Biotechnological process for production of beta-dipeptides from cyanophycin on a technical scale and its optimization

Bridging Nature and Engineering: Protein-Derived Materials for Bio-Inspired Applications

The sophisticated, elegant protein-polymers designed by nature can serve as inspiration to redesign and biomanufacture protein-based materials using synthetic biology. Historically, petro-based polymeric materials have dominated industrial activities, consequently transforming our way of living. While this benefits humans, the fabrication and disposal of these materials causes environmental sustainability challenges. Fortunately, protein-based biopolymers can compete with and potentially surpass the performance of petro-based polymers because they can be biologically produced and degraded in an environmentally friendly fashion. This paper reviews four groups of protein-based polymers, including fibrous proteins (collagen, silk fibroin, fibrillin, and keratin), elastomeric proteins (elastin, resilin, and wheat glutenin), adhesive/matrix proteins (spongin and conchiolin), and cyanophycin. We discuss the connection between protein sequence, structure, function, and biomimetic applications. Protein engineering techniques, such as directed evolution and rational design, can be used to improve the functionality of natural protein-based materials. For example, the inclusion of specific protein domains, particularly those observed in structural proteins, such as silk and collagen, enables the creation of novel biomimetic materials with exceptional mechanical properties and adaptability. This review also discusses recent advancements in the production and application of new protein-based materials through the approach of synthetic biology combined biomimetics, providing insight for future research and development of cutting-edge bio-inspired products. Protein-based polymers that utilize nature's designs as a base, then modified by advancements at the intersection of biology and engineering, may provide mankind with more sustainable products.

Recombinant Multi-l-Arginyl-Poly-l-Aspartate (Cyanophycin) as an Emerging Biomaterial

Multi-l-arginyl-poly-l-aspartate (MAPA) is a non-ribosomal polypeptide which synthesis is directed by cyanophycin synthetase, and its production can be achieved using recombinant microorganisms carrying the cphA gene. Along its poly-aspartate backbone, arginine or lysine links to each aspartate via an isopeptide bond. MAPA is a zwitterionic polyelectrolyte full of charged carboxylic, amine, and guanidino groups. In aqueous solution, MAPA exhibits dual thermal and pH responses similar to those stimuli-responsive polymers. Being biocompatible, the films containing MAPA can support cell proliferation and elicits minimal immune response in macrophages. Dipeptides from MAPA after enzymatic treatments can provide nutritional benefits. In light of the increasing interest in MAPA, this article focuses on the recent discovery of the function of cyanophycin synthetase and the potentials of MAPA as a biomaterial.

Theoretical Studies of Cyanophycin Dipeptides as Inhibitors of Tyrosinases

The three-dimensional structure of tyrosinase has been crystallized from many species but not from Homo sapiens. Tyrosinase is a key enzyme in melanin biosynthesis, being an important target for melanoma and skin-whitening cosmetics. Several studies employed the structure of tyrosinase from Agaricus bisporus as a model enzyme. Recently, 98% of human genome proteins were elucidated by AlphaFold. Herein, the AlphaFold structure of human tyrosinase and the previous model were compared. Moreover, tyrosinase-related proteins 1 and 2 were included, along with inhibition studies employing kojic and cinnamic acids. Peptides are widely studied for their inhibitory activity of skin-related enzymes. Cyanophycin is an amino acid polymer produced by cyanobacteria and is built of aspartic acid and arginine; arginine can be also replaced by other amino acids. A new set of cyanophycin-derived dipeptides was evaluated as potential inhibitors. Aspartate–glutamate showed the strongest interaction and was chosen as a leading compound for future studies.

Cyanophycin Modifications-Widening the Application Potential

A circular bioeconomy approach is essential to slowing down the fearsome ongoing climate change. Replacing polymers derived from fossil fuels with biodegradable biobased polymers is one crucial part of this strategy. Cyanophycin is a polymer consisting of amino acids produced by cyanobacteria with many potential applications. It consists mainly of aspartic acid and arginine, however, its composition may be changed at the production stage depending on the conditions of the polymerization reaction, as well as the characteristics of the enzyme cyanophycin synthetase, which is the key enzyme of catalysis. Cyanophycin synthetases from many sources were expressed heterologously in bacteria, yeast and plants aiming at high yields of the polymer or at introducing different amino acids into the structure. Furthermore, cyanophycin can be modified at the post-production level by chemical and enzymatic methods. In addition, cyanophycin can be combined with other compounds to yield hybrid materials. Although cyanophycin is an attractive polymer for industry, its usage as a sole material remains so far limited. Finding new variants of cyanophycin may bring this polymer closer to real-world applications. This short review summarizes all modifications of cyanophycin and its variants that have been reported within the literature until now, additionally addressing their potential applications.

Incorporation of alternative amino acids into cyanophycin by different cyanophycin synthetases heterologously expressed in Corynebacterium glutamicum

Cyanophycin (multi-L-arginyl-poly-L-aspartic acid; also known as cyanophycin grana peptide [CGP]) is a biopolymer that could be used in various fields, for example, as a potential precursor for the synthesis of polyaspartic acid or for the production of CGP-derived dipeptides. To extend the applications of this polymer, it is therefore of interest to synthesize CGP with different compositions. A recent re-evaluation of the CGP synthesis in C. glutamicum has shown that C. glutamicum is a potentially interesting microorganism for CGP synthesis with a high content of alternative amino acids. This study shows that the amount of alternative amino acids can be increased by using mutants of C. glutamicum with altered amino acid biosynthesis. With the DM1729 mutant, the lysine content in the polymer could be increased up to 33.5 mol%. Furthermore, an ornithine content of up to 12.6 mol% was achieved with ORN2(P_gdh4). How much water-soluble or insoluble CGP is synthesized is strongly related to the used cyanophycin synthetase. CphA_Dh synthesizes soluble CGP exclusively. However, soluble CGP could also be isolated from cells expressing CphA₆₃₀₈Δ1 or CphA₆₃₀₈Δ1_C595S in addition to insoluble CGP in all examined strains. The point mutation in CphA₆₃₀₈Δ1_C595S partially resulted in a higher lysine content. In addition, the CGP content could be increased to 36% of the cell dry weight under optimizing growth conditions in C. glutamicum ATCC13032. All known alternative major amino acids for CGP synthesis (lysine, ornithine, citrulline, and glutamic acid) could be incorporated into CGP in C. glutamicum.

Characterization of an efficient extracellular cyanophycinase and its encoding cphEStrept. gene from Streptomyces pratensis strain YSM

Until now, no enzymes were described that hydrolyze cyanophycin granular protein (CGP) from a species of the genus Streptomyces. An isolate able to hydrolyze CGP was identified as Streptomyces pratensis strain YSM. The CGPase from S. pratensis strain YSM had an optimum activity at 42 °C and pH 8.5, and was able to degrade CGP at a rate of 12 ± 0.3 μg/mL min. Additionally, this CGPase hydrolyzes water-soluble CGP significantly faster than water-insoluble CGP. The molecular mass of CGPase subunits from S. pratensis strain YSM as determined by SDS-PAGE was about 43 kDa, and the enzyme was entirely inhibited by serine-protease inhibitors. The CGPase coding gene (cphE_Strept.) was amplified from genomic DNA using primers designed form consensus sequence of putative CGPase sequences. The cphE_Strept. was 1427 bp encoding a CGPase of 420 amino acids that showed about 44% and 22% similarities to CGPase from Pseudomonas anguilliseptica BI and Synechocystis sp. PCC 6803, respectively. The catalytic triad and serine-protease residues (GXSXG) were identified in the CphE_Strept. sequence. Dipeptides and tetrapeptides were identified as hydrolysis products. Biotechnological exploitation of S. pratensis strain YSM for CGPase production might have an advantage due to the reduction of separation costs and its ability to degrade CGP in phosphate buffer saline using actively growing or resting cells.

Xylose-Inducible Promoter Tools for Pseudomonas Species and Their Use in Implicating a Role for the Type II Secretion System Protein XcpQ in the Inhibition of Corneal Epithelial Wound Closure

Tunable control of gene expression is an invaluable tool for biological experiments. In this study, we describe a new xylose-inducible promoter system and evaluate it in both Pseudomonas aeruginosa and Pseudomonas fluorescens The P_xut promoter, derived from the P. fluorescens xut operon, was incorporated into a broad-host-range pBBR1-based plasmid and was compared to the Escherichia coli-derived P_BAD promoter using gfp as a reporter. Green fluorescent protein (GFP) fluorescence from the P_xut promoter was inducible in both Pseudomonas species, but not in E. coli, which may facilitate the cloning of genes toxic to E. coli to generate plasmids. The P_xut promoter was activated at a lower inducer concentration than P_BAD in P. fluorescens, and higher gfp levels were achieved using P_xut Flow cytometry analysis indicated that P_xut was leakier than P_BAD in the Pseudomonas species tested but was expressed in a higher proportion of cells when induced. d-Xylose as a sole carbon source did not support the growth of P. aeruginosa or P. fluorescens and is less expensive than many other commonly used inducers, which could facilitate large-scale applications. The efficacy of this system was demonstrated by its use to reveal a role for the P. aeruginosa type II secretion system gene xcpQ in bacterial inhibition of corneal epithelial cell wound closure. This study introduces a new inducible promoter system for gene expression for use in Pseudomonas species.IMPORTANCE Pseudomonas species are enormously important in human infections, in biotechnology, and as model systems for investigating basic science questions. In this study, we have developed a xylose-inducible promoter system, evaluated it in P. aeruginosa and P. fluorescens, and found it to be suitable for the strong induction of gene expression. Furthermore, we have demonstrated its efficacy in controlled gene expression to show that a type II secretion system protein from P. aeruginosa, XcpQ, is important for host-pathogen interactions in a corneal wound closure model.

Strategy for the Biosynthesis of Short Oligopeptides: Green and Sustainable Chemistry

Short oligopeptides are some of the most promising and functionally important amide bond-containing components, with widespread applications. Biosynthesis of these oligopeptides may potentially become the ultimate strategy because it has better cost efficiency and environmental-friendliness than conventional solid phase peptide synthesis and chemo-enzymatic synthesis. To successfully apply this strategy for the biosynthesis of structurally diverse amide bond-containing components, the identification and selection of specific biocatalysts is extremely important. Given that perspective, this review focuses on the current knowledge about the typical enzymes that might be potentially used for the synthesis of short oligopeptides. Moreover, novel enzymatic methods of producing desired peptides via metabolic engineering are highlighted. It is believed that this review will be helpful for technological innovation in the production of desired peptides.

Microbial production of cyanophycin: From enzymes to biopolymers

Cyanophycin is an attractive biopolymer with chemical and material properties that are suitable for industrial applications in the fields of food, medicine, cosmetics, nutrition, and agriculture. For efficient production of cyanophycin, considerable efforts have been exerted to characterize cyanophycin synthetases (CphAs) and optimize fermentations and downstream processes. In this paper, we review the characteristics of diverse CphAs from cyanobacteria and non-cyanobacteria. Furthermore, strategies for cyanophycin production in microbial strains, including Escherichia coli, Pseudomonas putida, Ralstonia eutropha, Rhizopus oryzae, and Saccharomyces cerevisiae, heterologously expressing different cphA genes are reviewed. Additionally, chemical and material properties of cyanophycin and its derivatives produced through biological or chemical modifications are reviewed in the context of their industrial applications. Finally, future perspectives on microbial production of cyanophycin are provided to improve its cost-effectiveness.

Re-evaluation of cyanophycin synthesis in Corynebacterium glutamicum and incorporation of glutamic acid and lysine into the polymer

Corynebacterium glutamicum was only examined in the early 2000s as a possible microorganism for the production of the polyamide cyanophycin (multi-L-arginyl-poly-[L-aspartic acid], CGP). CGP is a potential precursor for the synthesis of polyaspartic acid and CGP-derived dipeptides which may be of use in peptide-based clinical diets, as dietary supplements, or in livestock feeds. In the past, C. glutamicum was disregarded for CGP production due to low CGP contents and difficulties in isolating the polymer. However, considering recent advances in CGP research, the capabilities of this organism were revisited. In this study, several cyanophycin synthetases (CphA) as well as expression vectors and cultivation conditions were evaluated. The ability of C. glutamicum to incorporate additional amino acids such as lysine and glutamic acid was also examined. The strains C. glutamicum pVWEx1::cphA_Δ1 and C. glutamicum pVWEx1::cphA_BP1 accumulated up to 14% of their dry weight CGP, including soluble CGP containing more than 40 mol% of the alternative side-chain amino acid lysine. The soluble, lysine-rich form of the polymer was not detected in C. glutamicum in previous studies. Additionally, an incorporation of up to 6 mol% of glutamic acid into the backbone of CGP synthesized by C. glutamicum pVWEx1::cphA_Dh was detected. The strain accumulated up to 17% of its dry weight in soluble CGP. Although glutamic acid had previously been found to replace arginine in the side chain, this is the first time that glutamic acid was found to substitute aspartic acid in the backbone.

Influence of the Membrane Dye R18 and of DMSO on Cell Penetration of Guanidinium-Rich Peptides

A quantitative analysis by confocal fluorescence microscopy of the entry into HEK293 and MCF-7 cells by fluorescein-labeled octaarginine (1) and by three octa-Adp derivatives (2 - 4, octamers of the β-Asp-Arg-dipeptide, derived from the biopolymer cyanophycin) is described, including the effects of the membrane dye R18 and of DMSO on cell penetration.

Cell Penetration, Herbicidal Activity, and in-vivo-Toxicity of Oligo-Arginine Derivatives and of Novel Guanidinium-Rich Compounds Derived from the Biopolymer Cyanophycin

Oligo-arginines are thoroughly studied cell-penetrating peptides (CPPs, Figures 1 and 2). Previous in-vitro investigations with the octaarginine salt of the phosphonate fosmidomycin (herbicide and anti-malaria drug) have shown a 40-fold parasitaemia inhibition with P. falciparum, compared to fosmidomycin alone (Figure 3). We have now tested this salt, as well as the corresponding phosphinate salt of the herbicide glufosinate, for herbicidal activity with whole plants by spray application, hoping for increased activities, i.e. decreased doses. However, both salts showed low herbicidal activity, indicating poor foliar uptake (Table 1). Another pronounced difference between in-vitro and in-vivo activity was demonstrated with various cell-penetrating octaarginine salts of fosmidomycin: intravenous injection to mice caused exitus of the animals within minutes, even at doses as low as 1.4 μmol/kg (Table 2). The results show that use of CPPs for drug delivery, for instance to cancer cells and tissues, must be considered with due care. The biopolymer cyanophycin is a poly-aspartic acid containing argininylated side chains (Figure 4); its building block is the dipeptide H-βAsp-αArg-OH (H-Adp-OH). To test and compare the biological properties with those of octaarginines we synthesized Adp8-derivatives (Figure 5). Intravenouse injection of H-Adp8-NH2 into the tail vein of mice with doses as high as 45 μmol/kg causes no symptoms whatsoever (Table 3), but H-Adp8-NH2 is not cell penetrating (HEK293 and MCF-7 cells, Figure 6). On the other hand, the fluorescently labeled octamers FAM-(Adp(OMe))8-NH2 and FAM-(Adp(NMe2))8-NH2 with ester and amide groups in the side chains exhibit mediocre to high cell-wall permeability (Figure 6), and are toxic (Table 3). Possible reasons for this behavior are discussed (Figure 7) and corresponding NMR spectra are presented (Figure 8).

In silico characterization of a cyanobacterial plant-type isoaspartyl aminopeptidase/asparaginase

Asparaginases are found in a range of organisms, although those found in cyanobacteria have been little studied, in spite of their great potential for biotechnological application. This study therefore sought to characterize the molecular structure of an L-asparaginase from the cyanobacterium Limnothrix sp. CACIAM 69d, which was isolated from a freshwater Amazonian environment. After homology modeling, model validation was performed using a Ramachandran plot, VERIFY3D, and the RMSD. We also performed molecular docking and dynamics simulations based on binding free-energy analysis. Structural alignment revealed homology with the isoaspartyl peptidase/asparaginase (EcAIII) from Escherichia coli. When compared to the template, our model showed full conservation of the catalytic site. In silico simulations confirmed the interaction of cyanobacterial isoaspartyl peptidase/asparaginase with its substrate, β-Asp-Leu dipeptide. We also observed that the residues Thr154, Thr187, Gly207, Asp218, and Gly237 were fundamental to protein-ligand complexation. Overall, our results suggest that L-asparaginase from Limnothrix sp. CACIAM 669d has similar properties to E. coli EcAIII asparaginase. Our study opens up new perspectives for the biotechnological exploitation of cyanobacterial asparaginases.

Effects of Reduced and Enhanced Glycogen Pools on Salt-Induced Sucrose Production in a Sucrose-Secreting Strain of Synechococcus elongatus PCC 7942

Sucrose and glycogen syntheses in cyanobacteria share the common precursor glucose-1-phosphate. It is generally assumed that lowering glycogen synthesis could drive more carbon toward sucrose synthesis that can be induced by salt stress among cyanobacteria. By using a theophylline-dependent riboswitch system, the expression of glgC, a key gene in glycogen synthesis, was downregulated in a quantitative manner in a sucrose-secreting strain of Synechococcus elongatus PCC 7942. We observed that the stepwise suppression of glycogen synthesis limited rather than stimulated sucrose production in the salt-stressed cells, suggesting that glycogen could serve as a carbon pool for the synthesis of sucrose. Accordingly, we generated glycogen-overproducing strains, but the increased glycogen pool alone did not stimulate sucrose production, indicating that alternative steps limit the carbon flux toward the synthesis of sucrose. Consistent with previous studies that showed that sucrose-phosphate synthase (SPS) catalyzes the rate-limiting step in sucrose synthesis, the combination of glycogen overproduction and sps overexpression resulted in increased sucrose production. Our results indicate that the glycogen and sucrose pools are closely linked in Synechococcus elongatus PCC 7942, and we propose that enhancing the glycogen pool could be a promising strategy for the improvement of sucrose production by cyanobacteria in the presence of a strong sucrose synthesis sink.IMPORTANCE Many cyanobacteria naturally synthesize and accumulate sucrose when stressed by NaCl, which provides novel possibilities for obtaining sugar feedstock by engineering of cyanobacteria. It has been assumed that glycogen synthesis competes with sucrose synthesis for the carbon flux. However, our results showed that the suppression of glycogen synthesis decreased rather than stimulated sucrose production in a sucrose-secreting strain of Synechococcus elongatus PCC 7942. This result suggests that glycogen could serve as a supportive rather than a competitive carbon pool for the synthesis of sucrose, providing new insights about the relation between glycogen synthesis and sucrose synthesis in cyanobacteria. This finding is also useful to guide metabolic engineering work to optimize the production of sucrose and possibly other products by cyanobacteria.

Expression of CphB- and CphE-type cyanophycinases in cyanophycin-producing tobacco and comparison of their ability to degrade cyanophycin in plant and plant extracts

Increasing the arginine (Arg) content in plants used as feed or food is of interest, since the supplementation of food with conditionally essential Arg has been shown to have nutritional benefits. An increase was achieved by the expression of the Arg-rich bacterial storage component, cyanophycin (CGP), in the chloroplast of transgenic plants. CGP is stable in plants and its degradation into β-aspartic acid (Asp)-Arg dipeptides, is solely catalyzed by bacterial cyanophycinases (CGPase). Dipeptides can be absorbed by animals even more efficiently than free amino acids (Matthews and Adibi 1976; Wenzel et al. 2001). The simultaneous production of CGP and CGPase in plants could be a source of β-Asp-Arg dipeptides if CGP degradation can be prevented in planta or if dipeptides are stable in the plants. We have shown for the first time that it is possible to co-express CGP and CGPase in the same plant without substrate degradation in planta by transient expression of the cyanobacterial CGPase CPHB (either in the plastid or cytosol), and the non-cyanobacterial CGPase CPHE (cytosol) in CGP-producing Nicotiana tabacum plants. We compared their ability to degrade CGP in planta and in crude plant extracts. No CGP degradation appeared prior to cell homogenization independent of the CGPase produced. In crude plant extracts, only cytosolic CPHE led to a fast degradation of CGP. CPHE also showed higher stability and in vitro activity compared to both CPHB variants. This work is the next step to increase Arg in forage plants using a stable, Arg-rich storage protein.

Stable production of cyanophycinase in Nicotiana benthamiana and its functionality to hydrolyse cyanophycin in the murine intestine

Food supplementation with the conditionally essential amino acid arginine (Arg) has been shown to have nutritional benefits. Degradation of cyanophycin (CGP), a peptide polymer used for nitrogen storage by cyanobacteria, requires cyanophycinase (CGPase) and results in the release of β-aspartic acid (Asp)-Arg dipeptides. The simultaneous production of CGP and CGPase in plants could be a convenient source of Arg dipeptides. Different variants of the cphB coding region from Thermosynechococcus elongatus BP-1 were transiently expressed in Nicotiana benthamiana plants. Translation and enzyme stability were optimized to produce high amounts of active CGPase. Protein stability was increased by the translational fusion of CGPase to the green fluorescent protein (GFP) or to the transit peptide of the small subunit of RuBisCO for peptide production in the chloroplasts. Studies in mice showed that plant-expressed CGP fed in combination with plant-made CGPase was hydrolysed in the intestine, and high levels of ß-Asp-Arg dipeptides were found in plasma, demonstrating dipeptide absorption. However, the lack of an increase in Asp and Arg or its metabolite ornithine in plasma suggests that Arg from CGP was not bioavailable in this mouse group. Intestinal degradation of CGP by CGPase led to low intestinal CGP content 4 h after consumption, but after ingestion of CGP alone, high CGP concentrations remained in the large intestine; this indicated that intact CGP was transported from the small to the large intestine and that CGP was resistant to colonic microbes.

Enzymatic Modification of Soluble Cyanophycin Using the Type II Peptidyl Arginine Deiminase from Oryctolagus cuniculus

An increased structural variety expands the number of putative applications for cyanophycin (multi-l-arginyl-poly-[l-aspartic acid], CGP). Therefore, structural modifications of CGP are of major interest; these are commonly obtained by modification and optimization of the bacterial producing strain or by chemical modification. In this study, an enzymatic modification of arginine side chains from lysine-rich CGP is demonstrated using the peptidyl arginine deiminase from Oryctolagus cuniculus, purified from Escherichia coli after heterologous expression. About 10% of the arginine side chains are converted to citrulline which corresponds to 4% of the polymer's total side chains. An inhibition of the reaction in the presence of small amounts of l-citrulline is observed, thereby explaining the low conversion rate. CGP dipeptides can be modified with about 7.5 mol% of the Asp-Arg dipeptides being converted to Asp-Cit. These results show that the enzymatic modification of CGP is feasible, opening up a whole new area of possible CGP modifications for further research.

Features of the biotechnologically relevant polyamide family "cyanophycins" and their biosynthesis in prokaryotes and eukaryotes

Cyanophycin, inclusions in cyanobacteria discovered by the Italian scientist Borzi in 1887, were characterized as a polyamide consisting of aspartic acid and arginine. Its synthesis in cyanobacteria was analyzed regarding growth conditions, responsible gene product, requirements, polymer structure and properties. Heterologous expression of diverse cyanophycin synthetases (CphA) in Escherichia coli enabled further enzyme characterization. Cyanophycin is a polyamide with variable composition and physiochemical properties dependent on host and cultivation conditions in contrast to the extracellular polyamides poly-γ-glutamic acid and poly-ε-l-lysine. Furthermore, recombinant prokaryotes and transgenic eukaryotes, including plants expressing different cphA genes, were characterized as suitable for production of insoluble cyanophycin regarding higher yields and modified composition for other requirements and applications. In addition, cyanophycin was characterized as a source for the synthesis of polyaspartic acid or N-containing bulk chemicals and dipeptides upon chemical treatment or degradation by cyanophycinases, respectively. Moreover, water-soluble cyanophycin derivatives with altered amino acid composition were isolated from transgenic plants, yeasts and recombinant bacteria. Thereby, the range of dipeptides could be extended by biological processes and by chemical modification, thus increasing the range of applications for cyanophycin and its dipeptides, including agriculture, food supplementations, medical and cosmetic purposes, synthesis of the polyacrylate substitute poly(aspartic acid) and other applications.

Guanidination of soluble lysine-rich cyanophycin yields a homoarginine-containing polyamide

Soluble cyanobacterial granule polypeptide (CGP), especially that isolated from recombinant Escherichia coli strains, consists of aspartic acid, arginine, and a greater amount of lysine than that in insoluble CGP isolated from cyanobacteria or various other recombinant bacteria. In vitro guanidination of lysine side chains of soluble CGP with o-methylisourea (OMIU) yielded the nonproteinogenic amino acid homoarginine. The modified soluble CGP consisted of 51 mol% aspartate, 14 mol% arginine, and 35 mol% homoarginine. The complete conversion of lysine residues to homoarginine was confirmed by (i) nuclear magnetic resonance spectrometry, (ii) coupled liquid chromatography-mass spectrometry, and (iii) high-performance liquid chromatography. Unlike soluble CGP, this new homoarginine-containing polyamide was soluble only under acidic or alkaline conditions and was insoluble in water or at a neutral pH. Thus, it showed solubility behavior similar to that of the natural insoluble polymer isolated from cyanobacteria, consisting of aspartic acid and arginine only. Polyacrylamide gel electrophoresis revealed similar degrees of polymerization of the native (12- to 40-kDa) and modified (10- to 35-kDa) polymers. This study showed that the chemical structure and properties of a biopolymer could be changed by in vitro introduction of a new functional group after biosynthesis of the native polymer. In addition, the modified CGP could be digested in vitro using the cyanophycinase from Pseudomonas alcaligenes strain DIP1, yielding a new dipeptide consisting of aspartate and homoarginine.

Production optimization of cyanophycinase ChpEal from Pseudomonas alcaligenes DIP1

Pseudomonas alcaligenes DIP1 produces an extracellular cyanophycinase (CphE_al). The corresponding gene (cphE_al) was identified from subclones of a genomic DNA gene library by heterologously expressing the functionally active enzyme in Escherichia coli. The nucleotide sequence of the gene (1260 base pairs) was determined indicating a theoretical mass of 43.6 kDa (mature CphE_al) plus a leader peptide of 2,6 kDa which corresponds well to the apparent molecular mass of 45 kDa as revealed by SDS-PAGE. The enzyme exhibited a high sequence identity of 91% with the extracellular cyanophycinase from P. anguilliseptica strain BI and carried an N-terminal Sec secretion signal peptide. Analysis of the amino acid sequence of cphE revealed a putative catalytic triad consisting of the serine motif GXSXG plus a histidine and a glutamate residue, suggesting a catalytic mechanism similar to serine-type proteases. The cyanophycinase (CphE_al) was heterologously produced in two different E. coli strains (Top10 and BL21(DE3)) from two plasmid vectors (pBBR1MCS-4 and pET-23a(+)). The signal peptide of CphE_al was cleaved in E. coli, suggesting active export of the protein at least to the periplasm. Substantial enzyme activity was also present in the culture supernatants. The extracellular cyanophycinase activities in E. coli were higher than activities in the wild type P. alcaligenes DIP1 in complex LB medium. Highest extracellular enzyme production was achieved with E. coli BL21(DE3) expressing CphE_al from pBBR1MCS-4. Using M9 minimal medium was less effective, but the relatively low cost of mineral salt media makes these results important for the industrial-scale production of dipeptides from cyanophycin.

Poly(aspartate) hydrolases: biochemical properties and applications

Thermally synthesized poly(aspartate) (tPAA) shows potential for use in a wide variety of products and applications as a biodegradable replacement for non-biodegradable polycarboxylates, such as poly(acrylate). The tPAA molecule has unnatural structures, and the relationship between its biodegradability and structures has been investigated. Two tPAA-degrading bacteria, Sphingomonas sp. KT-1 and Pedobacter sp. KP-2, were isolated from river water; from them, two PAA-hydrolyzing enzymes, PAA hydrolases-1 and -2, were purified and biologically and genetically characterized. Interestingly, not only are PAA hydrolases-1 from those two strains novel in terms of structural genes and substrate specificities (they specifically cleave the amide bond between β-aspartate units in tPAA), they also probably play a central role in tPAA biodegradation by both strains. In green polymer chemistry, one active area of research is the use of purified enzymes for the enzyme-catalyzed synthesis of polypeptides by taking advantage of their substrate specificities. Recently, β-peptides have attracted academic and industrial interest as functional materials as they possess both functions of α-peptides and excellent metabolic stability. As one of the attractive applications of PAA hydrolases, we report here the enzyme-catalyzed synthesis of poly(α-ethyl β-aspartate), which is composed of only β-linkages and belongs to β-peptides, using the unique substrate specificity of the enzyme from Pedobacter sp. KP-2.

Nanostructured bacterial materials for innovative medicines

The development of innovative medicines and personalized biomedical approaches require the identification and implementation of new biocompatible materials produced by methodologically simple and cheap fabrication methods. The biological fabrication of materials, mostly carried out by microorganisms, has historically provided organic compounds with wide-spectrum biomedical applications, including hyaluronic acid, poly(gamma-glutamic acid) and polyhydroxyalkanoates. Additionally, the implementation of new methodological platforms such as metabolic engineering and systems biology have facilitated the controlled production of natural nanoparticles produced by bacteria, including metallic deposits of Au, Ag, Cd, Zn or Fe, virus-like particles or other nanoscale protein-only entities. The unexpected potential of such self-organized and functional materials in nanomedical scenarios (especially in drug delivery, imaging and tissue engineering) prompts serious consideration of further exploitation of bacterial cell factories as convenient alternatives to chemical synthesis and as sources of novel bioproducts that could dramatically expand the existing catalog of biomedical materials.

Establishment of cyanophycin biosynthesis in Pichia pastoris and optimization by use of engineered cyanophycin synthetases

Two strains of the methylotrophic yeast Pichia pastoris were used to establish cyanophycin (multi-L-arginyl-poly-L-aspartic acid [CGP]) synthesis and to explore the applicability of this industrially widely used microorganism for the production of this polyamide. Therefore, the CGP synthetase gene from the cyanobacterium Synechocystis sp. strain PCC 6308 (cphA(6308)) was expressed under the control of the alcohol oxidase 1 promoter, yielding CGP contents of up to 10.4% (wt/wt), with the main fraction consisting of the soluble form of the polymer. To increase the polymer contents and to obtain further insights into the structural or catalytic properties of the enzyme, site-directed mutagenesis was applied to cphA(6308) and the mutated gene products were analyzed after expression in P. pastoris and Escherichia coli, respectively. CphA(6308)Delta1, which was truncated by one amino acid at the C terminus; point mutated CphA(6308)C595S; and the combined double-mutant CphA(6308)Delta1C595S protein were purified. They exhibited up to 2.5-fold higher enzyme activities of 4.95 U/mg, 3.20 U/mg, and 4.17 U/mg, respectively, than wild-type CphA(6308) (2.01 U/mg). On the other hand, CphA proteins truncated by two (CphA(6308)Delta2) or three (CphA(6308)Delta3) amino acids at the C terminus showed similar or reduced CphA enzyme activity in comparison to CphA(6308). In flask experiments, a maximum of 14.3% (wt/wt) CGP was detected after the expression of CphA(6308)Delta1 in P. pastoris. For stabilization of the expression plasmid, the his4 gene from Saccharomyces cerevisiae was cloned into the expression vector used and the constructs were transferred to histidine auxotrophic P. pastoris strain GS115. Parallel fermentations at a one-to-one scale revealed 26 degrees C and 6.0 as the optimal temperature and pH, respectively, for CGP synthesis. After optimization of fermentation parameters, medium composition, and the length of the cultivation period, CGP contents could be increased from 3.2 to 13.0% (wt/wt) in cells of P. pastoris GS115 expressing CphA(6308) and up to even 23.3% (wt/wt) in cells of P. pastoris GS115 expressing CphA(6308)Delta1.

Establishment of a simple and effective isolation method for cyanophycin from recombinant Saccharomyces cerevisiae

An efficient, time-saving, and cost-effective method for isolation of the polyamide cyanophycin from recombinant Saccharomyces cerevisiae was established. Due to its simple procedure, this isolation method may be also applicable at industrial scale and also to other intracellular compounds in this yeast. Production of cyanophycin gained preferential interest in the past, as degradation products thereof are of pharmaceutical and technical interest. Recently, it was shown that Saccharomyces cerevisiae represents a putative candidate for cyanophycin synthesis at industrial scale. For identification of optimal isolation procedures, several parameters such as heat stress, freeze drying, and freeze/thaw cycles of transgenic yeast cells were compared for their effectiveness of cyanophycin isolation. Additionally, optimal resuspension solutions for the applied cells and minimal required materials or chemicals were determined to make the process most environmentally and economically friendly. Maximal cyanophycin granule polypeptide yields of 21% (w/w) were obtained after incubation of dry cells at 70 degrees C or 80 degrees C and precipitation of the polymer with two volumes of ethanol.

Metabolic engineering of Saccharomyces cerevisiae for production of novel cyanophycins with an extended range of constituent amino acids

Cyanophycin (multi-l-arginyl-poly-l-aspartic acid; also known as cyanophycin grana peptide [CGP]) is a putative precursor for numerous biodegradable technically used chemicals. Therefore, the biosynthesis and production of the polymer in recombinant organisms is of special interest. The synthesis of cyanophycin derivatives consisting of a wider range of constituents would broaden the applications of this polymer. We applied recombinant Saccharomyces cerevisiae strains defective in arginine metabolism and expressing the cyanophycin synthetase of Synechocystis sp. strain PCC 6308 in order to synthesize CGP with citrulline and ornithine as constituents. Strains defective in arginine degradation (Car1 and Car2) accumulated up to 4% (wt/wt) CGP, whereas strains defective in arginine synthesis (Arg1, Arg3, and Arg4) accumulated up to 15.3% (wt/wt) of CGP, which is more than twofold higher than the previously content reported in yeast and the highest content ever reported in eukaryotes. Characterization of the isolated polymers by different analytical methods indicated that CGP synthesized by strain Arg1 (with argininosuccinate synthetase deleted) consisted of up to 20 mol% of citrulline, whereas CGP from strain Arg3 (with ornithine carbamoyltransferase deleted) consisted of up to 8 mol% of ornithine, and CGP isolated from strain Arg4 (with argininosuccinate lyase deleted) consisted of up to 16 mol% lysine. Cultivation experiments indicated that the incorporation of citrulline or ornithine is enhanced by the addition of low amounts of arginine (2 mM) and also by the addition of ornithine or citrulline (10 to 40 mM), respectively, to the medium.