Expanding chemistry through in vitro and in vivo biocatalysis.

Living systems contain a vast network of metabolic reactions, providing a wealth of enzymes and cells as potential biocatalysts for chemical processes. The properties of protein and cell biocatalysts-high selectivity, the ability to control reaction sequence and operation in environmentally benign conditions-offer approaches to produce molecules at high efficiency while lowering the cost and environmental impact of industrial chemistry. Furthermore, biocatalysis offers the opportunity to generate chemical structures and functions that may be inaccessible to chemical synthesis. Here we consider developments in enzymes, biosynthetic pathways and cellular engineering that enable their use in catalysis for new chemistry and beyond.

Biocatalytic control of site-selectivity and chain length-selectivity in radical amino acid halogenases.

Biocatalytic C-H activation has the potential to merge enzymatic and synthetic strategies for bond formation. FeII/αKG-dependent halogenases are particularly distinguished for their ability both to control selective C-H activation as well as to direct group transfer of a bound anion along a reaction axis separate from oxygen rebound, enabling the development of new transformations. In this context, we elucidate the basis for the selectivity of enzymes that perform selective halogenation to yield 4-Cl-lysine (BesD), 5-Cl-lysine (HalB), and 4-Cl-ornithine (HalD), allowing us to probe how site-selectivity and chain length selectivity are achieved. We now report the crystal structure of the HalB and HalD, revealing the key role of the substrate-binding lid in positioning the substrate for C4 vs C5 chlorination and recognition of lysine vs ornithine. Targeted engineering of the substrate-binding lid further demonstrates that these selectivities can be altered or switched, showcasing the potential to develop halogenases for biocatalytic applications.

Engineering nonphotosynthetic carbon fixation for production of bioplastics by methanogenic archaea.

The conversion of CO2 to value-added products allows both capture and recycling of greenhouse gas emissions. While plants and other photosynthetic organisms play a key role in closing the global carbon cycle, their dependence on light to drive carbon fixation can be limiting for industrial chemical synthesis. Methanogenic archaea provide an alternative platform as an autotrophic microbial species capable of non-photosynthetic CO2 fixation, providing a potential route to engineered microbial fermentation to synthesize chemicals from CO2 without the need for light irradiation. One major challenge in this goal is to connect upstream carbon-fixation pathways with downstream biosynthetic pathways, given the distinct differences in metabolism between archaea and typical heterotrophs. We engineered the model methanogen, Methanococcus maripaludis, to divert acetyl-coenzyme A toward biosynthesis of value-added chemicals, including the bioplastic polyhydroxybutyrate (PHB). A number of studies implicated limitations in the redox pool, with NAD(P)(H) pools in M. maripaludis measured to be <15% of that of Escherichia coli, likely since methanogenic archaea utilize F420 and ferredoxins instead. Multiple engineering strategies were used to precisely target and increase the cofactor pool, including heterologous expression of a synthetic nicotinamide salvage pathway as well as an NAD+-dependent formate dehydrogenase from Candida boidinii. Engineered strains of M. maripaludis with improved NADH pools produced up to 171 ± 4 mg/L PHB and 24.0 ± 1.9% of dry cell weight. The metabolic engineering strategies presented in this study broaden the utility of M. maripaludis for sustainable chemical synthesis using CO2 and may be transferable to related archaeal species.

Reaction pathway engineering converts a radical hydroxylase into a halogenase.

FeII/α-ketoglutarate (FeII/αKG)-dependent enzymes offer a promising biocatalytic platform for halogenation chemistry owing to their ability to functionalize unactivated C-H bonds. However, relatively few radical halogenases have been identified to date, limiting their synthetic utility. Here, we report a strategy to expand the palette of enzymatic halogenation by engineering a reaction pathway rather than substrate selectivity. This approach could allow us to tap the broader class of FeII/αKG-dependent hydroxylases as catalysts by their conversion to halogenases. Toward this goal, we discovered active halogenases from a DNA shuffle library generated from a halogenase-hydroxylase pair using a high-throughput in vivo fluorescent screen coupled to an alkyne-producing biosynthetic pathway. Insights from sequencing halogenation-active variants along with the crystal structure of the hydroxylase enabled engineering of a hydroxylase to perform halogenation with comparable activity and higher selectivity than the wild-type halogenase, showcasing the potential of harnessing hydroxylases for biocatalytic halogenation.

Engineering site-selective incorporation of fluorine into polyketides.

Although natural products and synthetic small molecules both serve important medicinal functions, their structures and chemical properties are relatively distinct. To expand the molecular diversity available for drug discovery, one strategy is to blend the effective attributes of synthetic and natural molecules. A key feature found in synthetic compounds that is rare in nature is the use of fluorine to tune drug behavior. We now report a method to site-selectively incorporate fluorine into complex structures to produce regioselectively fluorinated full-length polyketides. We engineered a fluorine-selective trans-acyltransferase to produce site-selectively fluorinated erythromycin precursors in vitro. We further demonstrated that these analogs could be produced in vivo in Escherichia coli on engineering of the fluorinated extender unit pool. By using engineered microbes, elaborate fluorinated compounds can be produced by fermentation, offering the potential for expanding the identification and development of bioactive fluorinated small molecules.

Biosynthesis of Strained Amino Acids by a PLP-Dependent Enzyme through Cryptic Halogenation.

Amino acids (AAs) are modular building blocks which nature uses to synthesize both macromolecules, such as proteins, and small molecule natural products, such as alkaloids and non-ribosomal peptides. While the 20 main proteinogenic AAs display relatively limited side chain diversity, a wide range of non-canonical amino acids (ncAAs) exist that are not used by the ribosome for protein synthesis, but contain a broad array of structural features and functional groups. In this communication, we report the discovery of the biosynthetic pathway for a new ncAA, pazamine, which contains a cyclopropane ring formed in two steps. In the first step, a chlorine is added onto the C4 position of lysine by a radical halogenase, PazA. The cyclopropane ring is then formed in the next step by a pyridoxal-5'-phosphate-dependent enzyme, PazB, via an SN2-like attack at C4 to eliminate chloride. Genetic studies of this pathway in the native host, Pseudomonas azotoformans, show that pazamine potentially inhibits ethylene biosynthesis in growing plants based on alterations in the root phenotype of Arabidopsis thaliana seedlings. We further show that PazB can be utilized to make an alternative cyclobutane-containing AA. These discoveries may lead to advances in biocatalytic production of specialty chemicals and agricultural biotechnology.

Synergistic Binding of the Halide and Cationic Prime Substrate of l-Lysine 4-Chlorinase, BesD, in Both Ferrous and Ferryl States.

An aliphatic halogenase requires four substrates: 2-oxoglutarate (2OG), halide (Cl- or Br-), the halogenation target ("prime substrate"), and dioxygen. In well-studied cases, the three nongaseous substrates must bind to activate the enzyme's Fe(II) cofactor for efficient capture of O2. Halide, 2OG, and (lastly) O2 all coordinate directly to the cofactor to initiate its conversion to a cis-halo-oxo-iron(IV) (haloferryl) complex, which abstracts hydrogen (H•) from the non-coordinating prime substrate to enable radicaloid carbon-halogen coupling. We dissected the kinetic pathway and thermodynamic linkage in binding of the first three substrates of the l-lysine 4-chlorinase, BesD. After addition of 2OG, subsequent coordination of the halide to the cofactor and binding of cationic l-Lys near the cofactor are associated with strong heterotropic cooperativity. Progression to the haloferryl intermediate upon the addition of O2 does not trap the substrates in the active site and, in fact, markedly diminishes cooperativity between halide and l-Lys. The surprising lability of the BesD•[Fe(IV)=O]•Cl•succinate•l-Lys complex engenders pathways for decay of the haloferryl intermediate that do not result in l-Lys chlorination, especially at low chloride concentrations; one identified pathway involves oxidation of glycerol. The mechanistic data imply (i) that BesD may have evolved from a hydroxylase ancestor either relatively recently or under weak selective pressure for efficient chlorination and (ii) that acquisition of its activity may have involved the emergence of linkage between l-Lys binding and chloride coordination following the loss of the anionic protein-carboxylate iron ligand present in extant hydroxylases.

Substrate-Triggered µ-Peroxodiiron(III) Intermediate in the 4-Chloro-l-Lysine-Fragmenting Heme-Oxygenase-like Diiron Oxidase (HDO) BesC: Substrate Dissociation from, and C4 Targeting by, the Intermediate.

The enzyme BesC from the ß-ethynyl-l-serine biosynthetic pathway in Streptomyces cattleya fragments 4-chloro-l-lysine (produced from l-Lysine by BesD) to ammonia, formaldehyde, and 4-chloro-l-allylglycine and can analogously fragment l-Lys itself. BesC belongs to the emerging family of O2-activating non-heme-diiron enzymes with the "heme-oxygenase-like" protein fold (HDOs). Here, we show that the binding of l-Lys or an analogue triggers capture of O2 by the protein's diiron(II) cofactor to form a blue µ-peroxodiiron(III) intermediate analogous to those previously characterized in two other HDOs, the olefin-installing fatty acid decarboxylase, UndA, and the guanidino-N-oxygenase domain of SznF. The ∼5- and ∼30-fold faster decay of the intermediate in reactions with 4-thia-l-Lys and (4RS)-chloro-dl-lysine than in the reaction with l-Lys itself and the primary deuterium kinetic isotope effects (D-KIEs) on decay of the intermediate and production of l-allylglycine in the reaction with 4,4,5,5-[2H4]-l-Lys suggest that the peroxide intermediate or a reversibly connected successor complex abstracts a hydrogen atom from C4 to enable olefin formation. Surprisingly, the sluggish substrate l-Lys can dissociate after triggering intermediate formation, thereby allowing one of the better substrates to bind and react. The structure of apo BesC and the demonstrated linkage between Fe(II) and substrate binding suggest that the triggering event involves an induced ordering of ligand-providing helix 3 (α3) of the conditionally stable HDO core. As previously suggested for SznF, the dynamic α3 also likely initiates the spontaneous degradation of the diiron(III) product cluster after decay of the peroxide intermediate, a trait emerging as characteristic of the nascent HDO family.

Biocatalytic Asymmetric Construction of Secondary and Tertiary Fluorides from ß-Fluoro-α-Ketoacids.

Fluorine is a critical element for the design of bioactive compounds, driving advances in selective and sustainable fluorination. However, stereogenic tertiary fluorides pose a synthetic challenge and are thus present in only a few approved drugs (fluticasone, solithromycin, and sofosbuvir). The aldol reaction of fluorinated donors provides an atom-economical approach to asymmetric C-F motifs via C-C bond formation. We report that the type II pyruvate aldolase HpcH and engineered variants perform addition of ß-fluoro-α-ketoacids (including fluoropyruvate, ß-fluoro-α-ketobutyrate, and ß-fluoro-α-ketovalerate) to diverse aldehydes. The reactivity of HpcH towards these fluoro-donors grants access to enantiopure secondary or tertiary fluorides. In addition to representing the first synthesis of tertiary fluorides via biocatalytic carboligation, the afforded products could improve the diversity of fluorinated building blocks and enable the synthesis of fluorinated drug analogs.

A family of radical halogenases for the engineering of amino-acid-based products.

The integration of synthetic and biological catalysis enables new approaches to the synthesis of small molecules by combining the high selectivity of enzymes with the reaction diversity offered by synthetic chemistry. While organohalogens are valued for their bioactivity and utility as synthetic building blocks, only a handful of enzymes that carry out the regioselective halogenation of unactivated [Formula: see text] bonds have previously been identified. In this context, we report the structural characterization of BesD, a recently discovered radical halogenase from the FeII/α-ketogluturate-dependent family that chlorinates the free amino acid lysine. We also identify and characterize additional halogenases that produce mono- and dichlorinated, as well as brominated and azidated, amino acids. The substrate selectivity of this new family of radical halogenases takes advantage of the central role of amino acids in metabolism and enables engineering of biosynthetic pathways to afford a wide variety of compound classes, including heterocycles, diamines, α-keto acids and peptides.

Entropy drives selective fluorine recognition in the fluoroacetyl-CoA thioesterase from Streptomyces cattleya.

Fluorinated small molecules play an important role in the design of bioactive compounds for a broad range of applications. As such, there is strong interest in developing a deeper understanding of how fluorine affects the interaction of these ligands with their targets. Given the small number of fluorinated metabolites identified to date, insights into fluorine recognition have been provided almost entirely by synthetic systems. The fluoroacetyl-CoA thioesterase (FlK) from Streptomyces cattleya thus provides a unique opportunity to study an enzyme-ligand pair that has been evolutionarily optimized for a surprisingly high 106 selectivity for a single fluorine substituent. In these studies, we synthesize a series of analogs of fluoroacetyl-CoA and acetyl-CoA to generate nonhydrolyzable ester, amide, and ketone congeners of the thioester substrate to isolate the role of fluorine molecular recognition in FlK selectivity. Using a combination of thermodynamic, kinetic, and protein NMR experiments, we show that fluorine recognition is entropically driven by the interaction of the fluorine substituent with a key residue, Phe-36, on the lid structure that covers the active site, resulting in an ∼5- to 20-fold difference in binding (KD). Although the magnitude of discrimination is similar to that found in designed synthetic ligand-protein complexes where dipolar interactions control fluorine recognition, these studies show that hydrophobic and solvation effects serve as the major determinant of naturally evolved fluorine selectivity.

Fluorothreonyl-tRNA deacylase prevents mistranslation in the organofluorine producer Streptomyces cattleya.

Fluorine is an element with unusual properties that has found significant utility in the design of synthetic small molecules, ranging from therapeutics to materials. In contrast, only a few fluorinated compounds made by living organisms have been found to date, most of which derive from the fluoroacetate/fluorothreonine biosynthetic pathway first discovered in Streptomyces cattleya While fluoroacetate has long been known to act as an inhibitor of the tricarboxylic acid cycle, the fate of the amino acid fluorothreonine is still not well understood. Here, we show that fluorothreonine can be misincorporated into protein in place of the proteinogenic amino acid threonine. We have identified two conserved proteins from the organofluorine biosynthetic locus, FthB and FthC, that are involved in managing fluorothreonine toxicity. Using a combination of biochemical, genetic, physiological, and proteomic studies, we show that FthB is a trans-acting transfer RNA (tRNA) editing protein, which hydrolyzes fluorothreonyl-tRNA 670-fold more efficiently than threonyl-RNA, and assign a role to FthC in fluorothreonine transport. While trans-acting tRNA editing proteins have been found to counteract the misacylation of tRNA with commonly occurring near-cognate amino acids, their role has yet to be described in the context of secondary metabolism. In this regard, the recruitment of tRNA editing proteins to biosynthetic clusters may have enabled the evolution of pathways to produce specialized amino acids, thereby increasing the diversity of natural product structure while also attenuating the risk of mistranslation that would ensue.

Elucidating the mechanism of fluorinated extender unit loading for improved production of fluorine-containing polyketides.

Polyketides are a large family of bioactive natural products synthesized by polyketide synthase (PKS) enzyme complexes predominantly from acetate and propionate. Given the structural diversity of compounds produced using these two simple building blocks, there has been longstanding interest in engineering the incorporation of alternative extender units. We have been investigating the mechanism of fluorinated monomer insertion by three of the six different modules of the PKS involved in erythromycin biosynthesis (6-deoxyerythronolide B synthase, DEBS) to begin understanding the contribution of different steps, such as enzyme acylation, transacylation, C-C bond formation, and chain transfer, to the overall selectivity and efficiency of this process. In these studies, we observe that inactivation of a cis-acyltransferase (AT) domain to circumvent its native extender unit preference leads concurrently to a change of mechanism in which chain extension with fluorine-substituted extender units switches largely to an acyl carrier protein (ACP)-independent mode. This result suggests that the covalent linkage between the growing polyketide chain and the enzyme is lost in these cases, which would limit efficient chain elongation after insertion of a fluorinated monomer. However, use of a standalone trans-acting AT to complement modules with catalytically deficient AT domains leads to enzyme acylation with the fluoromalonyl-CoA extender unit. Formation of the canonical ACP-linked intermediate with fluoromalonyl-CoA allows insertion of fluorinated extender units at 43% of the yield of the wild-type system while also amplifying product yield in single chain-extension experiments and enabling multiple chain extensions to form multiply fluorinated products.

Engineering in Vivo Production of α-Branched Polyesters.

Polymers are an important class of materials that are used for a broad range of applications, from drug delivery to packaging. Given their widespread use, a major challenge in this area is the development of technology for their production from renewable sources and efforts to promote their efficient recycling and biodegradation. In this regard, the synthesis of polyesters based on the natural polyhydroxyalkanoate (PHA) pathway offers an attractive route for producing sustainable polymers. However, monomer diversity in naturally occurring polyesters can be limited with respect to the design of polymers with material properties suitable for various applications. In this work, we have engineered a pathway to produce α-methyl-branched PHA. In the course of this work, we have also identified a PHA polymerase (CapPhaEC) from activated sludge from wastewater treatment that demonstrates a higher capacity for incorporation of α-branched monomer units than those previously identified or engineered. Production in Escherichia coli allows the construction of microbial strains that produce the copolyesters with 21-36% branched monomers using glucose and propionate as carbon sources. These polymers have typical weight-average molar masses (Mw) in the range (1.7-2.0) × 105 g mol-1 and display no observable melting transition, only relatively low glass transition temperatures from -13 to -20 °C. The lack of a melting transition indicates that these polymers are amorphous materials with no crystallinity, which is in contrast to the natural poly(hydroxybutyrate) homopolymer. Our results expand the utility of PHA-based pathways and provide biosynthetic access to α-branched polyesters to enrich the properties of bio-based sustainable polymers.

MamO Is a Repurposed Serine Protease that Promotes Magnetite Biomineralization through Direct Transition Metal Binding in Magnetotactic Bacteria.

Many living organisms transform inorganic atoms into highly ordered crystalline materials. An elegant example of such biomineralization processes is the production of nano-scale magnetic crystals in magnetotactic bacteria. Previous studies implicated the involvement of two putative serine proteases, MamE and MamO, during the early stages of magnetite formation in Magnetospirillum magneticum AMB-1. Here, using genetic analysis and X-ray crystallography, we show that MamO has a degenerate active site, rendering it incapable of protease activity. Instead, MamO promotes magnetosome formation through two genetically distinct, noncatalytic activities: activation of MamE-dependent proteolysis of biomineralization factors and direct binding to transition metal ions. By solving the structure of the protease domain bound to a metal ion, we identify a surface-exposed di-histidine motif in MamO that contributes to metal binding and show that it is required to initiate biomineralization in vivo. Finally, we find that pseudoproteases are widespread in magnetotactic bacteria and that they have evolved independently in three separate taxa. Our results highlight the versatility of protein scaffolds in accommodating new biochemical activities and provide unprecedented insight into the earliest stages of biomineralization.

Structural insight into magnetochrome-mediated magnetite biomineralization.

Magnetotactic bacteria align along the Earth's magnetic field using an organelle called the magnetosome, a biomineralized magnetite (Fe(II)Fe(III)2O4) or greigite (Fe(II)Fe(III)2S4) crystal embedded in a lipid vesicle. Although the need for both iron(II) and iron(III) is clear, little is known about the biological mechanisms controlling their ratio. Here we present the structure of the magnetosome-associated protein MamP and find that it is built on a unique arrangement of a self-plugged PDZ domain fused to two magnetochrome domains, defining a new class of c-type cytochrome exclusively found in magnetotactic bacteria. Mutational analysis, enzyme kinetics, co-crystallization with iron(II) and an in vitro MamP-assisted magnetite production assay establish MamP as an iron oxidase that contributes to the formation of iron(III) ferrihydrite eventually required for magnetite crystal growth in vivo. These results demonstrate the molecular mechanisms of iron management taking place inside the magnetosome and highlight the role of magnetochrome in iron biomineralization.

Chemoenzymatic Platform for Synthesis of Chiral Organofluorines Based on Type†II Aldolases.

Aldolases are C-C bond forming enzymes that have become prominent tools for sustainable synthesis of complex synthons. However, enzymatic methods of fluorine incorporation into such compounds are lacking due to the rarity of fluorine in nature. Recently, the use of fluoropyruvate as a non-native aldolase substrate has arisen as a solution. Here, we report that the type II HpcH aldolases efficiently catalyze fluoropyruvate addition to diverse aldehydes, with exclusive (3S)-selectivity at fluorine that is rationalized by DFT calculations on a mechanistic model. We also measure the kinetic parameters of aldol addition and demonstrate engineering of the hydroxyl group stereoselectivity. Our aldolase collection is then employed in the chemoenzymatic synthesis of novel fluoroacids and ester derivatives in high stereopurity (d.r. 80-98 %). The compounds made available by this method serve as precursors to fluorinated analogs of sugars, amino acids, and other valuable chiral building blocks.

Structural and Biochemical Studies of Substrate Selectivity in Ascaris suum Thiolases.

Thiolases are a class of carbon-carbon bond forming enzymes with important applications in biotechnology and metabolic engineering as they provide a general method for the condensation of two acyl coenzyme A (CoA) substrates. As such, developing a greater understanding of their substrate selectivity would expand our ability to engineer the enzymatic or microbial production of a broad range of small-molecule targets. Here, we report the crystal structures and biochemical characterization of Acat2 and Acat5, two biosynthetic thiolases from Ascaris suum with varying selectivity toward branched compared to linear compounds. The structure of the Acat2-C91S mutant bound to propionyl-CoA shows that the terminal methyl group of the substrate, representing the α-branch point, is directed toward the conserved Phe 288 and Met 158 residues. In Acat5, the Phe ring is rotated to accommodate a hydroxyl-π interaction with an adjacent Thr side chain, decreasing space in the binding pocket and possibly accounting for its strong preference for linear substrates compared to Acat2. Comparison of the different Acat thiolase structures shows that Met 158 is flexible, adopting alternate conformations with the side chain rotated toward or away from a covering loop at the back of the active site. Mutagenesis of residues in the covering loop in Acat5 with the corresponding residues from Acat2 allows for highly increased accommodation of branched substrates, whereas the converse mutations do not significantly affect Acat2 substrate selectivity. Our results suggest an important contribution of second-shell residues to thiolase substrate selectivity and offer insights into engineering this enzyme class.

Hybrid bioinorganic approach to solar-to-chemical conversion.

Natural photosynthesis harnesses solar energy to convert CO2 and water to value-added chemical products for sustaining life. We present a hybrid bioinorganic approach to solar-to-chemical conversion in which sustainable electrical and/or solar input drives production of hydrogen from water splitting using biocompatible inorganic catalysts. The hydrogen is then used by living cells as a source of reducing equivalents for conversion of CO2 to the value-added chemical product methane. Using platinum or an earth-abundant substitute, α-NiS, as biocompatible hydrogen evolution reaction (HER) electrocatalysts and Methanosarcina barkeri as a biocatalyst for CO2 fixation, we demonstrate robust and efficient electrochemical CO2 to CH4 conversion at up to 86% overall Faradaic efficiency for ≥ 7 d. Introduction of indium phosphide photocathodes and titanium dioxide photoanodes affords a fully solar-driven system for methane generation from water and CO2, establishing that compatible inorganic and biological components can synergistically couple light-harvesting and catalytic functions for solar-to-chemical conversion.