic and kinetic studies have revealed that the phenylalanine 91 (F91) and tryptophan 93 (W93) residues are critical for in vivo activity, presumably by blocking access to the lower axial ligand site of the Co(II)rrinoid substrate. To further assess the importance of the F91 and W93 residues with respect to enzymatic function, we have characterized various SeCobA active-site variants using electronic absorption, magnetic circular dichroism, and electron paramagnetic resonance spectroscopies. Our data provide unprecedented insight into the mechanism by which SeCobA converts the Co(II)rrinoid substrate to 4c species, with the hydrophobicity, size, and ability to participate in offset π-stacking interactions of key active-site residues all being critical for activity. The structural changes that occur upon Co(II)rrinoid binding also appear to be crucial for properly orienting the transiently generated Co(I) "supernucleophile" for rapid reaction with cosubstrate ATP.

The extracellular heme-binding protein HbpS from the soil bacterium Streptomyces reticuli is an aquo-cobalamin binder

The extracellular protein HbpS from Streptomyces reticuli interacts with iron ions and heme. It also acts in concert with the two-component sensing system SenS-SenR in response to oxidative stress. Sequence comparisons suggested that the protein may bind a cobalamin. UV-visible spectroscopy confirmed binding (Kd = 34 μm) to aquo-cobalamin (H2OCbl(+)) but not to other cobalamins. Competition experiments with the H2OCbl(+)-coordinating ligand CN(-) and comparison of mutants identified a histidine residue (His-156) that coordinates the cobalt ion of H2OCbl(+) and substitutes for water. HbpS·Cobalamin lacks the Asp-X-His-X-X-Gly motif seen in some cobalamin binding enzymes. Preliminary tests showed that a related HbpS protein from a different species also binds H2OCbl(+). Furthermore, analyses of HbpS-heme binding kinetics are consistent with the role of HbpS as a heme-sensor and suggested a role in heme transport. Given the high occurrence of HbpS-like sequences among Gram-positive and Gram-negative bacteria, our findings suggest a great functional versatility among these proteins.

Diverse bacterial microcompartment organelles

Bacterial microcompartments (MCPs) are sophisticated protein-based organelles used to optimize metabolic pathways. They consist of metabolic enzymes encapsulated within a protein shell, which creates an ideal environment for catalysis and facilitates the channeling of toxic/volatile intermediates to downstream enzymes. The metabolic processes that require MCPs are diverse and widely distributed and play important roles in global carbon fixation and bacterial pathogenesis. The protein shells of MCPs are thought to selectively control the movement of enzyme cofactors, substrates, and products (including toxic or volatile intermediates) between the MCP interior and the cytoplasm of the cell using both passive electrostatic/steric and dynamic gated mechanisms. Evidence suggests that specialized shell proteins conduct electrons between the cytoplasm and the lumen of the MCP and/or help rebuild damaged iron-sulfur centers in the encapsulated enzymes. The MCP shell is elaborated through a family of small proteins whose structural core is known as a bacterial microcompartment (BMC) domain. BMC domain proteins oligomerize into flat, hexagonally shaped tiles, which assemble into extended protein sheets that form the facets of the shell. Shape complementarity along the edges allows different types of BMC domain proteins to form mixed sheets, while sequence variation provides functional diversification. Recent studies have also revealed targeting sequences that mediate protein encapsulation within MCPs, scaffolding proteins that organize lumen enzymes and the use of private cofactor pools (NAD/H and coenzyme A [HS-CoA]) to facilitate cofactor homeostasis. Although much remains to be learned, our growing understanding of MCPs is providing a basis for bioengineering of protein-based containers for the production of chemicals/pharmaceuticals and for use as molecular delivery vehicles.

Analyses of cobalt-ligand and potassium-ligand bond lengths in metalloproteins: trends and patterns

Cobalt and potassium are biologically important metal elements that are present in a large array of proteins. Cobalt is mostly found in vivo associated with a corrin ring, which represents the core of the vitamin B12 molecule. Potassium is the most abundant metal in the cytosol, and it plays a crucial role in maintaining membrane potential as well as correct protein function. Here, we report a thorough analysis of the geometric properties of cobalt and potassium coordination spheres that was performed with high resolution on a representative set of structures from the Protein Data Bank and complemented by quantum mechanical calculations realized at the DFT level of theory (B3LYP/ SDD) on mononuclear model systems. The results allowed us to draw interesting conclusions on the structural characteristics of both Co and K centers, and to evaluate the importance of effects such as their association energies and intrinsic thermodynamic stabilities. Overall, the results obtained provide useful data for enhancing the atomic models normally applied in theoretical and computational studies of Co or K proteins performed at the quantum mechanical level, and for developing molecular mechanical parameters for treating Co or K coordination spheres in molecular mechanics or molecular dynamics studies.

Essential roles of nucleotide-switch and metal-coordinating residues for chaperone function of diol dehydratase-reactivase

Diol dehydratase-reactivase (DD-R) is a molecular chaperone that reactivates inactivated holodiol dehydratase (DD) by cofactor exchange. Its ADP-bound and ATP-bound forms are high-affinity and low-affinity forms for DD, respectively. Among DD-Rs mutated at the nucleotide-binding site, neither the Dα8N nor Dα413N mutant was effective as a reactivase. Although Dα413N showed ATPase activity, it did not mediate cyanocobalamin (CN-Cbl) release from the DD·CN-Cbl complex in the presence of ATP or ADP and formed a tight complex with apoDD even in the presence of ATP, suggesting the involvement of Aspα413 in the nucleotide switch. In contrast, Dα8N showed very low ATPase activity and did not mediate CN-Cbl release from the complex in the presence of ATP, but it did cause about 50% release in the presence of ADP. The complex formation of this mutant with DD was partially reversed by ATP, suggesting that Aspα8 is involved in the ATPase activity but only partially in the nucleotide switch. Among DD-Rs mutated at the Mg(2+)-binding site, only Eβ31Q was about 30% as active as wild-type DD-R and formed a tight complex with apoDD, indicating that the DD-R β subunit is not absolutely required for reactivation. If subunit swapping occurs between the DD-R β and DD β subunits, Gluβ97 of DD would coordinate to Mg(2+). The complex of Eβ97Q DD with CN-Cbl was not activated by wild-type DD-R. No complex was formed between this mutant and wild-type DD-R, indicating that the coordination of Gluβ97 to Mg(2+) is essential for subunit swapping and therefore for (re)activation.

Radical use of Rossmann and TIM barrel architectures for controlling coenzyme B12 chemistry

The ability of enzymes to harness free-radical chemistry allows for some of the most amazing transformations in nature, including reduction of ribonucleotides and carbon skeleton rearrangements. Enzyme cofactors involved in this chemistry can be large and complex, such as adenosylcobalamin (coenzyme B(12)), simpler, such as S-adenosylmethionine and an iron-sulfur cluster (i.e., poor man's B(12)), or very small, such as one nonheme iron atom coordinated by protein ligands. Although the chemistry catalyzed by these enzyme-bound cofactors is unparalleled, it does come at a price. The enzyme must be able to control these radical reactions, preventing unwanted chemistry and protecting the enzyme active site from damage. Here, we consider a set of radical folds: the (β/α)(8) or TIM barrel, combined with a Rossmann domain for coenzyme B(12)-dependent chemistry. Using specific enzyme examples, we consider how nature employs the common TIM barrel fold and its Rossmann domain partner for radical-based chemistry.

Growth inhibition of Sporomusa ovata by incorporation of benzimidazole bases into cobamides

Phenolyl cobamides are unique members of a class of cobalt-containing cofactors that includes vitamin B12 (cobalamin). Cobamide cofactors facilitate diverse reactions in prokaryotes and eukaryotes. Phenolyl cobamides are structurally and chemically distinct from the more commonly used benzimidazolyl cobamides such as cobalamin, as the lower axial ligand is a phenolic group rather than a benzimidazole. The functional significance of this difference is not well understood. Here we show that in the bacterium Sporomusa ovata, the only organism known to synthesize phenolyl cobamides, several cobamide-dependent acetogenic metabolisms have a requirement or preference for phenolyl cobamides. The addition of benzimidazoles to S. ovata cultures results in a decrease in growth rate when grown on methanol, 3,4-dimethoxybenzoate, H2 plus CO2, or betaine. Suppression of native p-cresolyl cobamide synthesis and production of benzimidazolyl cobamides occur upon the addition of benzimidazoles, indicating that benzimidazolyl cobamides are not functionally equivalent to the phenolyl cobamide cofactors produced by S. ovata. We further show that S. ovata is capable of incorporating other phenolic compounds into cobamides that function in methanol metabolism. These results demonstrate that S. ovata can incorporate a wide range of compounds as cobamide lower ligands, despite its preference for phenolyl cobamides in the metabolism of certain energy substrates. To our knowledge, S. ovata is unique among cobamide-dependent organisms in its preferential utilization of phenolyl cobamides.

Crystallographic studies on B12 binding proteins in eukaryotes and prokaryotes

The X-ray crystal structures of several important vitamin B12 binding proteins that have been solved in recent years have enhanced our current understanding in the vitamin B12 field. These structurally diverse groups of B12 binding proteins perform various important biological activities, both by transporting B12 as well as catalyzing various biological reactions. An in-depth comparative analysis of these structures was carried out using PDB coordinates of a carefully chosen database of B12 binding proteins to correlate the overall folding of the molecule with phylogeny, the B12 interactions, and with their biological function. The structures of these proteins are discussed in the context of this comparative analysis.

[Computational mutation analysis of enzymatic reaction]

Density functional theory (DFT) calculations are established as a useful research tool to investigate the structures and reactivity of biological systems; however, their high computational costs still restrict their applicability to systems of several tens up to a few hundred atoms. Recently, a combined quantum mechanical/molecular mechanical (QM/MM) approach has become an important method to study enzymatic reactions. In the past several years, we have investigated B12-dependent diol dehydratase using QM/MM calculations. The enzyme catalyzes chemically difficult reactions by utilizing the high reactivity of free radicals. In this paper, we explain our QM/MM calculations for the structure and reactivity of diol dehydratase and report key findings with respect to the catalytic roles of the active-site amino acid residues, computational mutational analysis of the active-site amino acid residues, assignment of the central metal ion, and function of the central metal ion. Our QM/MM calculations can correctly describe the structures and activation barriers of intermediate and transition states in the protein environment. Moreover, predicted relative activities of mutants are consistent with experimentally observed reactivity. These results will encourage the application of QM/MM research to the mechanistic study of enzymatic reactions, functional analysis of active-site residues, and rational design of enzymes with new catalytic functions.

Corrin ring-induced redox tuning

The density functional calculations suggest that the expansion of the corrin macrocycle's N(4) core by 0.06-0.10 Å leads to an appreciable lowering of 100-150 mV vs. saturated calomel electrode in the reduction potentials of two biologically important B(12) cofactors namely methylcobalamin and adenosylcobalamin respectively. This redox tuning of B(12) cofactors may encourage the electron transfer-based activation mechanism for B(12)-dependent enzymes.