Genome Mining for New Enzyme Chemistry

A revolution in the field of biocatalysis has enabled scalable access to compounds of high societal values using enzymes. The construction of biocatalytic routes relies on the reservoir of available enzymatic transformations. A review of uncharacterized proteins predicted from genomic sequencing projects shows that a treasure trove of enzyme chemistry awaits to be uncovered. This Review highlights enzymatic transformations discovered through various genome mining methods and showcases their potential future applications in biocatalysis.

Intracellular Application of an Asparaginyl Endopeptidase for Producing Recombinant Head-to-Tail Cyclic Proteins

Peptide backbone cyclization is commonly observed in nature and is increasingly applied to proteins and peptides to improve thermal and chemical stability and resistance to proteolytic enzymes and enhance biological activity. However, chemical synthesis of head-to-tail cyclic peptides and proteins is challenging, is often low yielding, and employs toxic and unsustainable reagents. Plant derived asparaginyl endopeptidases such as OaAEP1 have been employed to catalyze the head-to-tail cyclization of peptides in vitro, offering a safer and more sustainable alternative to chemical methods. However, while asparaginyl endopeptidases have been used in vitro and in native and transgenic plant species, they have never been used to generate recombinant cyclic proteins in live recombinant organisms outside of plants. Using dihydrofolate reductase as a proof of concept, we show that a truncated OaAEP1 variant C247A is functional in the Escherichia coli physiological environment and can therefore be coexpressed with a substrate protein to enable concomitant in situ cyclization. The bacterial system is ideal for cyclic protein production owing to the fast growth rate, durability, ease of use, and low cost. This streamlines cyclic protein production via a biocatalytic process with fast kinetics and minimal ligation scarring, while negating the need to purify the enzyme, substrate, and reaction mixtures individually. The resulting cyclic protein was characterized in vitro, demonstrating enhanced thermal stability compared to the corresponding linear protein without impacting enzyme activity. We anticipate this convenient method for generating cyclic peptides will have broad utility in a range of biochemical and chemical applications.

A Promiscuous rSAM Enzyme Enables Diverse Peptide Cross-linking

Ribosomally produced and post-translationally modified polypeptides (RiPPs) are a diverse group of natural products that are processed by a variety of enzymes to their biologically relevant forms. PapB is a member of the radical S-adenosyl-l-methionine (rSAM) superfamily that introduces thioether cross-links between Cys and Asp residues in the PapA RiPP. We report that PapB has high tolerance for variations in the peptide substrate. Our results demonstrate that branched side chains in the thiol- and carboxylate-containing residues are processed and that lengthening of these groups to homocysteine and homoglutamate does not impair the ability of PapB to form thioether cross-links. Remarkably, the enzyme can even cross-link a peptide substrate where the native Asp carboxylate moiety is replaced with a tetrazole. We show that variations to residues embedded between the thiol- and carboxylate-containing residues are tolerated by PapB, as peptides containing both bulky (e.g., Phe) and charged (e.g., Lys) side chains in both natural L- and unnatural D-forms are efficiently cross-linked. Diastereomeric peptides bearing (2S,3R)- and (2S,3S)-methylaspartate are processed by PapB to form cyclic thioethers with markedly different rates, suggesting the enzymatic hydrogen atom abstraction event for the native Asp-containing substrate is diastereospecific. Finally, we synthesized two diastereomeric peptide substrates bearing E- and Z-configured γ,δ-dehydrohomoglutamate and show that PapB promotes addition of the deoxyadenosyl radical (dAdo•) instead of hydrogen atom abstraction. In the Z-configured γ,δ-dehydrohomoglutamate substrate, a fraction of the dAdo-adduct peptide is thioether cross-linked. In both cases, there is evidence for product inhibition of PapB, as the dAdo-adducts likely mimic the native transition state where dAdo• is poised to abstract a substrate hydrogen atom. Collectively, these findings provide critical insights into the arrangement of reacting species in the active site of the PapB, reveal unusual promiscuity, and highlight the potential of PapB as a tool in the development peptide therapeutics.

Engineering lanthipeptides by introducing a large variety of RiPP modifications to obtain new-to-nature bioactive peptides

Natural bioactive peptide discovery is a challenging and time-consuming process. However, advances in synthetic biology are providing promising new avenues in peptide engineering that allow for the design and production of a large variety of new-to-nature peptides with enhanced or new bioactivities, using known peptides as templates. Lanthipeptides are ribosomally synthesized and post-translationally modified peptides (RiPPs). The modularity of posttranslational modification enzymes and ribosomal biosynthesis inherent to lanthipeptides, enable their engineering and screening in a high-throughput manner. The field of RiPPs research is rapidly evolving, with many novel post-translational modifications (PTMs) and their associated modification enzymes being identified and characterized. The modularity presented by these diverse and promiscuous modification enzymes has made them promising tools for further in vivo engineering of lanthipeptides, allowing for the diversification of their structures and activities. In this review, we explore the diverse modifications occurring in RiPPs and discuss the potential applications and feasibility of combining various modification enzymes for lanthipeptide engineering. We highlight the prospect of lanthipeptide- and RiPP engineering to produce and screen novel peptides, including mimics of potent non-ribosomally produced antimicrobial peptides (NRPs) such as daptomycin, vancomycin and teixobactin, which offer high therapeutic potential.

Recent Advances in Discovery, Bioengineering, and Bioactivity-Evaluation of Ribosomally Synthesized and Post-translationally Modified Peptides

Ribosomally synthesized and post-translationally modified peptides (RiPPs) are of increasing interest in natural products as well as drug discovery. This empowers not only the unique chemical structures and topologies in natural products but also the excellent bioactivities such as antibacteria, antifungi, antiviruses, and so on. Advances in genomics, bioinformatics, and chemical analytics have promoted the exponential increase of RiPPs as well as the evaluation of biological activities thereof. Furthermore, benefiting from their relatively simple and conserved biosynthetic logic, RiPPs are prone to be engineered to obtain diverse analogues that exhibit distinct physiological activities and are difficult to synthesize. This Review aims to systematically address the variety of biological activities and/or the mode of mechanisms of novel RiPPs discovered in the past decade, albeit the characteristics of selective structures and biosynthetic mechanisms are briefly covered as well. Almost one-half of the cases are involved in anti-Gram-positive bacteria. Meanwhile, an increasing number of RiPPs related to anti-Gram-negative bacteria, antitumor, antivirus, etc., are also discussed in detail. Last but not least, we sum up some disciplines of the RiPPs' biological activities to guide genome mining as well as drug discovery and optimization in the future.

Enzymatic Pyridine Aromatization during Thiopeptide Biosynthesis

Thiazole-containing pyritides (thiopeptides) are ribosomally synthesized and post-translationally modified peptides (RiPPs) that have attracted interest owing to their potent biological activities and structural complexity. The class-defining feature of a thiopeptide is a six-membered, nitrogenous heterocycle formed by an enzymatic [4 + 2]-cycloaddition. In rare cases, piperidine or dehydropiperidine (DHP) is present; however, the aromatized pyridine is considerably more common. Despite significant effort, the mechanism by which the central pyridine is formed remains poorly understood. Building on our recent observation of the Bycroft-Gowland intermediate (i.e., the direct product of the [4 + 2]-cycloaddition), we interrogated thiopeptide pyridine synthases using a combination of targeted mutagenesis, kinetic assays, substrate analogs, enzyme-substrate cross-linking, and chemical rescue experiments. Collectively, our data delineate roles for several conserved residues in thiopeptide pyridine synthases. A critical tyrosine facilitates the final aromatization step of pyridine formation. This work provides a foundation for further exploration of the [4 + 2]-cycloaddition reaction and future customization of pyridine-containing macrocyclic peptides.

Applying Promiscuous RiPP Enzymes to Peptide Backbone N-Methylation Chemistry

The methylation of peptide backbone amides is a hallmark of bioactive natural products, and it also greatly modifies the pharmacology of synthetic peptides. Usually, bioactive N-methylated peptides are cyclic. However, there is very limited knowledge about how post-translational enzymes can be applied to the synthesis of designed N-methylated peptides or peptide libraries. Here, driven by the established ability of some RiPP enzymes to process diverse substrates, we sought to define catalysts for the in vivo and in vitro macrocyclization of backbone-methylated peptides. We developed efficient methods in which short, synthetic N-methylated peptides could be modified using side chain and mainchain macrocyclases, PsnB and PCY1 from plesiocin and orbitide biosynthetic pathways, respectively. Most significantly, a strategy for PsnB cyclase was designed enabling simple in vitro methods compatible with solid-phase peptide synthesis. We show that cyanobactin N-terminal protease PatA is a broadly useful catalyst that is also compatible with N-methylation chemistry, but that cyanobactin macrocyclase PatG is strongly biased against N-methylated substrates. Finally, we sought to marry these macrocyclase tools with an enzyme that N-methylates its core peptide: OphMA from the omphalotin pathway. However, instead, we reveal some limitations of OphMA and demonstrate that it unexpectedly and extensively modified the enzyme itself in vivo. Together, these results demonstrate proof-of-concept for enzymatic synthesis of N-methylated peptide macrocycles.

Antimicrobial Peptides Produced by Selective Pressure Incorporation of Non-canonical Amino Acids

Nature has a variety of possibilities to create new protein functions by modifying the sequence of the individual amino acid building blocks. However, all variations are based on the 20 canonical amino acids (cAAs). As a way to introduce additional physicochemical properties into polypeptides, the incorporation of non-canonical amino acids (ncAAs) is increasingly used in protein engineering. Due to their relatively short length, the modification of ribosomally synthesized and post-translationally modified peptides by ncAAs is particularly attractive. New functionalities and chemical handles can be generated by specific modifications of individual residues. The selective pressure incorporation (SPI) method utilizes auxotrophic host strains that are deprived of an essential amino acid in chemically defined growth media. Several structurally and chemically similar amino acid analogs can then be activated by the corresponding aminoacyl-tRNA synthetase and provide residue-specific cAA(s) → ncAA(s) substitutions in the target peptide or protein sequence. Although, in the context of the SPI method, ncAAs are also incorporated into the host proteome during the phase of recombinant gene expression, the majority of the cell's resources are assigned to the expression of the target gene. This enables efficient residue-specific incorporation of ncAAs often accompanied with high amounts of modified target. The presented work describes the in vivo incorporation of six proline analogs into the antimicrobial peptide nisin, a lantibiotic naturally produced by Lactococcus lactis. Antimicrobial properties of nisin can be changed and further expanded during its fermentation and expression in auxotrophic Escherichia coli strains in defined growth media. Thereby, the effects of residue-specific replacement of cAAs with ncAAs can deliver changes in antimicrobial activity and specificity. Antimicrobial activity assays and fluorescence microscopy are used to test the new nisin variants for growth inhibition of a Gram-positive Lactococcus lactis indicator strain. Mass spectroscopy is used to confirm ncAA incorporation in bioactive nisin variants.

Structural Insights into Thioether Bond Formation in the Biosynthesis of Sactipeptides

Sactipeptides are ribosomally synthesized peptides that contain a characteristic thioether bridge (sactionine bond) that is installed posttranslationally and is absolutely required for their antibiotic activity. Sactipeptide biosynthesis requires a unique family of radical SAM enzymes, which contain multiple [4Fe-4S] clusters, to form the requisite thioether bridge between a cysteine and the α-carbon of an opposing amino acid through radical-based chemistry. Here we present the structure of the sactionine bond-forming enzyme CteB, from Clostridium thermocellum ATCC 27405, with both SAM and an N-terminal fragment of its peptidyl-substrate at 2.04 Å resolution. CteB has the (β/α)6-TIM barrel fold that is characteristic of radical SAM enzymes, as well as a C-terminal SPASM domain that contains two auxiliary [4Fe-4S] clusters. Importantly, one [4Fe-4S] cluster in the SPASM domain exhibits an open coordination site in absence of peptide substrate, which is coordinated by a peptidyl-cysteine residue in the bound state. The crystal structure of CteB also reveals an accessory N-terminal domain that has high structural similarity to a recently discovered motif present in several enzymes that act on ribosomally synthesized and post-translationally modified peptides (RiPPs), known as a RiPP precursor peptide recognition element (RRE). This crystal structure is the first of a sactionine bond forming enzyme and sheds light on structures and mechanisms of other members of this class such as AlbA or ThnB.