Engineered bacterial microcompartments: apps for programming metabolism

Enhanced precision and efficiency in metabolic regulation: Compartmentalized metabolic engineering

Metabolic engineering has witnessed remarkable advancements, enabling successful large-scale, cost-effective and efficient production of numerous compounds. However, the predominant expression of heterologous genes in the cytoplasm poses limitations, such as low substrate concentration, metabolic competition and product toxicity. To overcome these challenges, compartmentalized metabolic engineering allows the spatial separation of metabolic pathways for the efficient and precise production of target compounds. Compartmentalized metabolic engineering and its common strategies are comprehensively described in this study, where various membranous compartments and membraneless compartments have been used for compartmentalization and constructive progress has been made. Additionally, the challenges and future directions are discussed in depth. This review is dedicated to providing compartmentalized, precise and efficient methods for metabolic production, and provides valuable guidance for further development in the field of metabolic engineering.

Interfacial Assembly of Bacterial Microcompartment Shell Proteins in Aqueous Multiphase Systems

Compartments are a fundamental feature of life, based variously on lipid membranes, protein shells, or biopolymer phase separation. Here, this combines self-assembling bacterial microcompartment (BMC) shell proteins and liquid-liquid phase separation (LLPS) to develop new forms of compartmentalization. It is found that BMC shell proteins assemble at the liquid-liquid interfaces between either 1) the dextran-rich droplets and PEG-rich continuous phase of a poly(ethyleneglycol)(PEG)/dextran aqueous two-phase system, or 2) the polypeptide-rich coacervate droplets and continuous dilute phase of a polylysine/polyaspartate complex coacervate system. Interfacial protein assemblies in the coacervate system are sensitive to the ratio of cationic to anionic polypeptides, consistent with electrostatically-driven assembly. In both systems, interfacial protein assembly competes with aggregation, with protein concentration and polycation availability impacting coating. These two LLPS systems are then combined to form a three-phase system wherein coacervate droplets are contained within dextran-rich phase droplets. Interfacial localization of BMC hexameric shell proteins is tunable in a three-phase system by changing the polyelectrolyte charge ratio. The tens-of-micron scale BMC shell protein-coated droplets introduced here can accommodate bioactive cargo such as enzymes or RNA and represent a new synthetic cell strategy for organizing biomimetic functionality.

Comparative Pore Structure and Dynamics for Bacterial Microcompartment Shell Protein Assemblies in Sheets or Shells

Bacterial microcompartments (BMCs) are protein-bound organelles found in some bacteria which encapsulate enzymes for enhanced catalytic activity. These compartments spatially sequester enzymes within semi-permeable shell proteins, analogous to many membrane-bound organelles. The shell proteins assemble into multimeric tiles; hexamers, trimers, and pentamers, and these tiles self-assemble into larger assemblies with icosahedral symmetry. While icosahedral shells are the predominant form in vivo, the tiles can also form nanoscale cylinders or sheets. The individual multimeric tiles feature central pores that are key to regulating transport across the protein shell. Our primary interest is to quantify pore shape changes in response to alternative component morphologies at the nanoscale. We use molecular modeling tools to develop atomically detailed models for both planar sheets of tiles and curved structures representative of the complete shells found in vivo. Subsequently, these models were animated using classical molecular dynamics simulations. From the resulting trajectories, we analyzed overall structural stability, water accessibility to individual residues, water residence time, and pore geometry for the hexameric and trimeric protein tiles from the Haliangium ochraceum model BMC shell. These exhaustive analyses suggest no substantial variation in pore structure or solvent accessibility between the flat and curved shell geometries. We additionally compare our analysis to hydroxyl radical footprinting data to serve as a check against our simulation results, highlighting specific residues where water molecules are bound for a long time. Although with little variation in morphology or water interaction, we propose that the planar and capsular morphology can be used interchangeably when studying permeability through BMC pores.

Designer protein compartments for microbial metabolic engineering

Protein compartments are distinct structures assembled in living cells via self-assembly or phase separation of specific proteins. Significant efforts have been made to discover their molecular structures and formation mechanisms, as well as their fundamental roles in spatiotemporal control of cellular metabolism. Here, we review the design and construction of synthetic protein compartments for spatial organization of target metabolic pathways toward increased efficiency and specificity. In particular, we highlight the compartmentalization strategies and recent examples to speed up desirable metabolic reactions, to reduce the accumulation of toxic metabolic intermediates, and to switch competing metabolic pathways. We also identify the most important challenges that need to be addressed for exploitation of these designer compartments as a versatile toolkit in metabolic reprogramming.

Modulation of Hybrid GRM2-type Bacterial Microcompartment Shells through BMC-H Shell Protein Fusion and Incorporation of Non-native BMC-T Shell Proteins

Bacterial microcompartments (BMCs) are organelle-like structures in bacteria that facilitate a wide range of enzymatic reactions. The microcompartment shell contains an encapsulated enzymatic core and, in contrast to phospholipid-based eukaryotic organelle membranes, has a pseudoicosahedral shape composed of BMC-H, BMC-T, and BMC-P proteins with conserved structures. This semipermeable microcompartment shell delineates the enzymatic core assemblies and the intermediates from the rest of the cell. It is also thought to function as a barrier against toxic intermediates as well as to increase the reaction rate. These properties of BMCs have made them intriguing candidates for biotechnological applications, for which it is important to explore the potential scope of the BMC shell modulation possibilities. In this work, we explore two BMC shell modulation mechanisms: first, confirming the incorporation of three trimeric BMC-T shell proteins and two truncated BMC-T shell proteins into Klebsiella pneumoniae GRM2-type BMC protein shells containing no representatives of this group, and second, producing BMC particles from double- and triple-fused hexameric BMC-H shell proteins. These results reveal the potential for "mix and match" synthetic BMC shell formation to ensure shell properties specifically suited to the encapsulated cargo and show for the first time the involvement of an essentially dimeric pseudohexameric shell protein in BMC shell formation.

Heterologous Assembly of Pleomorphic Bacterial Microcompartment Shell Architectures Spanning the Nano- to Microscale

Many bacteria use protein-based organelles known as bacterial microcompartments (BMCs) to organize and sequester sequential enzymatic reactions. Regardless of their specialized metabolic function, all BMCs are delimited by a shell made of multiple structurally redundant, yet functionally diverse, hexameric (BMC-H), pseudohexameric/trimeric (BMC-T), or pentameric (BMC-P) shell protein paralogs. When expressed without their native cargo, shell proteins have been shown to self-assemble into 2D sheets, open-ended nanotubes, and closed shells of ≈40 nm diameter that are being developed as scaffolds and nanocontainers for applications in biotechnology. Here, by leveraging a strategy for affinity-based purification, it is demonstrated that a wide range of empty synthetic shells, many differing in end-cap structures, can be derived from a glycyl radical enzyme-associated microcompartment. The range of pleomorphic shells observed, which span ≈2 orders of magnitude in size from ≈25 nm to ≈1.8 µm, reveal the remarkable plasticity of BMC-based biomaterials. In addition, new capped nanotube and nanocone morphologies are observed that are consistent with a multicomponent geometric model in which architectural principles are shared among asymmetric carbon, viral protein, and BMC-based structures.

Theoretical and Practical Aspects of Multienzyme Organization and Encapsulation

The advent of biotechnology has enabled metabolic engineers to assemble heterologous pathways in cells to produce a variety of products of industrial relevance, often in a sustainable way. However, many pathways face challenges of low product yield. These pathways often suffer from issues that are difficult to optimize, such as low pathway flux and off-target pathway consumption of intermediates. These issues are exacerbated by the need to balance pathway flux with the health of the cell, particularly when a toxic intermediate builds up. Nature faces similar challenges and has evolved spatial organization strategies to increase metabolic pathway flux and efficiency. Inspired by these strategies, bioengineers have developed clever strategies to mimic spatial organization in nature. This review explores the use of spatial organization strategies, including protein scaffolding and protein encapsulation inside of proteinaceous shells, toward overcoming bottlenecks in metabolic engineering efforts.

Microbial cell factories based on filamentous bacteria, yeasts, and fungi

BACKGROUND

Advanced DNA synthesis, biosensor assembly, and genetic circuit development in synthetic biology and metabolic engineering have reinforced the application of filamentous bacteria, yeasts, and fungi as promising chassis cells for chemical production, but their industrial application remains a major challenge that needs to be solved.

RESULTS

As important chassis strains, filamentous microorganisms can synthesize important enzymes, chemicals, and niche pharmaceutical products through microbial fermentation. With the aid of metabolic engineering and synthetic biology, filamentous bacteria, yeasts, and fungi can be developed into efficient microbial cell factories through genome engineering, pathway engineering, tolerance engineering, and microbial engineering. Mutant screening and metabolic engineering can be used in filamentous bacteria, filamentous yeasts (Candida glabrata, Candida utilis), and filamentous fungi (Aspergillus sp., Rhizopus sp.) to greatly increase their capacity for chemical production. This review highlights the potential of using biotechnology to further develop filamentous bacteria, yeasts, and fungi as alternative chassis strains.

CONCLUSIONS

In this review, we recapitulate the recent progress in the application of filamentous bacteria, yeasts, and fungi as microbial cell factories. Furthermore, emphasis on metabolic engineering strategies involved in cellular tolerance, metabolic engineering, and screening are discussed. Finally, we offer an outlook on advanced techniques for the engineering of filamentous bacteria, yeasts, and fungi.

Toward a glycyl radical enzyme containing synthetic bacterial microcompartment to produce pyruvate from formate and acetate

The enormous complexity of metabolic pathways, in both their regulation and propensity for metabolite cross-talk, represents a major obstacle for metabolic engineering. Self-assembling, catalytically programmable and genetically transferable bacterial microcompartments (BMCs) offer solutions to decrease this complexity through compartmentalization of enzymes within a selectively permeable protein shell. Synthetic BMCs can operate as autonomous metabolic modules decoupled from the cell’s regulatory network, only interfacing with the cell’s metabolism via the highly engineerable proteinaceous shell. Here, we build a synthetic, modular, multienzyme BMC. It functions not only as a proof-of-concept for next-generation metabolic engineering, but also provides the foundation for subsequent tuning, with the goal to create a microanaerobic environment protecting an oxygen-sensitive reaction in aerobic growth conditions that could be deployed.

Formate has great potential to function as a feedstock for biorefineries because it can be sustainably produced by a variety of processes that don’t compete with agricultural production. However, naturally formatotrophic organisms are unsuitable for large-scale cultivation, difficult to engineer, or have inefficient native formate assimilation pathways. Thus, metabolic engineering needs to be developed for model industrial organisms to enable efficient formatotrophic growth. Here, we build a prototype synthetic formate utilizing bacterial microcompartment (sFUT) encapsulating the oxygen-sensitive glycyl radical enzyme pyruvate formate lyase and a phosphate acyltransferase to convert formate and acetyl-phosphate into the central biosynthetic intermediate pyruvate. This metabolic module offers a defined environment with a private cofactor coenzyme A that can cycle efficiently between the encapsulated enzymes. To facilitate initial design-build-test-refine cycles to construct an active metabolic core, we used a “wiffleball” architecture, defined as an icosahedral bacterial microcompartment (BMC) shell with unoccupied pentameric vertices to freely permit substrate and product exchange. The resulting sFUT prototype wiffleball is an active multi enzyme synthetic BMC functioning as platform technology.

Advances in the bacterial organelles for CO2 fixation

Carboxysomes are a family of bacterial microcompartments (BMCs), present in all cyanobacteria and some proteobacteria, which encapsulate the primary CO₂-fixing enzyme, Rubisco, within a virus-like polyhedral protein shell. Carboxysomes provide significantly elevated levels of CO₂ around Rubisco to maximize carboxylation and reduce wasteful photorespiration, thus functioning as the central CO₂-fixation organelles of bacterial CO₂-concentration mechanisms. Their intriguing architectural features allow carboxysomes to make a vast contribution to carbon assimilation on a global scale. In this review, we discuss recent research progress that provides new insights into the mechanisms of how carboxysomes are assembled and functionally maintained in bacteria and recent advances in synthetic biology to repurpose the metabolic module in diverse applications.

Physiological limitations and opportunities in microbial metabolic engineering

Metabolic engineering can have a pivotal role in increasing the environmental sustainability of the transportation and chemical manufacturing sectors. The field has already developed engineered microorganisms that are currently being used in industrial-scale processes. However, it is often challenging to achieve the titres, yields and productivities required for commercial viability. The efficiency of microbial chemical production is usually dependent on the physiological traits of the host organism, which may either impose limitations on engineered biosynthetic pathways or, conversely, boost their performance. In this Review, we discuss different aspects of microbial physiology that often create obstacles for metabolic engineering, and present solutions to overcome them. We also describe various instances in which natural or engineered physiological traits in host organisms have been harnessed to benefit engineered metabolic pathways for chemical production.

A catalog of the diversity and ubiquity of bacterial microcompartments

Bacterial microcompartments (BMCs) are organelles that segregate segments of metabolic pathways which are incompatible with surrounding metabolism. BMCs consist of a selectively permeable shell, composed of three types of structurally conserved proteins, together with sequestered enzymes that vary among functionally distinct BMCs. Genes encoding shell proteins are typically clustered with those for the encapsulated enzymes. Here, we report that the number of identifiable BMC loci has increased twenty-fold since the last comprehensive census of 2014, and the number of distinct BMC types has doubled. The new BMC types expand the range of compartmentalized catalysis and suggest that there is more BMC biochemistry yet to be discovered. Our comprehensive catalog of BMCs provides a framework for their identification, correlation with bacterial niche adaptation, experimental characterization, and development of BMC-based nanoarchitectures for biomedical and bioengineering applications.

Bacterial microcompartments (BMCs) are organelles consisting of a protein shell in which certain metabolic reactions take place separated from the cytoplasm. Here, Sutter et al. present a comprehensive catalog of BMC loci, substantially expanding the number of known BMCs and describing distinct types and compartmentalized reactions.

Introducing noncanonical amino acids for studying and engineering bacterial microcompartments

Bacterial microcompartments (BMCs) with selectively permeable shells and encapsulated enzyme cores are well-suited candidates for nano-bioreactors because of their advantages of enhancing pathway flux and protection against toxic products. To better study and engineer protein-based BMCs, a series of protein chemistry approaches are adopted. As one of the most advanced techniques, genetic code expansion can introduce various noncanonical amino acids (ncAAs) with diverse functional groups into target proteins, thus providing powerful tools for protein studies and engineering. This review summarizes and proposes useful tools based on current development of the genetic code expansion technique towards challenges in BMC studies and engineering.

Ethanolamine bacterial microcompartments: from structure, function studies to bioengineering applications

Two decades of structural and functional studies have revealed functions, structures and diversity of bacterial microcompartments. The protein-based organelles encapsulate diverse metabolic pathways in semipermeable, icosahedral or pseudo-icosahedral shells. One of the first discovered and characterized microcompartments are those involved in ethanolamine degradation. This review will summarize their function and assembly along with shared and unique characteristics with other microcompartment types. The modularity and self-assembling properties of their shell proteins make them valuable targets for bioengineering. Advances and prospects for shell protein engineering in vivo and in vitro for synthetic biology and biotechnology applications will be discussed.

Organizing Multi-Enzyme Systems into Programmable Materials for Biocatalysis

Significant advances in enzyme discovery, protein and reaction engineering have transformed biocatalysis into a viable technology for the industrial scale manufacturing of chemicals. Multi-enzyme catalysis has emerged as a new frontier for the synthesis of complex chemicals. However, the in vitro operation of multiple enzymes simultaneously in one vessel poses challenges that require new strategies for increasing the operational performance of enzymatic cascade reactions. Chief among those strategies is enzyme co-immobilization. This review will explore how advances in synthetic biology and protein engineering have led to bioinspired co-localization strategies for the scaffolding and compartmentalization of enzymes. Emphasis will be placed on genetically encoded co-localization mechanisms as platforms for future autonomously self-organizing biocatalytic systems. Such genetically programmable systems could be produced by cell factories or emerging cell-free systems. Challenges and opportunities towards self-assembling, multifunctional biocatalytic materials will be discussed.

Mechanisms of Scaffold-Mediated Microcompartment Assembly and Size Control

This article describes a theoretical and computational study of the dynamical assembly of a protein shell around a complex consisting of many cargo molecules and long, flexible scaffold molecules. Our study is motivated by bacterial microcompartments, which are proteinaceous organelles that assemble around a condensed droplet of enzymes and reactants. As in many examples of cytoplasmic liquid-liquid phase separation, condensation of the microcompartment interior cargo is driven by flexible scaffold proteins that have weak multivalent interactions with the cargo. Our results predict that the shell size, amount of encapsulated cargo, and assembly pathways depend sensitively on properties of the scaffold, including its length and valency of scaffold-cargo interactions. Moreover, the ability of self-assembling protein shells to change their size to accommodate scaffold molecules of different lengths depends crucially on whether the spontaneous curvature radius of the protein shell is smaller or larger than a characteristic elastic length scale of the shell. Beyond natural microcompartments, these results have important implications for synthetic biology efforts to target alternative molecules for encapsulation by microcompartments or viral shells. More broadly, the results elucidate how cells exploit coupling between self-assembly and liquid-liquid phase separation to organize their interiors.

Manipulating microcompartment operons to study mechanism and function

The gene systems that encode functional bacterial microcompartments (BMCs) are typically comprised of between 10-23 genes, often in a contiguous operon. BMC genes can be studied as whole native operons or as subsets of genes that form structures for specific applications. Recent examples of such studies highlight the flexible modular nature of BMC operons/genes and the options that exist to harness their functions via manipulation at the DNA level. This work also demonstrates the transfer and functional expression of BMC operons/genes across bacterial species. Recombineering, DNA synthesis technology, and advanced cloning techniques have all been applied in creative ways to study the nature of BMC mechanism and function.