A consensus-guided approach yields a heat-stable alkane-producing enzyme and identifies residues promoting thermostability

Proteomics and metabolic burden analysis to understand the impact of recombinant protein production in E. coli

The impact of recombinant protein production (RPP) on host cells and the metabolic burden associated with it undermine the efficiency of the production system. This study utilized proteomics to investigate the dynamics of parent and recombinant cells induced at different time points for RPP. The results revealed significant changes in both transcriptional and translational machinery that may have impacted the metabolic burden, growth rate of the culture and the RPP. The timing of protein synthesis induction also played a critical role in the fate of the recombinant protein within the host cell, affecting protein and product yield. The study identified significant differences in the expression of proteins involved in fatty acid and lipid biosynthesis pathways between two E. coli host strains (M15 and DH5⍺), with the E. coli M15 strain demonstrating superior expression characteristics for the recombinant protein. Overall, these findings contribute to the knowledge base for rational strain engineering for optimized recombinant protein production.

Acid-resistant enzymes: the acquisition strategies and applications

Enzymes have promising applications in chemicals, food, pharmaceuticals, and other variety products because of their high efficiency, specificity, and environmentally friendly properties. However, due to the complexity of raw materials, pH, temperature, solvents, etc., the application range of enzymes is greatly limited in the industry. Protein engineering and enzyme immobilization are classical strategies to overcome the limitations of industrial applications. Although the pH tendency of enzymes has been extensively researched, the mechanism underlying enzyme acid resistance is unclear, and a less practical strategy for altering the pH propensity of enzymes has been suggested. This review proposes that the optimum pH of enzyme is determined by the pKa values of active center ionizable amino acid residues. Three levels of acquiring acid-resistant enzymes are summarized: mining from extreme environments and enzyme databases, modification with protein engineering and enzyme microenvironment engineering, and de novo synthesis. The industrial applications of acid-resistant enzymes in chemicals, food, and pharmaceuticals are also summarized. KEY POINTS: • The mechanism of enzyme acid resistance is fundamentally determined. • The three aspects of the method for acquiring acid-resistant enzymes are summarized. • Computer-aided strategies and artificial intelligence are used to obtain acid-resistant enzymes.

Thermostability enhancement and insight of L-asparaginase from Mycobacterium sp. via consensus-guided engineering

Acrylamide alleviation in food has represented as a critical issue due to its neurotoxic effect on human health. L-Asparaginase (ASNase, EC 3.5.1.1) is considered a potential additive for acrylamide alleviation in food. However, low thermal stability hinders the application of ASNase in thermal food processing. To obtain highly thermal stable ASNase for its industrial application, a consensus-guided approach combined with site-directed saturation mutation (SSM) was firstly reported to engineer the thermostability of Mycobacterium gordonae L-asparaginase (GmASNase). The key residues Gly97, Asn159, and Glu249 were identified for improving thermostability. The combinatorial triple mutant G97T/N159Y/E249Q (TYQ) displayed significantly superior thermostability with half-life values of 61.65 ± 8.69 min at 50 °C and 5.12 ± 1.66 min at 55 °C, whereas the wild-type was completely inactive at these conditions. Moreover, its T_m value increased by 8.59 °C from parent wild-type. Interestingly, TYQ still maintained excellent catalytic efficiency and specific activity. Further molecular dynamics and structure analysis revealed that the additional hydrogen bonds, increased hydrophobic interactions, and favorable electrostatic potential were essential for TYQ being in a more rigid state for thermostability enhancement. These results suggested that our strategy was an efficient engineering approach for improving fundamental properties of GmASNase and offering GmASNase as a potential agent for efficient acrylamide mitigation in food industry. KEY POINTS: • The thermostability of GmASNase was firstly improved by consensus-guided engineering. • The half-life and T_m value of triple mutant TYQ were significantly increased. • Insight on improved thermostability of TYQ was revealed by MD and structure analysis.

Strategies for tailoring pH performances of glycoside hydrolases

Glycoside hydrolases (GHs) exhibit high activity and stability under harsh conditions, such as high temperatures and extreme pHs, given their wide use in industrial biotechnology. However, strategies for improving the acidophilic and alkalophilic adaptations of GHs are poorly summarized due to the complexity of the mechanisms of these adaptations. This review not only highlights the adaptation mechanisms of acidophilic and alkalophilic GHs under extreme pH conditions, but also summarizes the recent advances in engineering the pH performances of GHs with a focus on four strategies of protein engineering, enzyme immobilization, chemical modification, and medium engineering (additives). The examples described here summarize the methods used in modulating the pH performances of GHs and indicate that methods integrated in different protein engineering techniques or methods are efficient to generate industrial biocatalysts with the desired pH performance and other adapted enzyme properties.

Insights into cyanobacterial alkane biosynthesis

Alkanes are high-energy molecules that are compatible with enduring liquid fuel infrastructures, which make them highly suitable for being next-generation biofuels. Though biological production of alkanes has been reported in various microorganisms, the reports citing photosynthetic cyanobacteria as natural producers have been the most consistent for the long-chain alkanes and alkenes (C15-C19). However, the production of alkane in cyanobacteria is low, leading to its extraction being uneconomical for commercial purposes. In order to make alkane production economically feasible from cyanobacteria, the titre and yield need to be increased by several orders of magnitude. In the recent past, efforts have been made to enhance alkane production, although with a little gain in yield, leaving space for much improvement. Genetic manipulation in cyanobacteria is considered challenging, but recent advancements in genetic engineering tools may assist in manipulating the genome in order to enhance alkane production. Further, advancement in a basic understanding of metabolic pathways and gene functioning will guide future research for harvesting the potential of these tiny photosynthetically efficient factories. In this review, our focus would be to highlight the current knowledge available on cyanobacterial alkane production, and the potential aspects of developing cyanobacterium as an economical source of biofuel. Further insights into different metabolic pathways and hosts explored so far, and possible challenges in scaling up the production of alkanes will also be discussed.

Biophysical and structural studies reveal marginal stability of a crucial hydrocarbon biosynthetic enzyme acyl ACP reductase

Acyl-ACP reductase (AAR) is one of the two key cyanobacterial enzymes along with aldehyde deformylating oxygenase (ADO) involved in the synthesis of long-chain alkanes, a drop-in biofuel. The enzyme is prone to aggregation when expressed in Escherichia coli, leading to varying alkane levels. The present work attempts to investigate the crucial structural aspects of AAR protein associated with its stability and folding. Characterization by dynamic light scattering experiment and intact mass spectrometry revealed that recombinantly expressed AAR in E. coli existed in multiple-sized protein particles due to diverse lipidation. Interestingly, while thermal- and urea-based denaturation of AAR showed 2-state unfolding transition in circular dichroism and intrinsic fluorescent spectroscopy, the unfolding process of AAR was a 3-state pathway in GdnHCl solution suggesting that the protein milieu plays a significant role in dictating its folding. Apparent standard free energy \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\left( {\Delta {\text{G}}_{{{\text{NU}}}}^{{{\text{H}}_{2} {\text{O}}}} } \right)$$\end{document}ΔGNUH2O of ~ 4.5 kcal/mol for the steady-state unfolding of AAR indicated borderline stability of the protein. Based on these evidences, we propose that the marginal stability of AAR are plausible contributing reasons for aggregation propensity and hence the low catalytic activity of the enzyme when expressed in E. coli for biofuel production. Our results show a path for building superior biocatalyst for higher biofuel production.

Microbial engineering to produce fatty alcohols and alkanes

Owing to their high energy density and composition, fatty acid-derived chemicals possess a wide range of applications such as biofuels, biomaterials, and other biochemical, and as a consequence, the global annual demand for products has surpassed 2 million tons. With the exhausting petroleum reservoirs and emerging environmental concerns on using petroleum feedstock, it has become indispensable to shift to a renewable-based industry. With the advancement in the field of synthetic biology and metabolic engineering, the use of microbes as factories for the production of fatty acid-derived chemicals is becoming a promising alternative approach for the production of these derivatives. Numerous metabolic approaches have been developed for conditioning the microbes to improve existing or develop new methodologies capable of efficient oleochemical production. However, there still exist several limitations that need to be addressed for the commercial viability of the microbial cell factory production. Though substantial advancement has been made toward successfully producing these fatty acids derived chemicals, a considerable amount of work needs to be done for improving the titers. In the present review, we aim to address the roadblocks impeding the heterologous production, the engineering pathway strategies implemented across the range of microbes in a detailed manner, and the commercial readiness of these molecules of immense application.

High-level expression of a β-mannanase (manB) in Pichia pastoris GS115 for mannose production with Penicillium brevicompactum fermentation pretreatment of soybean meal

An endo-1,4-β-mannanase gene (manB) from a Bacillus pumilus Nsic-2 grown in a stinky tofu emulsion was cloned and expressed in Pichia pastoris GS115. After characterized, the endo-1,4-β-mannanase (manB) show maximum activity at pH 6.0 and 50 °C with LBG as substrate and perform high stability at a range of pH 6-8. After applying for a shake flask fermentation, the specific activity of manB reached 3462 U/mg. To produce mannose, the soybean meal (SBM) was pretreated by biological fermentation for 11 days with Penicillium brevicompactum, and then hydrolyzed by manB. As a result, mannose yield reached 3.58 g per 1 kg SBM which indicated that 0.358% SBM was converted into mannose after hydrolyzation, and mean a total 20% mannan of SBM converting into mannose, while the control group demonstrated only 1.78% conversion. An effective β-mannanase for the bioconversion of mannan-rich biomasses and an efficient method to produce mannose with soybean meal were introduced.

Tailoring an aldo-keto reductase KmAKR for robust thermostability and catalytic efficiency by stepwise evolution and structure-guided consensus engineering

t-Butyl 6-cyano-(3R,5R)-dihydroxyhexanoate ((3R,5R)-2) is an advanced chiral diol intermediate of the cholesterol-lowering drug atorvastatin. KmAKR_M5 (W297H/Y296W/K29H/Y28A/T63M) constructed in our previous work, displayed good biocatalytic performance on (3R,5R)-2. In the present work, stepwise evolution was applied to further enhance the thermostability and activity of KmAKR_M5. For thermostability enhancement, N109 and S196 located far from the active site were picked out by structure-guided consensus engineering, and mutated by site-directed mutagenesis (SDM). For catalytic efficiency improvement, the residues A30 and T302 adjacent to the substrate-binding pocket were subjected to site-saturation mutagenesis (SSM). As a result, the "best" mutant KmAKR_M9 (W297H/Y296W/K29H/Y28A/T63M/A30P/T302S/N109K/S196C) was developed, of which T₅₀¹⁵ and T_m were 5.0 °C and 8.2 °C higher than those of KmAKR_M5. Moreover, compared to KmAKR_M5, KmAKR_M9 displayed a 1.9-fold (846 vs 2436 min) and 6.7-fold (126 vs 972 min) longer half-lives at 40 and 50 °C, respectively. Structural analysis suggested that beneficial mutations introduced additional hydrophobic interactions and hydrogen bonds, contributing rigidification of the flexible loops and the increase of internal forces, hence increasing the thermostability and activity. 5 g DCW (dry cell weight) L^-1KmAKR_M9 completely reduced 350 g L^-1t-butyl 6-cyano-(5R)-hydroxy-3-oxo-hexanoate ((5R)-1), within 3.7 h at 40 °C, yielding optically pure (3R,5R)-2 (d.e._p > 99.5%) with a space-time yield (STY) of 1.82 kg L^-1 d^-1. Hence, KmAKR_M9 is a robust biocatalyst for the synthesis of (3R,5R)-2.

Cyanobacterial aldehyde deformylating oxygenase: Structure, function, and potential in biofuels production

The conversion of aldehydes to valuable alkanes via cyanobacterial aldehyde deformylating oxygenase is of great interest. The availability of fossil reserves that keep on decreasing due to human exploitation is worrying, and even more troubling is the combustion emission from the fuel, which contributes to the environmental crisis and health issues. Hence, it is crucial to use a renewable and eco-friendly alternative that yields compound with the closest features as conventional petroleum-based fuel, and that can be used in biofuels production. Cyanobacterial aldehyde deformylating oxygenase (ADO) is a metal-dependent enzyme with an α-helical structure that contains di‑iron at the active site. The substrate enters the active site of every ADO through a hydrophobic channel. This enzyme exhibits catalytic activity toward converting C_n aldehyde to C_n-1 alkane and formate as a co-product. These cyanobacterial enzymes are small and easy to manipulate. Currently, ADOs are broadly studied and engineered for improving their enzymatic activity and substrate specificity for better alkane production. This review provides a summary of recent progress in the study of the structure and function of ADO, structural-based engineering of the enzyme, and highlight its potential in producing biofuels.

Protein Sequence Selection Method That Enables Full Consensus Design of Artificial l-Threonine 3-Dehydrogenases with Unique Enzymatic Properties

Exponentially increasing protein sequence data enables artificial enzyme design using sequence-based protein design methods, including full-consensus protein design (FCD). The success of artificial enzyme design is strongly dependent on the nature of the sequences used. Hence, sequences must be selected from databases and curated libraries prepared to enable a successful design by FCD. In this study, we proposed a selection approach regarding several key residues as sequence motifs. We used l-threonine 3-dehydrogenase (TDH) as a model to test the validity of this approach. In the classification, four residues (143, 174, 188, and 214) were used as key residues. We classified thousands of TDH homologous sequences into five groups containing hundreds of sequences. Utilizing sequences in the libraries, we designed five artificial TDHs by FCD. Among the five, we successfully expressed four in soluble form. Biochemical analysis of artificial TDHs indicated that their enzymatic properties vary; half of the maximum measured enzyme activity (t_1/2) and activation energies were distributed from 53 to 65 °C and from 38 to 125 kJ/mol, respectively. The artificial TDHs had unique kinetic parameters, distinct from one another. Structural analysis indicates that consensus mutations are mainly introduced in the secondary or outer shell. The functional diversity of the artificial TDHs is due to the accumulation of mutations that affect their physicochemical properties. Taken together, our findings indicate that our proposed approach can help generate artificial enzymes with unique enzymatic properties.

In vivo Quantification of Alkanes in Escherichia coli

Microbial production of alkanes employing synthetic biology tools has gained tremendous attention owing to the high energy density and similarity of alkanes to existing petroleum fuels. One of the most commonly studied pathways includes the production of alkanes by AAR (acyl-ACP (acyl carrier protein) reductase)-ADO (aldehyde deformylating oxygenase) pathway. Here, the intermediates of fatty acid synthesis pathway are used as substrate by the AAR enzyme to make fatty aldehyde, which is then deformylated by ADO to make linear chain alkane. However, the variation in substrate availability to the first enzyme of the pathway, i.e., AAR, via fatty acid synthesis pathway and low turnover of the ADO enzyme make calculation of yields and titers under in vivo conditions extremely difficult. In vivo assay employing external addition of defined substrates for ADO enzyme into the medium helps to monitor the influx of substrate hence providing a more accurate measurement of the product yields. In this protocol, we include a detailed guide for implementing the in vivo assay for monitoring alkane production in E. coli.

Building cell factories for the production of advanced fuels

Synthetic biology-based engineering strategies are being extensively employed for microbial production of advanced fuels. Advanced fuels, being comparable in energy efficiency and properties to conventional fuels, have been increasingly explored as they can be directly incorporated into the current fuel infrastructure without the need for reconstructing the pre-existing set-up rendering them economically viable. Multiple metabolic engineering approaches have been used for rewiring microbes to improve existing or develop newly programmed cells capable of efficient fuel production. The primary challenge in using these approaches is improving the product yield for the feasibility of the commercial processes. Some of the common roadblocks towards enhanced fuel production include - limited availability of flux towards precursors and desired pathways due to presence of competing pathways, limited cofactor and energy supply in cells, the low catalytic activity of pathway enzymes, obstructed product transport, and poor tolerance of host cells for end products. Consequently, despite extensive studies on the engineering of microbial hosts, the costs of industrial-scale production of most of these heterologously produced fuel compounds are still too high. Though considerable progress has been made towards successfully producing some of these biofuels, a substantial amount of work needs to be done for improving the titers of others. In this review, we have summarized the different engineering strategies that have been successfully used for engineering pathways into commercial hosts for the production of advanced fuels and different approaches implemented for tuning host strains and pathway enzymes for scaling up production levels.

Thermostabilization of Membrane Proteins by Consensus Mutation: A Case Study for a Fungal Δ8-7 Sterol Isomerase

Membrane proteins are generally challenging to work with because of their notorious instability. Protein engineering has been used increasingly to thermostabilize labile membrane proteins such as G-protein-coupled receptors for structural and functional studies in recent years. Two major strategies exist. Scanning mutagenesis systematically eliminates destabilizing residues, whereas the consensus approach assembles mutants with the most frequent residues among selected homologs, bridging sequence conservation with stability. Here, we applied the consensus concept to stabilize a fungal homolog of the human sterol Δ8-7 isomerase, a 26.4 kDa protein with five transmembrane helices. The isomerase is also called emopamil-binding protein (EBP), as it binds this anti-ischemic drug with high affinity. The wild-type had an apparent melting temperature (T_m) of 35.9 °C as measured by the fluorescence-detection size-exclusion chromatography-based thermostability assay. A total of 87 consensus mutations sourced from 22 homologs gained expression level and thermostability, increasing the apparent T_m to 69.9 °C at the cost of partial function loss. Assessing the stability and activity of several systematic chimeric constructs identified a construct with an apparent T_m of 79.8 °C and two regions for function rescue. Further back-mutations of the chimeric construct in the two target regions yielded the final construct with similar apparent activity to the wild-type and an elevated T_m of 88.8 °C, totaling an increase of 52.9 °C. The consensus approach is effective and efficient because it involves fewer constructs compared with scanning mutagenesis. Our results should encourage more use of the consensus strategy for membrane protein thermostabilization.

A novel C-terminal degron identified in bacterial aldehyde decarbonylases using directed evolution

BACKGROUND

Aldehyde decarbonylases (ADs), which convert acyl aldehydes into alkanes, supply promising solution for producing alkanes from renewable feedstock. However the instability of ADs impedes their further application. Therefore, the current study aimed to investigate the degradation mechanism of ADs and engineer it towards high stability.

RESULTS

Here, we describe the discovery of a degradation tag (degron) in the AD from marine cyanobacterium Prochlorococcus marinus using error-prone PCR-based directed evolution system. Bioinformatic analysis revealed that this C-terminal degron is common in bacterial ADs and identified a conserved C-terminal motif, RMSAYGLAAA, representing the AD degron (ADcon). Furthermore, we demonstrated that the ATP-dependent proteases ClpAP and Lon are involved in the degradation of AD-tagged proteins in E. coli, thereby limiting alkane production. Deletion or modification of the degron motif increased alkane production in vivo.

CONCLUSION

This work revealed the presence of a novel degron in bacterial ADs responsible for its instability. The in vivo experiments proved eliminating or modifying the degron could stabilize AD, thereby producing higher titers of alkanes.

Identification of non-conserved residues essential for improving the hydrocarbon-producing activity of cyanobacterial aldehyde-deformylating oxygenase

BACKGROUND

Cyanobacteria produce hydrocarbons corresponding to diesel fuels by means of aldehyde-deformylating oxygenase (ADO). ADO catalyzes a difficult and unusual reaction in the conversion of aldehydes to hydrocarbons and has been widely used for biofuel production in metabolic engineering; however, its activity is low. A comparison of the amino acid sequences of highly active and less active ADOs will elucidate non-conserved residues that are essential for improving the hydrocarbon-producing activity of ADOs.

RESULTS

Here, we measured the activities of ADOs from 10 representative cyanobacterial strains by expressing each of them in Escherichia coli and quantifying the hydrocarbon yield and amount of soluble ADO. We demonstrated that the activity was highest for the ADO from Synechococcus elongatus PCC 7942 (7942ADO). In contrast, the ADO from Gloeobacter violaceus PCC 7421 (7421ADO) had low activity but yielded high amounts of soluble protein, resulting in a high production level of hydrocarbons. By introducing 37 single amino acid substitutions at the non-conserved residues of the less active ADO (7421ADO) to make its sequence more similar to that of the highly active ADO (7942ADO), we found 20 mutations that improved the activity of 7421ADO. In addition, 13 other mutations increased the amount of soluble ADO while maintaining more than 80% of wild-type activity. Correlation analysis showed a solubility-activity trade-off in ADO, in which activity was negatively correlated with solubility.

CONCLUSIONS

We succeeded in identifying non-conserved residues that are essential for improving ADO activity. Our results may be useful for generating combinatorial mutants of ADO that have both higher activity and higher amounts of the soluble protein in vivo, thereby producing higher yields of biohydrocarbons.

Cyanobacterial Enzymes for Bioalkane Production

Cyanobacterial biosynthesis of alkanes is an attractive way of producing substitutes for petroleum-based fuels. Key enzymes for bioalkane production in cyanobacteria are acyl-ACP reductase (AAR) and aldehyde-deformylating oxygenase (ADO). AAR catalyzes the reduction of the fatty acyl-ACP/CoA substrates to fatty aldehydes, which are then converted into alkanes/alkenes by ADO. These enzymes have been widely used for biofuel production by metabolic engineering of cyanobacteria and other organisms. However, both proteins, particularly ADO, have low enzymatic activities, and their catalytic activities are desired to be improved for use in biofuel production. Recently, progress has been made in the basic sciences and in the application of AAR and ADO in alkane production. This chapter provides an overview of recent advances in the study of the structure and function of AAR and ADO, protein engineering of these enzymes for improving activity and modifying substrate specificities, and examples of metabolic engineering of cyanobacteria and other organisms using AAR and ADO for biofuel production.