Vinayak Agarwal

Biosynthesis of polybrominated aromatic organic compounds by marine bacteria

Polybrominated diphenyl ethers (PBDEs) and polybrominated bipyrroles are natural products that bioaccumulate in the marine food chain. PBDEs have attracted widespread attention due to their persistence in the environment and potential toxicity to humans. However, the natural origins of PBDE biosynthesis are not known. Here we report marine bacteria as producers of PBDEs and establish a genetic and molecular foundation for their production that unifies paradigms for the elaboration of bromophenols and bromopyrroles abundant in marine biota. We provide biochemical evidence of marine brominase enzymes revealing decarboxylative-halogenation enzymology previously unknown among halogenating enzymes. Biosynthetic motifs discovered in our study were used to mine sequence databases to discover unrealized marine bacterial producers of organobromine compounds.

Mutations that stabilize the open state of the Erwinia chrisanthemi ligand-gated ion channel fail to change the conformation of the pore domain in crystals

The determination of structural models of the various stable states of an ion channel is a key step toward the characterization of its conformational dynamics. In the case of nicotinic-type receptors, different structures have been solved but, thus far, these different models have been obtained from different members of the superfamily. In the case of the bacterial member ELIC, a cysteamine-gated channel from Erwinia chrisanthemi, a structural model of the protein in the absence of activating ligand (and thus, conceivably corresponding to the closed state of this channel) has been previously generated. In this article, electrophysiological characterization of ELIC mutants allowed us to identify pore mutations that slow down the time course of desensitization to the extent that the channel seems not to desensitize at all for the duration of the agonist applications (>20 min). Thus, it seems reasonable to conclude that the probability of ELIC occupying the closed state is much lower for the ligand-bound mutants than for the unliganded wild-type channel. To gain insight into the conformation adopted by ELIC under these conditions, we solved the crystal structures of two of these mutants in the presence of a concentration of cysteamine that elicits an intracluster open probability of >0.9. Curiously, the obtained structural models turned out to be nearly indistinguishable from the model of the wild-type channel in the absence of bound agonist. Overall, our findings bring to light the limited power of functional studies in intact membranes when it comes to inferring the functional state of a channel in a crystal, at least in the case of the nicotinic-receptor superfamily.

Circular Logic: Nonribosomal Peptide-like Macrocyclization with a Ribosomal Peptide Catalyst

A protease from ribosomal peptide biosynthesis macrocyclizes diverse substrates, including those resembling nonribosomal peptide and hybrid polyketide-peptide products. The proposed mechanism is analogous to thioesterase-catalyzed chemistry, but the substrates are amide bonds rather than thioesters.

Enzymatic Halogenation and Dehalogenation Reactions: Pervasive and Mechanistically Diverse

Naturally produced halogenated compounds are ubiquitous across all domains of life where they perform a multitude of biological functions and adopt a diversity of chemical structures. Accordingly, a diverse collection of enzyme catalysts to install and remove halogens from organic scaffolds has evolved in nature. Accounting for the different chemical properties of the four halogen atoms (fluorine, chlorine, bromine, and iodine) and the diversity and chemical reactivity of their organic substrates, enzymes performing biosynthetic and degradative halogenation chemistry utilize numerous mechanistic strategies involving oxidation, reduction, and substitution. Biosynthetic halogenation reactions range from simple aromatic substitutions to stereoselective C-H functionalizations on remote carbon centers and can initiate the formation of simple to complex ring structures. Dehalogenating enzymes, on the other hand, are best known for removing halogen atoms from man-made organohalogens, yet also function naturally, albeit rarely, in metabolic pathways. This review details the scope and mechanism of natures halogenation and dehalogenation enzymatic strategies, highlights gaps in our understanding, and posits where new advances in the field might arise in the near future.

Structure of the enzyme-acyl carrier protein (ACP) substrate gatekeeper complex required for biotin synthesis

Although the pimeloyl moiety was long known to be a biotin precursor, the mechanism of assembly of this C7 α,ω-dicarboxylic acid was only recently elucidated. In Escherichia coli, pimelate is made by bypassing the strict specificity of the fatty acid synthetic pathway. BioC methylates the free carboxyl of a malonyl thioester, which replaces the usual acetyl thioester primer. This atypical primer is transformed to pimeloyl-acyl carrier protein (ACP) methyl ester by two cycles of fatty acid synthesis. The question is, what stops this product from undergoing further elongation? Although BioH readily cleaves this product in vitro, the enzyme is nonspecific, which made assignment of its physiological substrate problematical, especially because another enzyme, BioF, could also perform this gatekeeping function. We report the 2.05-Å resolution cocrystal structure of a complex of BioH with pimeloyl-ACP methyl ester and use the structure to demonstrate that BioH is the gatekeeper and its physiological substrate is pimeloyl-ACP methyl ester.

Structures of Cyanobactin Maturation Enzymes Define a Family of Transamidating Proteases

Cyanobactins, a class of ribosomally encoded macrocylic natural products, are biosynthesized through the proteolytic processing and subsequent N-C macrocylization of ribosomal peptide precursors. Macrocylization occurs through a two-step process in which the first protease (PatA) removes the amino terminal flanking sequence from the precursor to yield a free N terminus of the precursor peptide, and the second protease (PatG) removes the C-terminal flanking sequence and then catalyzes the transamidation reaction to yield an N-C cyclized product. Here, we present the crystal structures of the protease domains of PatA and PatG from the patellamide cluster and of PagA from the prenylagaramide cluster. A comparative structural and biochemical analysis of the transamidating PatG protease reveals the presence of a unique structural element distinct from canonical subtilisin proteases, which may facilitate the N-C macrocylization of the peptide substrate.

Biochemical and Structural Insights into Xylan Utilization by the Thermophilic Bacterium Caldanaerobius polysaccharolyticus

Background: Caldanaerobius polysaccharolyticus is a thermophile with a hemicellulose utilization gene cluster. Results: The cluster is induced by xylan. The ligand-binding cleft of XBP1 is optimized for binding xylotriose. Conclusion: This gene cluster encodes all of the proteins required to degrade xylan, transport the fragments, and metabolize them via the pentose-phosphate pathway. Significance: This gene cluster could be designed as a cassette to impart a capacity for utilizing hemicellulose. Hemicellulose is the next most abundant plant cell wall component after cellulose. The abundance of hemicellulose such as xylan suggests that their hydrolysis and conversion to biofuels can improve the economics of bioenergy production. In an effort to understand xylan hydrolysis at high temperatures, we sequenced the genome of the thermophilic bacterium Caldanaerobius polysaccharolyticus. Analysis of the partial genome sequence revealed a gene cluster that contained both hydrolytic enzymes and also enzymes key to the pentose-phosphate pathway. The hydrolytic enzymes in the gene cluster were demonstrated to convert products from a large endoxylanase (Xyn10A) predicted to anchor to the surface of the bacterium. We further use structural and calorimetric studies to demonstrate that the end products of Xyn10A hydrolysis of xylan are recognized and bound by XBP1, a putative solute-binding protein, likely for transport into the cell. The XBP1 protein showed preference for xylo-oligosaccharides as follows: xylotriose > xylobiose > xylotetraose. To elucidate the structural basis for the oligosaccharide preference, we solved the co-crystal structure of XBP1 complexed with xylotriose to a 1.8-Å resolution. Analysis of the biochemical data in the context of the co-crystal structure reveals the molecular underpinnings of oligosaccharide length specificity.

Biosynthesis of coral settlement cue tetrabromopyrrole in marine bacteria by a uniquely adapted brominase-thioesterase enzyme pair

Significance The majority of pharmaceuticals are inspired by natural product scaffolds that are functionalized by tailoring enzymes, such as halogenases. The degree of halogenation is an important determinant of natural product bioactivity, yet little is known regarding the molecular basis for the exquisite control exhibited by tailoring halogenases. Known pyrrole halogenases commonly perform up to two halogenations on the pyrrole. Our study of tetrabromopyrrole biosynthesis revealed a uniquely adapted halogenase–thioesterase enzyme pair that catalyzes an unprecedented series of halogenations on a pyrrole. Structural comparison of the pyrrole tetrahalogenase to a pyrrole dihalogenase revealed key residues involved in controlling the degree of halogenation. Our findings provide fundamental insights that might be applied in the rational design of biocatalysts toward directed biosynthesis of new chemicals. Halogenated pyrroles (halopyrroles) are common chemical moieties found in bioactive bacterial natural products. The halopyrrole moieties of mono- and dihalopyrrole-containing compounds arise from a conserved mechanism in which a proline-derived pyrrolyl group bound to a carrier protein is first halogenated and then elaborated by peptidic or polyketide extensions. This paradigm is broken during the marine pseudoalteromonad bacterial biosynthesis of the coral larval settlement cue tetrabromopyrrole (1), which arises from the substitution of the proline-derived carboxylate by a bromine atom. To understand the molecular basis for decarboxylative bromination in the biosynthesis of 1, we sequenced two Pseudoalteromonas genomes and identified a conserved four-gene locus encoding the enzymes involved in its complete biosynthesis. Through total in vitro reconstitution of the biosynthesis of 1 using purified enzymes and biochemical interrogation of individual biochemical steps, we show that all four bromine atoms in 1 are installed by the action of a single flavin-dependent halogenase: Bmp2. Tetrabromination of the pyrrole induces a thioesterase-mediated offloading reaction from the carrier protein and activates the biosynthetic intermediate for decarboxylation. Insights into the tetrabrominating activity of Bmp2 were obtained from the high-resolution crystal structure of the halogenase contrasted against structurally homologous halogenase Mpy16 that forms only a dihalogenated pyrrole in marinopyrrole biosynthesis. Structure-guided mutagenesis of the proposed substrate-binding pocket of Bmp2 led to a reduction in the degree of halogenation catalyzed. Our study provides a biogenetic basis for the biosynthesis of 1 and sets a firm foundation for querying the biosynthetic potential for the production of 1 in marine (meta)genomes.

Unusual flavoenzyme catalysis in marine bacteria.

Ever since the discovery of the flavin cofactor more than 80 years ago, flavin-dependent enzymes have emerged as ubiquitous and versatile redox catalysts in primary metabolism. Yet, the recent advances in the discovery and characterization of secondary metabolic pathways exposed new roles for flavin-mediated catalysis in the generation of structurally complex natural products. Here, we review a selection of key biosynthetic flavoenzymes from marine bacterial secondary metabolism and illustrate how their functional and mechanistic investigation expanded our view of the cofactors chemical repertoire and led to the discovery of a previously unknown flavin redox state.

Structural and functional insight into an unexpectedly selective N-methyltransferase involved in plantazolicin biosynthesis

Plantazolicin (PZN), a polyheterocyclic, Nα,Nα-dimethylarginine–containing antibiotic, harbors remarkably specific bactericidal activity toward strains of Bacillus anthracis, the causative agent of anthrax. Previous studies demonstrated that genetic deletion of the S-adenosyl-l-methionine–dependent methyltransferase from the PZN biosynthetic gene cluster results in the formation of desmethylPZN, which is devoid of antibiotic activity. Here we describe the in vitro reconstitution, mutational analysis, and X-ray crystallographic structure of the PZN methyltransferase. Unlike all other known small molecule methyltransferases, which act upon diverse substrates in vitro, the PZN methyltransferase is uncharacteristically limited in substrate scope and functions only on desmethylPZN and close derivatives. The crystal structures of two related PZN methyltransferases, solved to 1.75 Å (Bacillus amyloliquefaciens) and 2.0 Å (Bacillus pumilus), reveal a deep, narrow cavity, putatively functioning as the binding site for desmethylPZN. The narrowness of this cavity provides a framework for understanding the molecular basis of the extreme substrate selectivity. Analysis of a panel of point mutations to the methyltransferase from B. amyloliquefaciens allowed the identification of residues of structural and catalytic importance. These findings further our understanding of one set of orthologous enzymes involved in thiazole/oxazole-modified microcin biosynthesis, a rapidly growing sector of natural products research.