Cell surface display of carbonic anhydrase on Escherichia coli using ice nucleation protein for CO₂ sequestration

Biosilica-coated carbonic anhydrase displayed on Escherichia coli: A novel design approach for efficient and stable biocatalyst for CO2 sequestration

A robust and stable carbonic anhydrase (CA) system is indispensable for effectively sequestering carbon dioxide to mitigate climate change. While microbial surface display technology has been employed to construct an economically promising cell-displayed CO2-capturing biocatalyst, the displayed CA enzymes were prone to inactivation due to their low stability in harsh conditions. Herein, drawing inspiration from biomineralized diatom frustules, we artificially introduced biosilica shell materials to the CA macromolecules displayed on Escherichia coli surfaces. Specifically, we displayed a fusion of CA and the diatom-derived silica-forming Sil3K peptide (CA-Sil3K) on the E. coli surface using the membrane anchor protein Lpp-OmpA linker. The displayed CA-Sil3K (dCA-Sil3K) fusion protein underwent a biosilicification reaction under mild conditions, resulting in nanoscale self-encapsulation of the displayed enzyme in biosilica. The biosilicified dCA-Sil3K (BS-dCA-Sil3K) exhibited improved thermal, pH, and protease stability and retained 63 % of its initial activity after ten reuses. Additionally, the BS-dCA-Sil3K biocatalyst significantly accelerated the CaCO3 precipitation rate, reducing the time required for the onset of CaCO3 formation by 92 % compared to an uncatalyzed reaction. Sedimentation of BS-dCA-Sil3K on a membrane filter demonstrated a reliable CO2 hydration application with superior long-term stability under desiccation conditions. This study may open new avenues for the nanoscale-encapsulation of enzymes with biosilica, offering effective strategies to provide efficient, stable, and economic cell-displayed biocatalysts for practical applications.

Surface display provides an efficient expression system for production of recombinant proteins and bacterial whole cell biosensor in E. coli

A novel bacterial display vector based on Escherichia coli has been engineered for recombinant protein production and purification. Accordingly, a construct harboring the enhanced green fluorescent protein (EGFP) and the ice nucleation protein (INP) was designed to produce EGFP via the surface display in E. coli cells. The fusion EGFP-expressed cells were then investigated using fluorescence measurement, SDS- and native-PAGE before and after TEV protease digestion. The displayed EGFP was obtained with a recovery of 57.7 % as a single band on SDS-PAGE. Next, the efficiency of the cell surface display for mutant EGFP (EGFP S202H/Q204H) was examined in sensing copper ions. Under optimal conditions, a satisfactorily linear range for copper ions concentrations up to 10 nM with a detection limit of 0.073 nM was obtained for cell-displayed mutant EGFP (mEGFP). In the presence of bacterial cell lysates and purified mEGFP, response to copper was linear in the 2-10 nM and 0.1-2 μM concentration range, respectively, with a 1.3 nM and 0.14 μM limit of detection. The sensitivity of bacterial cell lysates and surface-displayed mEGFP in the detection of copper ions is higher than the purified mEGFP.

Characteristics of cold-adapted carbonic anhydrase and efficient carbon dioxide capture based on cell surface display technology

Carbonic anhydrase (CA) is currently under investigation because of its potential to capture CO2. A novel N-domain of ice nucleoproteins (INPN)-mediated surface display technique was developed to produce CA with low-temperature capture CO2 based on the mining and characterization of Colwellia sp. CA (CsCA) with cold-adapted enzyme structural features and catalytic properties. CsCA and INPN were effectively integrated into the outer membrane of the cell as fusion proteins. Throughout the display process, the integrity of the membrane of engineered bacteria BL21/INPN-CsCA was maintained. Notably, the study affirmed positive applicability, wherein 94 % activity persisted after 5 d at 15 °C, and 73 % of the activity was regained after 5 cycles of CO2 capture. BL21/INPN-CsCA displayed a high CO2 capture capacity of 52 mg of CaCO3/mg of whole-cell biocatalysts during CO2 mineralization at 25 °C. Therefore, the CsCA functional cell surface display technology could contribute significantly to environmentally friendly CO2 capture.

Surface expression of carbonic anhydrase on E. coli as a sustainable approach for enzymatic CO2 capture

The utilisation of carbonic anhydrase (CA) in CO2 sequestration is becoming prominent as an efficient, environment friendly and rapid catalyst for capturing CO2 from industrial emissions. However, the application of CA enzyme in soluble form is constrained due to its poor stability in operational conditions of CO2 capture and also production cost of the enzyme. Addressing these limitations, the present study focuses on the surface display of CA from Bacillus halodurans (BhCA) on E coli aiming to contribute to the cost-effectiveness of carbon capture through CA technology. This involved the fusion of the BhCA-encoding gene with the adhesion molecule involved in diffuse adherence (AIDA-I) autotransporter, resulting in the efficient display of BhCA (595 ± 60 U/gram dry cell weight). Verification of the surface display of BhCA was accomplished by conjugating with FITC labelled anti-his antibody followed by fluorescence-activated cell sorting (FACS) and cellular fractionation in conjunction with zymography. Biochemical characterisation of whole-cell biocatalyst revealed a noteworthy enhancement in thermostability, improvement in the thermostability with T1/2 of 90 ± 1.52 minutes at 50 ˚C, 36 ± 2.51 minutes at 60 ˚C and18 ± 1.52 minutes at 80˚C. Surface displayed BhCA displayed remarkable reusability retaining 100% activity even after 15 cycles. Surface displayed BhCA displayed highly alkali stable nature like free counterpart in solution. The alkali stability of the surface-displayed BhCA was comparable to its free counterpart in solution. Furthermore, the study investigated the impact of different metal ions, modulators, and detergents on the whole-cell biocatalysts. The present work represents the first report on surface display of CA utilising the AIDA-1 autotransporter.

Construction of Cupriavidus necator displayed with superoxide dismutases for enhanced growth in bioelectrochemical systems

It is of great significance to utilize CO2 as feedstock to synthesize biobased products, particularly single cell protein (SCP) as the alternative food and feed. Bioelectrochemical system (BES) driven by clean electric energy has been regarded as a promising way for Cupriavidus necator to produce SCP from CO2 directly. At present, the key problem of culturing C. necator in BES is that reactive oxygen species (ROS) generated in cathode chamber are harmful to bacterial growth. Therefore, it is necessary to find a solution to mitigate the negative effect of ROS. In this study, we constructed a number of C. necator strains displayed with superoxide dismutase (SOD), which allowed the decomposition of superoxide anion radical. The effects of promoters and signal peptides on the cell surface displayed SOD were analyzed. The proteins displayed on the surface were further verified by the fluorescence experiment. Finally, the growth of C. necator CMS incorporating a pBAD-SOD-E-tag-IgAβ plasmid could achieve 4.9 ± 1.0 of OD600 by 7 days, equivalent to 1.7 ± 0.3 g/L dry cell weight (DCW), and the production rate was 0.24 ± 0.04 g/L/d DCW, around 2.7-fold increase than the original C. necator CMS (1.8 ± 0.3 of OD600). This study can provide an effective and novel strategy of cultivating strains for the production of CO2-derived SCP or other chemicals in BES.

Highly efficient low-temperature biodegradation of polyethylene microplastics by using cold-active laccase cell-surface display system

To eliminate efficiency restriction of polyethylene microplastics low-temperature biodegradation, a novel InaKN-mediated Escherichia coli surface display platform for cold-active degrading laccase PsLAC production was developed. Display efficiency of 88.0% for engineering bacteria BL21/pET-InaKN-PsLAC was verified via subcellular extraction and protease accessibility, exhibiting an activity load of 29.6 U/mg. Cell growth and membrane integrity revealed BL21/pET-InaKN-PsLAC maintained stable growth and intact membrane structure during the display process. The favorable applicability was confirmed, with 50.0% activity remaining in 4 days at 15 °C, and 39.0% activity recovery retention after 15 batches of activity substrate oxidation reactions. Moreover, BL21/pET-InaKN-PsLAC possessed high polyethylene low-temperature depolymerizing capacity. Bioremediation experiments proved that the degradation rate was 48.0% within 48 h at 15 °C, and reached 66.0% after 144 h. Collectively, cold-active PsLAC functional surface display technology and its significant contributions to polyethylene microplastics low-temperature degradation constitute an effective improvement strategy for biomanufacturing and microplastics cold remediation.

Biocatalytic Membranes for Carbon Capture and Utilization

Innovative carbon capture technologies that capture CO₂ from large point sources and directly from air are urgently needed to combat the climate crisis. Likewise, corresponding technologies are needed to convert this captured CO₂ into valuable chemical feedstocks and products that replace current fossil-based materials to close the loop in creating viable pathways for a renewable economy. Biocatalytic membranes that combine high reaction rates and enzyme selectivity with modularity, scalability, and membrane compactness show promise for both CO₂ capture and utilization. This review presents a systematic examination of technologies under development for CO₂ capture and utilization that employ both enzymes and membranes. CO₂ capture membranes are categorized by their mode of action as CO₂ separation membranes, including mixed matrix membranes (MMM) and liquid membranes (LM), or as CO₂ gas-liquid membrane contactors (GLMC). Because they selectively catalyze molecular reactions involving CO₂, the two main classes of enzymes used for enhancing membrane function are carbonic anhydrase (CA) and formate dehydrogenase (FDH). Small organic molecules designed to mimic CA enzyme active sites are also being developed. CO₂ conversion membranes are described according to membrane functionality, the location of enzymes relative to the membrane, which includes different immobilization strategies, and regeneration methods for cofactors. Parameters crucial for the performance of these hybrid systems are discussed with tabulated examples. Progress and challenges are discussed, and perspectives on future research directions are provided.

Promoter-regulated in vivo asymmetric self-assembly strategy to synthesize heterogeneous nanoparticles for signal amplification

Signal amplification is commonly used to enhance the sensitivity of biological analysis. Here, we present a strategy involving in vivo asymmetric self-assembly combined with promoter strength regulation to synthesize heterogeneous nanoparticles for signal amplification. Two expression vectors were constructed by genetically inserting, respectively, signal and binding molecules into the hepatitis B core antigen protein (HBcAg) structure. Because of differential expression of the two recombinant proteins in the presence of a strong promoter (T7) and a weak promoter (Tac-1) and spontaneous asymmetric self-assembly in vivo, heterogeneous HBcAg nanoparticles (NPs) with a high ratio of signal-bearing to target-binding molecules were obtained. These nanoparticles contained a large number of green fluorescent proteins as signal molecules and a small number of B1 immunoglobulin-binding domains from protein G for antibody binding, thus enabling sensitive immunoassays. As a proof of concept, improved sensitivity for antibody detection was achieved using the heterogeneous nanoparticle conjugated with a secondary antibody molecule.

Immobilization of carbonic anhydrase for CO2 capture and utilization

Carbonic anhydrase (CA) is an excellent candidate for novel biocatalytic processes based on the capture and utilization of CO₂. The setup of efficient methods for enzyme immobilization makes CA utilization in continuous bioreactors increasingly attractive and opens up new opportunities for the industrial use of CA. The development of efficient processes for CO₂ capture and utilization (CCU) is one of the most challenging targets of modern chemical reaction engineering. In the general frame of CCU processes, the interest in the utilization of immobilized CA as a biocatalyst for augmentation of CO₂ reactive absorption has grown consistently over the last decade. The present mini-review surveys and discusses key methodologies for CA immobilization aimed at the development of heterogeneous biocatalysts for CCU. Advantages and drawbacks of covalent attachment on fine granular solids, immobilization as cross-linked enzyme aggregates, and "in vivo" immobilization methods are presented. In particular, criteria for optimal selection of CA-biocatalyst and design of CO₂ absorption units are presented and discussed to highlight the most effective solutions. Perspectives on biocatalytic CCU processes that can include the use of CA in an enzymatic reactive CO₂ absorption step are eventually presented with a special focus on two examples of CO₂ fixation pathways: hybrid enzyme-microalgae process and enzyme cascade for the production of carboxylic acids. KEY POINTS: • Covalent immobilization techniques applied to CA are effective for CO₂ ERA. • Biocatalyst type and morphology must be selected considering CO₂ ERA conditions. • Immobilized CA can offer novel routes to CO₂ capture and direct utilization.

Surface display of carbonic anhydrase on Escherichia coli for CO2 capture and mineralization

Mineralization catalyzed by carbonic anhydrase (CA) is one of the most promising technologies for capturing CO2. In this work, Escherichia coli BL21(DE3) was used as the host, and the N-terminus of ice nucleation protein (INPN) was used as the carrier protein. Different fusion patterns and vectors were used to construct CA surface display systems for α-carbonic anhydrase (HPCA) from Helicobacter pylori 26695 and α-carbonic anhydrase (SazCA) from Sulfurihydrogenibium azorense. The surface display system in which HPCA was fused with INPN via a flexible linker and intermediate repeat sequences showed higher whole-cell enzyme activity, while the enzyme activity of the SazCA expression system was significantly higher than that of the HPCA expression system. The pET22b vector with the signal peptide PelB was more suitable for the cell surface display of SazCA. Cell fractionation and western-blot analysis indicated that SazCA and INPN were successfully anchored on the cell's outer membrane as a fusion protein. The enzyme activity of the surface display strain E-22b-IRLS (11.43 U·mL-1OD600 -1) was significantly higher than that of the intracellular expression strain E-22b-S (8.355 U·mL-1OD600 -1) under optimized induction conditions. Compared with free SazCA, E-22b-IRLS had higher thermal and pH stability. The long-term stability of SazCA was also significantly improved by surface display. When the engineered strain and free enzyme were used for CO2 mineralization, the amount of CaCO3 deposition catalyzed by the strain E-22b-IRLS on the surface (241 mg) was similar to that of the free SazCA and was significantly higher than the intracellular expression strain E-22b-S (173 mg). These results demonstrate that the SazCA surface display strain can serve as a whole-cell biocatalyst for CO2 capture and mineralization.

Non-Photosynthetic CO2 Utilization to Increase Fatty Acid Production in Yarrowia lipolytica

Metabolic engineering of non-photosynthetic microorganisms to increase the utilization of CO₂ has been focused on as a green strategy to convert CO₂ into valuable products such as fatty acids. In this study, a CO₂ utilization pathway involving carbonic anhydrase and biotin carboxylase was formed to recycle CO₂ in the oleaginous yeast Yarrowia lipolytica, thereby increasing the production of fatty acids. In the recombinant strain in which the CO₂ utilization pathway was introduced, the production of fatty acids was 10.7 g/L, which was 1.5-fold higher than that of the wild-type strain. The resulting strain had a 1.4-fold increase in dry cell mass compared to the wild-type strain. In addition, linoleic acid was 47.7% in the fatty acid composition of the final strain, which was increased by 11.6% compared to the wild-type strain. These results can be applied as an essential technology for developing efficient and eco-friendly processes by directly utilizing CO₂.

In vivo immobilized carbonic anhydrase and its effect on the enhancement of CO2 absorption rate

Reactive absorption into aqueous solutions promoted by carbonic anhydrase (CA, E.C. 4.2.1.1.) has been often proposed as a post-combustion CO₂ capture process. The state of the art reveals the need for efficient biocatalyst based on carbonic anhydrase that can be used to further develop CO₂ capture and utilization technologies. The present study is focused on the use of a thermostable CA-based biocatalyst. The carbonic anhydrase SspCA, from the thermophilic bacterium Sulfurihydrogenibium yellowstonense, was in vivo immobilized as membrane-anchored protein (INPN-SspCA) on the outer membrane of Escherichia coli cells. The dispersed biocatalyst, made by cell membrane debris, was characterized in terms of its contribution to the enhancement of CO₂ absorption in carbonate/bicarbonate alkaline buffer at operating conditions relevant for industrial CO₂ capture processes. The amount of immobilized enzyme, estimated by SDS-PAGE, resulted in about 1 mg enzyme/g membrane debris. The apparent kinetics of the biocatalyst was characterized through CO₂ absorption tests in a stirred cell lab-scale reactor assuming a pseudo-homogeneous behaviour of the biocatalyst. At 298 K, the assessed values of the second-order kinetic constant ranged between 0.176 and 0.555 L∙mg^-1∙s^-1. Reusability of the biocatalyst after 24 h showed the absence of free enzyme release in the alkaline solvent. Moreover, the equilibration of dispersed cell membrane debris against the alkaline buffer positively affected the performances of the heterogeneous biocatalyst. These results encourage further studies on the in vivo immobilized SspCA aimed at optimizing the enzyme loading on the cell membrane and the handling of the biocatalyst in the CO₂ absorption reactors.

Display of lead-binding proteins on Escherichia coli surface for lead bioremediation

Cell surface display of heavy metal-binding proteins has been used to enhance the adsorption capacity of heavy metals and the engineered microbial cells can be potentially used for the bioremediation of heavy metals. In this study, the proteins PbrR, PbrR691, and PbrD from the Cupriavidus metallidurans strain CH34 were displayed on the extracellular membrane of Escherichia coli BL21 cells, with the N-domain of ice-nucleation protein as the anchor protein to achieve specific adsorption of lead ions (Pb²⁺ ) and bioremediation of lead in the soil. The localization of fusion proteins was confirmed by western blot analysis. We investigated the effects of fusion pattern, expression level, heavy metal concentration, and the presence of other heavy metal ions on the adsorption of Pb²⁺ by these engineered bacteria, and the optimal linker peptide (flexible linker) and inducer concentration (0.5 mM) were obtained. The engineered bacteria showed specific selectivity and strong adsorption capacity for Pb²⁺ . The maximum Pb²⁺ adsorption capacity of strains displaying the three proteins (PbrR, PbrR691, and PbrD) were 942.1-, 754.3-, and 864.8-μmol/g cell dry weight, respectively, which was the highest reported to date. The engineered E. coli bacteria were also applied to Pb²⁺ -contaminated soil and the detoxification effects were observed via the seed germination test and the growth of Nicotiana benthamiana in comparison with the control BL21, which provides the proof-of-concept for in situ remediations of Pb²⁺ -contaminated water or soil.

Chromosomal expression of CadR on Pseudomonas aeruginosa for the removal of Cd(II) from aqueous solutions

Genetically engineered bacteria for pollution control of heavy metal have been widely studied, however, using Pseudomonas aeruginosa (P. aeruginosa) that can adapt to various circumstances to remediate heavy metal pollution is rarely reported. In this study, we employed CadR, a cadmium (Cd)-specific binding protein, displaying on the surface of P. aeruginosa with chromosomal expression. The genetically engineered (GE) P. aeruginosa still flourished in the 30th generation in the LB broth which contained 100 μM Cd(II), exhibiting an excellent genetic stability. Chromosomally expressed P. aeruginosa showed an adsorption capacity of up to 131.9 μmol/g of Cd(II). In addition, the low concentration of the coexisting two valence ions has no significant effect on adsorption capacity of Cd(II). This study provides a direction for application of P. aeruginosa in environment remediation.

Simultaneous hydrolysis of carbaryl and chlorpyrifos by Stenotrophomonas sp. strain YC-1 with surface-displayed carbaryl hydrolase

Many sites are often co-contaminated with multiple pesticides. To date, there are no reports on simultaneous degradation of different classes of pesticides by a natural microorganism. In this work, we aim at constructing a live biocatalyst able to simultaneously hydrolyze carbaryl and chlorpyrifos. For this purpose, carbaryl hydrolase (CH) was displayed on the cell surface of a chlorpyrifos-degrading bacterium Stenotrophomonas sp. strain YC-1 using N- and C-terminal domain of ice nucleation protein (INPNC) from Pseudomonas syringae INA5 as an anchoring motif. The localization of INPNC-CH fusion protein in the outer membrane fraction was demonstrated by cell fractionation followed by Western blot analysis. Surface display of INPNC-CH was further confirmed by proteinase accessibility experiment and immunofluorescence microscope. CH was present in an active form on cell surface without causing any growth inhibition, suggesting that the INP-based display system is a useful tool for surface expression of macromolecular heterologous proteins on the bacterial cell surface. Because surface-displayed CH has free access to pesticides, this bacterium can be used as a whole-cell biocatalyst for efficient hydrolysis of pesticides.

A one-step procedure for immobilising the thermostable carbonic anhydrase (SspCA) on the surface membrane of Escherichia coli

The carbonic anhydrase superfamily (CA, EC 4.2.1.1) of metalloenzymes is present in all three domains of life (Eubacteria, Archaea, and Eukarya), being an interesting example of convergent/divergent evolution, with its seven families (α-, β-, γ-, δ-, ζ-, η-, and θ-CAs) described so far. CAs catalyse the simple, but physiologically crucial reaction of carbon dioxide hydration to bicarbonate and protons. Recently, our groups characterised the α-CA from the thermophilic bacterium, Sulfurihydrogenibium yellowstonense finding a very high catalytic activity for the CO₂ hydration reaction (k_cat = 9.35 × 10⁵ s^-1 and k_cat/K_m = 1.1 × 10⁸ M^-1s^-1) which was maintained after heating the enzyme at 80 °C for 3 h. This highly thermostable SspCA was covalently immobilised within polyurethane foam and onto the surface of magnetic Fe₃O₄ nanoparticles. Here, we describe a one-step procedure for immobilising the thermostable SspCA directly on the surface membrane of Escherichia coli, using the INPN domain of Pseudomonas syringae. This strategy has clear advantages with respect to other methods, which require as the first step the production and the purification of the biocatalyst, and as the second step the immobilisation of the enzyme onto a specific support. Our results demonstrate that thermostable SspCA fused to the INPN domain of P. syringae ice nucleation protein (INP) was correctly expressed on the outer membrane of engineered E. coli cells, affording for an easy approach to design biotechnological applications for this highly effective thermostable catalyst.

Co-localization of proteins with defined sequential order and controlled stoichiometric ratio on magnetic nanoparticles

Co-immobilization of enzymes used in cascade reactions is important for improving the overall catalytic efficiency. In this work, we employed scaffoldins as a bridge and succeeded in a highly-ordered co-localization of multiple proteins on magnetic nanoparticles with a loading capacity of ∼0.831 μmol g^-1 supports.

Functional display of ice nucleation protein InaZ on the surface of bacterial ghosts

In a concept study the ability to induce heterogeneous ice formation by Bacterial Ghosts (BGs) from Escherichia coli carrying ice nucleation protein InaZ from Pseudomonas syringae in their outer membrane was investigated by a droplet-freezing assay of ultra-pure water. As determined by the median freezing temperature and cumulative ice nucleation spectra it could be demonstrated that both the living recombinant E. coli and their corresponding BGs functionally display InaZ on their surface. Under the production conditions chosen both samples belong to type II ice-nucleation particles inducing ice formation at a temperature range of between -5.6 °C and -6.7 °C, respectively. One advantage for the application of such BGs over their living recombinant mother bacteria is that they are non-living native cell envelopes retaining the biophysical properties of ice nucleation and do no longer represent genetically modified organisms (GMOs).

Carbon dioxide capture using Escherichia coli expressing carbonic anhydrase in a foam bioreactor

The present study reports CO2 capture and conversion to bicarbonate using Escherichia coli expressing carbonic anhydrase (CA) on its cell surface in a novel foam bioreactor. The very large gas-liquid interfacial area in the foam bioreactor promoted rapid CO2 absorption while the CO2 in the aqueous phase was subsequently converted to bicarbonate ions by the CA. CO2 gas removal in air was investigated at various conditions such as gas velocity, cell density and CO2 inlet concentration. Regimes for kinetic and mass transfer limitations were defined. Very high removal rates of CO2 were observed: 9570 g CO2 m(-3) bioreactor h(-1) and a CO2 removal efficiency of 93% at 4% inlet CO2 when the gas retention time was 24 s, and cell concentration was 4 gdw L(-1). These performances are superior to earlier reports of experimental bioreactors using CA for CO2 capture. Overall, this bioreactor system has significant potential as an alternative CO2 capture technology.

Site-Specific and High-Loading Immobilization of Proteins by Using Cohesin-Dockerin and CBM-Cellulose Interactions

Immobilization of enzymes enhances their properties for application in industrial processes as reusable and robust biocatalysts. Here, we developed a new immobilization method by mimicking the natural cellulosome system. A group of cohesin and carbohydrate-binding module (CBM)-containing scaffoldins were genetically engineered, and their length was controlled by cohesin number. To use green fluorescent protein (GFP) as an immobilization model, its C-terminus was fused with a dockerin domain. GFP was able to specifically bind to scaffoldin via cohesin-dockerin interaction, while the scaffoldin could attach to cellulose by CBM-cellulose interaction. Our results showed that this mild and convenient approach was able to achieve site-specific immobilization, and the maximum GFP loading capacity reached ∼0.508 μmol/g cellulose.

Engineering yeast with bifunctional minicellulosome and cellodextrin pathway for co-utilization of cellulose-mixed sugars

BACKGROUND

Consolidated bioprocessing (CBP), integrating cellulase production, cellulose saccharification, and fermentation into one step has been widely considered as the ultimate low-cost configuration for producing second-generation fuel ethanol. However, the requirement of a microbial strain able to hydrolyze cellulosic biomass and convert the resulting sugars into high-titer ethanol limits CBP application.

RESULTS

In this work, cellulolytic yeasts were developed by engineering Saccharomyces cerevisiae with a heterologous cellodextrin utilization pathway and bifunctional minicellulosomes. The cell-displayed minicellulosome was two-scaffoldin derived, and contained an endoglucanase and an exoglucanase, while the intracellular cellodextrin pathway consisted of a cellodextrin transporter and a β-glucosidase, which mimicked the unique cellulose-utilization system in Clostridium thermocellum and allowed S. cerevisiae to degrade and use cellulose without glucose inhibition/repression on cellulases and mixed-sugar uptake. Consequently, only a small inoculation of the non-induced yeast cells was required to efficiently co-convert both cellulose and galactose to ethanol in a single-step co-fermentation process, achieving a high specific productivity of ~62.61 mg cellulosic ethanol/g cell·h from carboxymethyl cellulose and ~56.37 mg cellulosic ethanol/g cell·h from phosphoric acid-swollen cellulose.

CONCLUSIONS

Our work provides a versatile engineering strategy for co-conversion of cellulose-mixed sugars to ethanol by S. cerevisiae, and the achievements in this work may further promote cellulosic biofuel production.

Effects of novel auto-inducible medium on growth, activity and CO₂ capture capacity of Escherichia coli expressing carbonic anhydrase

A glucose-based auto-inducible medium (glucose-AIM) has been developed to enhance both growth and expression of lac operon-linked carbonic anhydrase (CA) expression in a recombinant strain of Escherichia coli. When the E. coli expressing CA was grown on various media, the glucose-based auto-inducible medium (glucose AIM) resulted in a CA activity of 1022 mU OD(600 nm)(-1) mL(-1) at 24 h and a specific growth rate of 0.082 h(-1). The CA activity was four to fourteen times higher than those by LB-IPTG. The E. coli expressing CA grown on the glucose-AIM showed highest activity at pH8.5 while it kept high stability up to 40°C and an inlet CO2 concentration of 6%. These findings indicate that the glucose-AIM would be a cost-effective medium to support high cell growth, CA activity and stability for effective CO2 capture.

Contemplating 3-Hydroxypropionic Acid Biosynthesis in Klebsiella pneumoniae

3-Hydroxypropionic acid (3-HP) is a commercially valuable platform chemical from which an array of C3 compounds can be generated. Klebsiella pneumoniae has been considered a promising species for biological production of 3-HP. Despite a wealth of reports related to 3-HP biosynthesis in K. pneumoniae, its commercialization is still in infancy. The major hurdle hindering 3-HP overproduction lies in the poor understanding of glycerol dissimilation in K. pneumoniae. To surmount this problem, this review aims to portray a picture of 3-HP biosynthesis, involving 3-HP-synthesizing strains, biochemical attributes, metabolic pathways and key enzymes. Inspired by the state-of-the-art advances in metabolic engineering and synthetic biology, here we advocate protocols for overproducing 3-HP in K. pneumoniae. These protocols range from cofactor regeneration, alleviation of metabolite toxicity, genome editing, remodeling of transport system, to carbon flux partition via logic gate. The feasibility for these protocols was also discussed.

Genetic manipulation of outer membrane permeability: generating porous heterogeneous catalyst analogs in Escherichia coli

The limited permeability of the E. coli outer membrane can significantly hinder whole-cell biocatalyst performance. In this study, the SARS coronavirus small envelope protein (SCVE) was expressed in E. coli cells previously engineered for periplasmic expression of carbonic anhydrase (CA) activity. This maneuver increased small molecule uptake by the cells, resulting in increased apparent CA activity of the biocatalysts. The enhancements in activity were quantified using methods developed for traditional heterogeneous catalysis. The expression of the SCVE protein was found to significantly reduce the Thiele moduli (ϕ), as well as increase the effectiveness factors (η), effective diffusivities (De), and permeabilities (P) of the biocatalysts. These catalytic improvements translated into superior performance of the biocatalysts for the precipitation of calcium carbonate from solution which is an attractive strategy for long-term sequestration of captured carbon dioxide. Overall, these results demonstrate that synthetic biology approaches can be used to enhance heterogeneous catalysts incorporated into microbial whole-cell scaffolds.

Surface Display of Small Peptides on Escherichia coli for Enhanced Calcite Precipitation Rates

Mineralization has emerged as a promising strategy for long-term carbon sequestration. These processes involve carbon dioxide hydration followed by mineral precipitation. We have explored the production of whole-cell biocatalysts engineered with carbonic anhydrase (CA) activity to accelerate the CO₂ hydration reaction. In this study, short polypeptides were displayed on the surface of E. coli cells and whole-cell biocatalysts containing periplasmically expressed CAs in an attempt to enhance calcite mineral formation. It was found that cells coexpressing recombinant periplasmic CA and surface-displayed GPA peptide (PEVPEGAFDTAI) outperformed other peptide-expressing biocatalysts evaluated in terms of the amount of precipitate formed, as well as the overall formation rate of solids. Cells expressing the Cab CA isoform (BLR-pCab) and Cam isoform (BLR-pCam) with the surface-displayed GPA peptide exhibited 36 and 59% improvements in precipitation amounts, as well as 18 and 60% improvements in overall formation rates, respectively, over similar biocatalysts without GPA expression. The biocatalyst with the best performance was BLR-pCam/GPA, which generated 0.15 g of CaCO₃, while BLR cells generated only 0.08 g of CaCO₃ under the same small batch reaction conditions. The BLR-pCam/GPA cells also exhibited the fastest formation rates, achieving the maximum change in solution turbidity after only 2.2 min, as opposed to 6.3 min for BLR cells. These results demonstrate that synthetic biology approaches can be used to create novel biocatalysts with the ability to enhance both catalysis and precipitation activities.

Carbonic anhydrases in industrial applications

Carbonic anhydrases (CAs) catalyze a fundamental reaction: the reversible hydration and dehydration of carbon dioxide (CO2) and bicarbonate ([Formula: see text]), respectively. Current methods for CO2 capture and sequestration are harsh, expensive, and require prohibitively large energy inputs, effectively negating the purpose of removing CO2 from the atmosphere. Due to CA's activity on CO2 there is increasing interest in using CAs for industrial applications such as carbon sequestration and biofuel production. A lot of work in the last decade has focused on immobilizing CA onto various supports for incorporation into CO2 scrubbing applications or devices. Although the proof of principle has been validated, current CAs being tested do not withstand the harsh industrial conditions. The advent of large-scale genome sequencing projects has resulted in several emerging efforts seeking out novel CAs from a variety of microorganisms, including bacteria, micro-, and macro-algae. CAs are also being investigated for their use in medical applications, such drug delivery systems and artificial lungs. This review also looks at possible downstream uses of captured and sequestered CO2, from using it to enhance oil recovery to incorporating it into useful and financially viable products.

Engineered Escherichia coli with periplasmic carbonic anhydrase as a biocatalyst for CO2 sequestration

Carbonic anhydrase is an enzyme that reversibly catalyzes the hydration of carbon dioxide (CO2). It has been suggested recently that this remarkably fast enzyme can be used for sequestration of CO2, a major greenhouse gas, making this a promising alternative for chemical CO2 mitigation. To promote the economical use of enzymes, we engineered the carbonic anhydrase from Neisseria gonorrhoeae (ngCA) in the periplasm of Escherichia coli, thereby creating a bacterial whole-cell catalyst. We then investigated the application of this system to CO2 sequestration by mineral carbonation, a process with the potential to store large quantities of CO2. ngCA was highly expressed in the periplasm of E. coli in a soluble form, and the recombinant bacterial cell displayed the distinct ability to hydrate CO2 compared with its cytoplasmic ngCA counterpart and previously reported whole-cell CA systems. The expression of ngCA in the periplasm of E. coli greatly accelerated the rate of calcium carbonate (CaCO3) formation and exerted a striking impact on the maximal amount of CaCO3 produced under conditions of relatively low pH. It was also shown that the thermal stability of the periplasmic enzyme was significantly improved. These results demonstrate that the engineered bacterial cell with periplasmic ngCA can successfully serve as an efficient biocatalyst for CO2 sequestration.

Engineered yeast for enhanced CO2 mineralization

In this work, a biologically catalyzed CO₂ mineralization process for the capture of CO₂ from point sources was designed, constructed at a laboratory scale, and, using standard chemical process scale-up protocols, was modeled and evaluated at an industrial scale. A yeast display system in Saccharomyces cerevisae was used to screen several carbonic anhydrase isoforms and mineralization peptides for their impact on CO₂ hydration, CaCO₃ mineralization, and particle settling rate. Enhanced rates for each of these steps in the CaCO₃ mineralization process were confirmed using quantitative techniques in lab-scale measurements. The effect of these enhanced rates on the CO₂ capture cost in an industrial scale CO₂ mineralization process using coal fly ash as the CaO source was evaluated. The model predicts a process using bCA2- yeast and fly ash is ~10% more cost effective per ton of CO₂ captured than a process with no biological molecules, a savings not realized by wild-type yeast and high-temperature stable recombinant CA2 alone or in combination. The levelized cost of electricity for a power plant using this process was calculated and scenarios in which this process compares favorably to CO₂ capture by MEA absorption process are presented.

Self-surface assembly of cellulosomes with two miniscaffoldins on Saccharomyces cerevisiae for cellulosic ethanol production

Yeast to directly convert cellulose and, especially, the microcrystalline cellulose into bioethanol, was engineered through display of minicellulosomes on the cell surface of Saccharomyces cerevisiae. The construction and cell surface attachment of cellulosomes were accomplished with two individual miniscaffoldins to increase the display level. All of the cellulases including a celCCA (endoglucanase), a celCCE (cellobiohydrolase), and a Ccel_2454 (β-glucosidase) were cloned from Clostridium cellulolyticum, ensuring the thermal compatibility between cellulose hydrolysis and yeast fermentation. Cellulases and one of miniscaffoldins were secreted by α-factor; thus, the assembly and attachment to anchoring miniscaffoldin were accomplished extracellularly. Immunofluorescence microscopy, flow cytometric analysis (FACS), and cellulosic ethanol fermentation confirmed the successful display of such complex on the yeast surface. Enzyme-enzyme synergy, enzyme-proximity synergy, and cellulose-enzyme-cell synergy were analyzed, and the length of anchoring miniscaffoldin was optimized. The engineered S. cerevisiae was applied in fermentation of carboxymethyl cellulose (CMC), phosphoric acid-swollen cellulose (PASC), or Avicel. It showed a significant hydrolytic activity toward microcrystalline cellulose, with an ethanol titer of 1,412 mg/L. This indicates that simultaneous saccharification and fermentation of crystalline cellulose to ethanol can be accomplished by the yeast, engineered with minicellulosome.