Programming interchangeable and reversible heterooligomeric protein self-assembly using a bifunctional ligand.

Protein design for self-assembly allows us to explore the emergence of protein-protein interfaces through various chemical interactions. Heterooligomers, unlike homooligomers, inherently offer a comprehensive range of structural and functional variations. Besides, the macromolecular repertoire and their applications would significantly expand if protein components could be easily interchangeable. This study demonstrates that a rationally designed bifunctional linker containing an enzyme inhibitor and maleimide can guide the formation of diverse protein heterooligomers in an easily applicable and exchangeable manner without extensive sequence optimizations. As proof of concept, we selected four structurally and functionally unrelated proteins, carbonic anhydrase, aldolase, acetyltransferase, and encapsulin, as building block proteins. The combinations of two proteins with the bifunctional linker yielded four two-component heterooligomers with discrete sizes, shapes, and enzyme activities. Besides, the overall size and formation kinetics of the heterooligomers alter upon adding metal chelators, acidic buffer components, and reducing agents, showing the reversibility and tunability in the protein self-assembly. Given that the functional groups of both the linker and protein components are readily interchangeable, our work broadens the scope of protein-assembled architectures and their potential applications as functional biomaterials.

Photocatalytic C-O Coupling Enzymes That Operate via Intramolecular Electron Transfer.

Efficient and environmentally friendly conversion of light energy for direct utilization in chemical production has been a long-standing goal in enzyme design. Herein, we synthesized artificial photocatalytic enzymes by introducing an Ir photocatalyst and a Ni(bpy) complex to an optimal protein scaffold in close proximity. Consequently, the enzyme generated C-O coupling products with up to 96% yields by harvesting visible light and performing intramolecular electron transfer between the two catalysts. We systematically modulated the catalytic activities of the artificial photocatalytic cross-coupling enzymes by tuning the electrochemical properties of the catalytic components, their positions, and distances within a protein. As a result, we discovered the best-performing mutant that showed broad substrate scopes under optimized conditions. This work explicitly demonstrated that we could integrate and control both the inorganic and biochemical components of photocatalytic biocatalysis to achieve high yield and selectivity in valuable chemical transformations.

Underlying Role of Hydrophobic Environments in Tuning Metal Elements for Efficient Enzyme Catalysis.

The catalytic functions of metalloenzymes are often strongly correlated with metal elements in the active sites. However, dioxygen-activating nonheme quercetin dioxygenases (QueD) are found with various first-row transition-metal ions when metal swapping inactivates their innate catalytic activity. To unveil the molecular basis of this seemingly promiscuous yet metal-specific enzyme, we transformed manganese-dependent QueD into a nickel-dependent enzyme by sequence- and structure-based directed evolution. Although the net effect of acquired mutations was primarily to rearrange hydrophobic residues in the active site pocket, biochemical, kinetic, X-ray crystallographic, spectroscopic, and computational studies suggest that these modifications in the secondary coordination spheres can adjust the electronic structure of the enzyme-substrate complex to counteract the effects induced by the metal substitution. These results explicitly demonstrate that such noncovalent interactions encrypt metal specificity in a finely modulated manner, revealing the underestimated chemical power of the hydrophobic sequence network in enzyme catalysis.

Discovery and Characterization of Polymyxin-Resistance Genes pmrE and pmrF from Sediment and Seawater Microbiome.

Polymyxins are the last-line antibiotics used to treat Gram-negative pathogens. Thus, the discovery and biochemical characterization of the resistance genes against polymyxins are urgently needed for diagnosis, treatment, and novel antibiotic design. Herein, we report novel polymyxin-resistance genes identified from sediment and seawater microbiome. Despite their low sequence identity against the known pmrE and pmrF, they show in vitro activities in UDP-glucose oxidation and l-Ara4N transfer to undecaprenyl phosphate, respectively, which occur as the part of lipid A modification that leads to polymyxin resistance. The expression of pmrE and pmrF also showed substantially high MICs in the presence of vanadate ions, indicating that they constitute polymyxin resistomes. IMPORTANCE Polymyxins are one of the last-resort antibiotics. Polymyxin resistance is a severe threat to combat multidrug-resistant pathogens. Thus, up-to-date identification and understanding of the related genes are crucial. Herein, we performed structure-guided sequence and activity analysis of five putative polymyxin-resistant metagenomes. Despite relatively low sequence identity to the previously reported polymyxin-resistance genes, at least four out of five discovered genes show reactivity essential for lipid A modification and polymyxin resistance, constituting antibiotic resistomes.

Design and directed evolution of noncanonical ß-stereoselective metalloglycosidases.

Metallohydrolases are ubiquitous in nearly all subclasses of hydrolases, utilizing metal elements to activate a water molecule and facilitate its subsequent dissociation of diverse chemical bonds. However, such a catalytic role of metal ions is rarely found with glycosidases that hydrolyze the glycosidic bonds in sugars. Herein, we design metalloglycosidases by constructing a hydrolytically active Zn-binding site within a barrel-shaped outer membrane protein OmpF. Structure- and mechanism-based redesign and directed evolution have led to the emergence of Zn-dependent glycosidases with catalytic proficiency of 2.8 × 109 and high ß-stereoselectivity. Biochemical characterizations suggest that the Zn-binding site constitutes a key catalytic motif along with at least one adjacent acidic residue. This work demonstrates that unprecedented metalloenzymes can be tailor-made, expanding the scope of inorganic reactivities in proteinaceous environments, resetting the structural and functional diversity of metalloenzymes, and providing the potential molecular basis of unidentified metallohydrolases and novel whole-cell biocatalysts.

Symmetry-Adapted Synthesis of Dicopper Oxidases with Divergent Dioxygen Reactivity.

Artificial metalloenzymes have fed our understanding of how inorganic reactivities emerge, evolve, and diversify in protein environments. Herein, we created dinuclear copper oxidases by genetically encoding a metal-ligating unnatural amino acid (bpy-Ala) per protomer in the vicinity of the innate C2 rotational axis of a homo-oligomeric protein. The inherent protein symmetry allows the precise multiplication and placement of two Cu(bpy) species. Depending on the location of bpy-Ala, the tailor-made metalloenzymes exhibited electronically uncoupled or coupled dicopper sites. Consequently, they displayed various reactivities with dioxygen associated with multiple protons and electrons, illustrating a diverse chemical repertoire of artificial copper-dependent enzymes.

Molecular mechanism underlying substrate recognition of the peptide macrocyclase PsnB.

Graspetides, also known as ω-ester-containing peptides (OEPs), are a family of ribosomally synthesized and post-translationally modified peptides (RiPPs) bearing side chain-to-side chain macrolactone or macrolactam linkages. Here, we present the molecular details of precursor peptide recognition by the macrocyclase enzyme PsnB in the biosynthesis of plesiocin, a group 2 graspetide. Biochemical analysis revealed that, in contrast to other RiPPs, the core region of the plesiocin precursor peptide noticeably enhanced the enzyme-precursor interaction via the conserved glutamate residues. We obtained four crystal structures of symmetric or asymmetric PsnB dimers, including those with a bound core peptide and a nucleotide, and suggest that the highly conserved Arg213 at the enzyme active site specifically recognizes a ring-forming acidic residue before phosphorylation. Collectively, this study provides insights into the mechanism underlying substrate recognition in graspetide biosynthesis and lays a foundation for engineering new variants.

Design of artificial metalloenzymes with multiple inorganic elements: The more the merrier.

A large fraction of metalloenzymes harbors multiple metal-centers that are electronically and/or functionally arranged within their proteinaceous environments. To explore the orchestration of inorganic and biochemical components and to develop bioinorganic catalysts and materials, we have described selected examples of artificial metalloproteins having multiple metallocofactors that were grouped depending on their initial protein scaffolds, the nature of introduced inorganic moieties, and the method used to propagate the number of metal ions within a protein. They demonstrated that diverse inorganic moieties can be selectively grafted and modulated in protein environments, providing a retrosynthetic bottom-up approach in the design of versatile and proficient biocatalysts and biomimetic model systems to explore fundamental questions in bioinorganic chemistry.

Folding of Circularly Permuted and Split Outer Membrane Protein F via Electrostatic Interactions with Terminal Residues.

Membrane proteins are essential targets in drug design, biosensing, and catalysis. In this study, we explored the folding of engineered outer membrane protein F (OmpF), an abundant and functional ß-barrel protein expressed in Gram-negative bacteria. We carried out circular permutation, splitting and self-complementation, and point mutation. The folding efficiency and kinetic analyses demonstrated that the N- and C-terminal residues of OmpF played critical roles in folding via electrostatic interactions with lipid headgroups. Our results indicate that native porins without charged terminal residues may be tightly downregulated to retain the integrity of the outer membrane, and this modification may facilitate the insertion and folding of modified membrane proteins under in vitro and in vivo conditions for various applications.

Symmetry-related residues as promising hotspots for the evolution of de novo oligomeric enzymes.

Directed evolution has provided us with great opportunities and prospects in the synthesis of tailor-made proteins. It, however, often requires at least mid to high throughput screening, necessitating more effective strategies for laboratory evolution. We herein demonstrate that protein symmetry can be a versatile criterion for searching for promising hotspots for the directed evolution of de novo oligomeric enzymes. The randomization of symmetry-related residues located at the rotational axes of artificial metallo-ß-lactamase yields drastic effects on catalytic activities, whereas that of non-symmetry-related, yet, proximal residues to the active site results in negligible perturbations. Structural and biochemical analysis of the positive hits indicates that seemingly trivial mutations at symmetry-related spots yield significant alterations in overall structures, metal-coordination geometry, and chemical environments of active sites. Our work implicates that numerous artificially designed and natural oligomeric proteins might have evolutionary advantages of propagating beneficial mutations using their global symmetry.

Genomic Determinants Encode the Reactivity and Regioselectivity of Flavin-Dependent Halogenases in Bacterial Genomes and Metagenomes.

Halogenases create diverse natural products by utilizing halide ions and are of great interest in the synthesis of potential pharmaceuticals and agrochemicals. An increasing number of halogenases discovered in microorganisms are annotated as flavin-dependent halogenases (FDHs), but their chemical reactivities are markedly different and the genomic contents associated with such functional distinction have not been revealed yet. Even though the reactivity and regioselectivity of FDHs are essential in the halogenation activity, these FDHs are annotated inaccurately in the protein sequence repositories without characterizing their functional activities. We carried out a comprehensive sequence analysis and biochemical characterization of FDHs. Using a probabilistic model that we built in this study, FDHs were discovered from 2,787 bacterial genomes and 17 sediment metagenomes. We analyzed the essential genomic determinants that are responsible for substrate binding and subsequent reactions: four flavin adenine dinucleotide-binding, one halide-binding, and four tryptophan-binding sites. Compared with previous studies, our study utilizes large-scale genomic information to propose a comprehensive set of sequence motifs that are related to the active sites and regioselectivity. We reveal that the genomic patterns and phylogenetic locations of the FDHs determine the enzymatic reactivities, which was experimentally validated in terms of the substrate scope and regioselectivity. A large portion of publicly available FDHs needs to be reevaluated to designate their correct functions. Our genomic models establish comprehensive links among genotypic information, reactivity, and regioselectivity of FDHs, thereby laying an important foundation for future discovery and classification of novel FDHs. IMPORTANCE Halogenases are playing an important role as tailoring enzymes in biosynthetic pathways. Flavin-dependent tryptophan halogenases (Trp-FDHs) are among the enzymes that have broad substrate scope and high selectivity. From bacterial genomes and metagenomes, we found highly diverse halogenase sequences by using a well-trained profile hidden Markov model built from the experimentally validated halogenases. The characterization of genotype, steady-state activity, substrate scope, and regioselectivity has established comprehensive links between the information encoded in the genomic sequence and reactivity of FDHs reported here. By constructing models for accurate and detailed sequence markers, our work should guide future discovery and classification of novel FDHs.

Proteins as diverse, efficient, and evolvable scaffolds for artificial metalloenzymes.

By combining synthetic catalysts and biochemical tools, numerous artificial metalloenzymes have been designed to exhibit high catalytic activity and selectivity in diverse chemical transformations. Out of the nearly infinite number of discovered or characterised proteins, however, only a handful of proteins have been employed as scaffolds for artificial metalloenzymes, implying that specific proteins are preferred owing to their native structural, functional, or biochemical properties. In the present review, we extract and group the biochemical and structural properties of proteins that are advantageous in the design of artificial metalloenzymes; protein stability, pre-existing metal centre, native binding affinity for small molecules, confined and empty space, well-defined secondary structure, and native cellular location. The desirable properties highlight proteins as the key players in the design of metal-dependent biocatalysts. We also propose rarely considered, yet promising, proteins that could be versatile and unique scaffolds for novel metalloenzymes.

Diverse protein assembly driven by metal and chelating amino acids with selectivity and tunability.

Proteins are versatile natural building blocks with highly complex and multifunctional architectures, and self-assembled protein structures have been created by the introduction of covalent, noncovalent, or metal-coordination bonding. Here, we report the robust, selective, and reversible metal coordination properties of unnatural chelating amino acids as the sufficient and dominant driving force for diverse protein self-assembly. Bipyridine-alanine is genetically incorporated into a D3 homohexamer. Depending on the position of the unnatural amino acid, 1-directional, crystalline and noncrystalline 2-directional, combinatory, and hierarchical architectures are effectively created upon the addition of metal ions. The length and shape of the structures is tunable by altering conditions related to thermodynamics and kinetics of metal-coordination and subsequent reactions. The crystalline 1-directional and 2-directional biomaterials retain their native enzymatic activities with increased thermal stability, suggesting that introducing chelating ligands provides a specific chemical basis to synthesize diverse protein-based functional materials while retaining their native structures and functions.

Emergence of metal selectivity and promiscuity in metalloenzymes.

Metal coordination with proteinaceous ligands has greatly expanded the chemical toolbox of proteins and their biological roles. The structure and function of natural metalloproteins have been determined according to the physicochemical properties of metal ions bound to the active sites. Concurrently, amino acid sequences are optimized for metal coordination geometry and/or dedicated action of metal ions in proteinaceous environments. In some occasions, however, natural enzymes exhibit promiscuous reactivity with more than one designated metal ion, under in vitro and/or in vivo conditions. In this review, we discuss selected examples of metalloenzymes that bind various first-row, mid- to late-transition metal ions for their native catalytic activities. From these examples, we suggest that environmental, inorganic, and biochemical factors, such as bioavailability, native organism, cellular compartment, reaction mechanism, binding affinity, protein sequence, and structure, might be responsible for determining metal selectivity or promiscuity. The current work proposes how natural metalloproteins might have emerged and adapted for specific metal incorporation under the given circumstances and may provide insights into the design and engineering of de novo metalloproteins.

Integrative metagenomic and biochemical studies on rifamycin ADP-ribosyltransferases discovered in the sediment microbiome.

Antibiotic resistance is a serious and growing threat to human health. The environmental microbiome is a rich reservoir of resistomes, offering opportunities to discover new antibiotic resistance genes. Here we demonstrate an integrative approach of utilizing gene sequence and protein structural information to characterize unidentified genes that are responsible for the resistance to the action of rifamycin antibiotic rifampin, a first-line antimicrobial agent to treat tuberculosis. Biochemical characterization of four environmental metagenomic proteins indicates that they are adenosine diphosphate (ADP)-ribosyltransferases and effective in the development of resistance to FDA-approved rifamycins. Our analysis suggests that even a single residue with low sequence conservation plays an important role in regulating the degrees of antibiotic resistance. In addition to advancing our understanding of antibiotic resistomes, this work demonstrates the importance of an integrative approach to discover new metagenomic genes and decipher their biochemical functions.

Importance of Scaffold Flexibility/Rigidity in the Design and Directed Evolution of Artificial Metallo-ß-lactamases.

We describe the design and evolution of catalytic hydrolase activity on a supramolecular protein scaffold, Zn4:C96RIDC14, which was constructed from cytochrome cb562 building blocks via a metal-templating strategy. Previously, we reported that Zn4:C96RIDC14 could be tailored with tripodal (His/His/Glu), unsaturated Zn coordination motifs in its interfaces to generate a variant termed Zn8:A104AB34, which in turn displayed catalytic activity for the hydrolysis of activated esters and ß-lactam antibiotics. Zn8:A104AB34 was subsequently subjected to directed evolution via an in vivo selection strategy, leading to a variant Zn8:A104/G57AB34 which displayed enzyme-like Michaelis-Menten behavior for ampicillin hydrolysis. A criterion for the evolutionary utility or designability of a new protein structure is its ability to accommodate different active sites. With this in mind, we examined whether Zn4:C96RIDC14 could be tailored with alternative Zn coordination sites that could similarly display evolvable catalytic activities. We report here a detailed structural and functional characterization of new variant Zn8:AB54, which houses similar, unsaturated Zn coordination sites to those in Zn8:A104/G57AB34, but in completely different microenvironments. Zn8:AB54 displays Michaelis-Menten behavior for ampicillin hydrolysis without any optimization. Yet, the subsequent directed evolution of Zn8:AB54 revealed limited catalytic improvement, which we ascribed to the local protein rigidity surrounding the Zn centers and the lack of evolvable loop structures nearby. The relaxation of local rigidity via the elimination of adjacent disulfide linkages led to a considerable structural transformation with a concomitant improvement in ß-lactamase activity. Our findings reaffirm previous observations that the delicate balance between protein flexibility and stability is crucial for enzyme design and evolution.

A designed supramolecular protein assembly with in vivo enzymatic activity.

The generation of new enzymatic activities has mainly relied on repurposing the interiors of preexisting protein folds because of the challenge in designing functional, three-dimensional protein structures from first principles. Here we report an artificial metallo-ß-lactamase, constructed via the self-assembly of a structurally and functionally unrelated, monomeric redox protein into a tetrameric assembly that possesses catalytic zinc sites in its interfaces. The designed metallo-ß-lactamase is functional in the Escherichia coli periplasm and enables the bacteria to survive treatment with ampicillin. In vivo screening of libraries has yielded a variant that displays a catalytic proficiency [(k(cat)/K(m))/k(uncat)] for ampicillin hydrolysis of 2.3 × 10(6) and features the emergence of a highly mobile loop near the active site, a key component of natural ß-lactamases to enable substrate interactions.

Interfacial metal coordination in engineered protein and peptide assemblies.

Metal ions are frequently found in natural protein-protein interfaces, where they stabilize quaternary or supramolecular protein structures, mediate transient protein-protein interactions, and serve as catalytic centers. Paralleling these natural roles, coordination chemistry of metal ions is being increasingly utilized in creative ways toward engineering and controlling the assembly of functional supramolecular peptide and protein architectures. Here we provide a brief overview of this emerging branch of metalloprotein/peptide engineering and highlight a few select examples from the recent literature that best capture the diversity and future potential of approaches that are being developed.