Tyrosine-based “Activatable Pro-Tag”: Enzyme-catalyzed protein capture and release

Enhancing the thermostability of Streptomyces cyaneofuscatus strain Ms1 tyrosinase by multi-factors rational design and molecular dynamics simulations

This study presents a multi-factor rational design strategy combined with molecular dynamics simulation to improve the thermostability of Streptomyces cyaneofuscatus strain Ms1 tyrosinase. Candidate mutation sites were identified using Discovery Studio and FoldX software, and the double mutant G124W/G137W was obtained. The mutant was heterogeneously expressed in Escherichia coli strain Rosetta2 (DE3), and its thermostability was verified. Results indicate that the rational design method, combined with molecular dynamics simulation and protein energy calculation, improved the enzyme's thermostability more accurately and effectively. The double mutant G124W/G137W had an optimum temperature of 60°C, about 5.0°C higher than that of the wild-type TYRwt, and its activity was 171.06% higher than the wild-type TYRwt. Its thermostability was enhanced, 42.78% higher than the wild-type at 50°C. These findings suggest that the rational design strategy applied in this study can facilitate the application of industrial enzymes in the pharmaceutical industry.

Electro-Biofabrication. Coupling Electrochemical and Biomolecular Methods to Create Functional Bio-Based Hydrogels

Twenty years ago, this journal published a review entitled "Biofabrication with Chitosan" based on the observations that (i) chitosan could be electrodeposited using low voltage electrical inputs (typically less than 5 V) and (ii) the enzyme tyrosinase could be used to graft proteins (via accessible tyrosine residues) to chitosan. Here, we provide a progress report on the coupling of electronic inputs with advanced biological methods for the fabrication of biopolymer-based hydrogel films. In many cases, the initial observations of chitosan's electrodeposition have been extended and generalized: mechanisms have been established for the electrodeposition of various other biological polymers (proteins and polysaccharides), and electrodeposition has been shown to allow the precise control of the hydrogel's emergent microstructure. In addition, the use of biotechnological methods to confer function has been extended from tyrosinase conjugation to the use of protein engineering to create genetically fused assembly tags (short sequences of accessible amino acid residues) that facilitate the attachment of function-conferring proteins to electrodeposited films using alternative enzymes (e.g., transglutaminase), metal chelation, and electrochemically induced oxidative mechanisms. Over these 20 years, the contributions from numerous groups have also identified exciting opportunities. First, electrochemistry provides unique capabilities to impose chemical and electrical cues that can induce assembly while controlling the emergent microstructure. Second, it is clear that the detailed mechanisms of biopolymer self-assembly (i.e., chitosan gel formation) are far more complex than anticipated, and this provides a rich opportunity both for fundamental inquiry and for the creation of high performance and sustainable material systems. Third, the mild conditions used for electrodeposition allow cells to be co-deposited for the fabrication of living materials. Finally, the applications have been expanded from biosensing and lab-on-a-chip systems to bioelectronic and medical materials. We suggest that electro-biofabrication is poised to emerge as an enabling additive manufacturing method especially suited for life science applications and to bridge communication between our biological and technological worlds.

Simple, rapidly electroassembled thiolated PEG-based sensor interfaces enable rapid interrogation of antibody titer and glycosylation

Process conditions established during the development and manufacture of recombinant protein therapeutics dramatically impacts their quality and clinical efficacy. Technologies that enable rapid assessment of product quality are critically important. Here, we describe the development of sensor interfaces that directly connect to electronics and enable near real-time assessment of antibody titer and N-linked galactosylation. We make use of a spatially resolved electroassembled thiolated polyethylene glycol hydrogel that enables electroactivated disulfide linkages. For titer assessment, we constructed a cysteinylated protein G that can be linked to the thiolated hydrogel allowing for robust capture and assessment of antibody concentration. For detecting galactosylation, the hydrogel is linked with thiolated sugars and their corresponding lectins, which enables antibody capture based on glycan pattern. Importantly, we demonstrate linear assessment of total antibody concentration over an industrially relevant range and the selective capture and quantification of antibodies with terminal β-galactose glycans. We also show that the interfaces can be reused after surface regeneration using a low pH buffer. Our functionalized interfaces offer advantages in their simplicity, rapid assembly, connectivity to electronics, and reusability. As they assemble directly onto electrodes that also serve as I/O registers, we envision incorporation into diagnostic platforms including those in manufacturing settings.

Protein Chemical Labeling Using Biomimetic Radical Chemistry

Chemical labeling of proteins with synthetic low-molecular-weight probes is an important technique in chemical biology. To achieve this, it is necessary to use chemical reactions that proceed rapidly under physiological conditions (i.e., aqueous solvent, pH, low concentration, and low temperature) so that protein denaturation does not occur. The radical reaction satisfies such demands of protein labeling, and protein labeling using the biomimetic radical reaction has recently attracted attention. The biomimetic radical reaction enables selective labeling of the C-terminus, tyrosine, and tryptophan, which is difficult to achieve with conventional electrophilic protein labeling. In addition, as the radical reaction proceeds selectively in close proximity to the catalyst, it can be applied to the analysis of protein–protein interactions. In this review, recent trends in protein labeling using biomimetic radical reactions are discussed.

pH-Responsive Polypeptide-Based Smart Nano-Carriers for Theranostic Applications

Smart nano-carriers have attained great significance in the biomedical field due to their versatile and interesting designs with different functionalities. The initial stages of the development of nanocarriers mainly focused on the guest loading efficiency, biocompatibility of the host and the circulation time. Later the requirements of less side effects with more efficacy arose by attributing targetability and stimuli-responsive characteristics to nano-carriers along with their bio- compatibility. Researchers are utilizing many stimuli-responsive polymers for the better release of the guest molecules at the targeted sites. Among these, pH-triggered release achieves increasing importance because of the pH variation in different organ and cancer cells of acidic pH. This specific feature is utilized to release the guest molecules more precisely in the targeted site by designing polymers having specific functionality with the pH dependent morphology change characteristics. In this review, we mainly concert on the pH-responsive polypeptides and some interesting nano-carrier designs for the effective theranostic applications. Also, emphasis is made on pharmaceutical application of the different nano-carriers with respect to the organ, tissue and cellular level pH environment.

Biofabricating Functional Soft Matter Using Protein Engineering to Enable Enzymatic Assembly

Biology often provides the inspiration for functional soft matter, but biology can do more: it can provide the raw materials and mechanisms for hierarchical assembly. Biology uses polymers to perform various functions, and biologically derived polymers can serve as sustainable, self-assembling, and high-performance materials platforms for life-science applications. Biology employs enzymes for site-specific reactions that are used to both disassemble and assemble biopolymers both to and from component parts. By exploiting protein engineering methodologies, proteins can be modified to make them more susceptible to biology's native enzymatic activities. They can be engineered with fusion tags that provide (short sequences of amino acids at the C- and/or N- termini) that provide the accessible residues for the assembling enzymes to recognize and react with. This "biobased" fabrication not only allows biology's nanoscale components (i.e., proteins) to be engineered, but also provides the means to organize these components into the hierarchical structures that are prevalent in life.

Modification and Assembly of a Versatile Lactonase for Bacterial Quorum Quenching

This work sets out to provide a self-assembled biopolymer capsule activated with a multi-functional enzyme for localized delivery. This enzyme, SsoPox, which is a lactonase and phosphotriesterase, provides a means of interrupting bacterial communication pathways that have been shown to mediate pathogenicity. Here we demonstrate the capability to express, purify and attach SsoPox to the natural biopolymer chitosan, preserving its activity to "neutralize" long-chain autoinducer-1 (AI-1) communication molecules. Attachment is shown via non-specific binding and by engineering tyrosine and glutamine affinity 'tags' at the C-terminus for covalent linkage. Subsequent degradation of AI-1, in this case N-(3-oxododecanoyl)-l-homoserine lactone (OdDHL), serves to "quench" bacterial quorum sensing (QS), silencing intraspecies communication. By attaching enzymes to pH-responsive chitosan that, in turn, can be assembled into various forms, we demonstrate device-based flexibility for enzyme delivery. Specifically, we have assembled quorum-quenching capsules consisting of an alginate inner core and an enzyme "decorated" chitosan shell that are shown to preclude bacterial QS crosstalk, minimizing QS mediated behaviors.

Bioorthogonal strategies for site-directed decoration of biomaterials with therapeutic proteins

Emerging strategies targeting site-specific protein modifications allow for unprecedented selectivity, fast kinetics and mild reaction conditions with high yield. These advances open exciting novel possibilities for the effective bioorthogonal decoration of biomaterials with therapeutic proteins. Site-specificity is particularly important to the therapeutics' end and translated by targeting specific functional groups or introducing new functional groups into the therapeutic at predefined positions. Biomimetic strategies are designed for modification of therapeutics emulating enzymatic strategies found in Nature. These strategies are suitable for a diverse range of applications - not only for protein-polymer conjugation, particle decoration and surface immobilization, but also for the decoration of complex biomaterials and the synthesis of bioresponsive drug delivery systems. This article reviews latest chemical and enzymatic strategies for the biorthogonal decoration of biomaterials with therapeutic proteins and inter-positioned linker structures. Finally, the numerous reports at the interface of biomaterials, linkers, and therapeutic protein decoration are integrated into practical advice for design considerations intended to support the selection of productive ligation strategies.

Connecting Biology to Electronics: Molecular Communication via Redox Modality

Biology and electronics are both expert at for accessing, analyzing, and responding to information. Biology uses ions, small molecules, and macromolecules to receive, analyze, store, and transmit information, whereas electronic devices receive input in the form of electromagnetic radiation, process the information using electrons, and then transmit output as electromagnetic waves. Generating the capabilities to connect biology-electronic modalities offers exciting opportunities to shape the future of biosensors, point-of-care medicine, and wearable/implantable devices. Redox reactions offer unique opportunities for bio-device communication that spans the molecular modalities of biology and electrical modality of devices. Here, an approach to search for redox information through an interactive electrochemical probing that is analogous to sonar is adopted. The capabilities of this approach to access global chemical information as well as information of specific redox-active chemical entities are illustrated using recent examples. An example of the use of synthetic biology to recognize external molecular information, process this information through intracellular signal transduction pathways, and generate output responses that can be detected by electrical modalities is also provided. Finally, exciting results in the use of redox reactions to actuate biology are provided to illustrate that synthetic biology offers the potential to guide biological response through electrical cues.

Incorporating LsrK AI-2 quorum quenching capability in a functionalized biopolymer capsule

Antibacterial resistance is an issue of increasing severity as current antibiotics are losing their effectiveness and fewer antibiotics are being developed. New methods for combating bacterial virulence are required. Modulating molecular communication among bacteria can alter phenotype, including attachment to epithelia, biofilm formation, and even toxin production. Intercepting and modulating communication networks provide a means to attenuate virulence without directly interacting with the bacteria of interest. In this work, we target communication mediated by the quorum sensing (QS) bacterial autoinducer-2, AI-2. We have assembled a capsule of biological polymers alginate and chitosan, attached an AI-2 processing kinase, LsrK, and provided substrate, ATP, for enzymatic alteration of AI-2 in culture fluids. Correspondingly, AI-2 mediated QS activity is diminished. All components of this system are "biofabricated"-they are biologically derived and their assembly is accomplished using biological means. Initially, component quantities and kinetics were tested as assembled in microtiter plates. Subsequently, the identical components and assembly means were used to create the "artificial cell" capsules. The functionalized capsules, when introduced into populations of bacteria, alter the dynamics of the AI-2 bacterial communication, attenuating QS activated phenotypes. We envision the assembly of these and other capsules or similar materials, as means to alter QS activity in a biologically compatible manner and in many environments, including in humans.

A Facile Two-Step Enzymatic Approach for Conjugating Proteins to Polysaccharide Chitosan at an Electrode Interface

Biological components are integrated with electronic devices to create microsystems with novel functions and chitosan, a naturally occurring biopolymer, can play a significant role as an interface material. Chitosan can be electrodeposited within confined geometries by cathodic charge and appropriate electrode design and proteins can be conjugated to chitosan. However, conjugation chemistries can be slow and chitosan, a polycationic polysaccharide, enables non-specific binding in biofabrication processes. There is a need to speed up the assembly process and reduce non-specific binding. Here, we have developed a two-step methodology that accelerates protein assembly, reduces background and increases specificity. We first "coated" the surface of chitosan with a Lys-Tyr-Lys (KYK) tripeptide in a slow step using tyrosinase-mediated conjugation chemistry and then conjugated proteins with C-terminal glutamine-tags to the saturating KYK tripeptide via transglutaminase. As a demonstration, we assembled a functioning two-enzyme bacterial metabolic pathway on an electrode chip. Results indicated a fivefold decrease in non-specific binding and an improvement in signal to noise ratio from 0.3 to 20. This transglutaminase-mediated approach is simple and quick, it requires no chemical reagents, no printing or stamping devices; it employs biological components and is biologically benign to the component parts-all characteristics of biofabricated devices.

Poly(colloid)s: "Polymerization" of Poly(l-tyrosine)-silica Composite Particles through the Photoinduced Cross-Linking of Unmodified Proteins Method

Photoinduced cross-linking of unmodified proteins, PICUP, was extended to core-shell silica-polypeptide composite particles to produce poly(colloid)s. Silica particles coated with poly(l-tyrosine), PTYR-SiO2, served as the monomer units. The PICUP reaction accomplished the formation of dityrosil linkages between the tyrosine units by illumination of photo-oxidizing ruthenium(II) bipyridyl catalyst under physiological conditions. The PICUP method was compared with an enzymatic route intermediated by horseradish peroxidase as catalyst. The PTYR-SiO2 particles feature high PTYR content in the shell, which facilitated the formation of heavily cross-linked but unstructured aggregates. After magnetic alignment of superparamagnetic PTYR-SiO2-cobalt composite particles, only the PICUP approach enabled the preparation of isolated chain-like poly(colloid)s. The cross-linking products were confirmed by FTIR. The native secondary structure of poly(l-tyrosine) is preserved in these poly(colloid)s. Because the PICUP reaction does not require the modification of the polypeptide structure, the cross-linked PTYR will retain its characteristic functions as a poly(amino acid). The PICUP method opens the door to a variety of PTYR-based poly(colloid) architectures.

Recombinant Tyrosinase from Polyporus arcularius: Overproduction in Escherichia coli, Characterization, and Use in a Study of Aurones as Tyrosinase Effectors

Tyrosinases act in the development of organoleptic properties of tea, raisins, etc., but also cause unwanted browning of fruits, vegetables, and mushrooms. The tyrosinase from Agaricus bisporus has been used as a model to study tyrosinase inhibitors, which are also indispensable in the treatment of skin pigmentation disorders. However, this model has disadvantages such as side enzyme activities and the presence of multiple isoenzymes. Therefore, we aimed to introduce a new tyrosinase model. The pro-tyrosinase from Polyporus arcularius was overproduced in Escherichia coli. Trypsin digestion led to a cleavage after R388 and hence enzyme activation. The tyrosinase was a homodimer and transformed L-DOPA and tert-butylcatechol preferentially. Various aurons were examined as effectors of this enzyme. 2'- and 3'-hydroxyaurones acted as its activators and 2',4'-dihydroxyaurone as an inhibitor, whereas 4'-hydroxyaurones were its substrates. The enzyme is a promising model for tyrosinase effector studies, being a single isoenzyme and void of side enzyme activities.

Enzymatic multi-functionalization of microparticles under aqueous neutral conditions

Enzymatic multi-functionalization of microparticles under aqueous neutral conditions using tyrosinase.

Tyrosinase-catalyzed site-specific immobilization of engineered C-phycocyanin to surface

Enzymatic crosslinking of proteins is often limited by the steric availability of the target residues, as of tyrosyl side chains in the case of tyrosinase. Carrying an N-terminal peptide-tag containing two tyrosine residues, the fluorescent protein C-phycocyanin HisCPC from Synechocystis sp. PCC6803 was crosslinked to fluorescent high-molecular weight forms with tyrosinase. Crosslinking with tyrosinase in the presence of L-tyrosine produced non fluorescent high-molecular weight products. Incubated in the presence of tyrosinase, HisCPC could also be immobilized to amino-modified polystyrene beads thus conferring a blue fluorescence. Crosslinking and immobilization were site-specific as both processes required the presence of the N-terminal peptide in HisCPC.

Synthetic biologists spring into action at the 245th American Chemical Society National Meeting

As the field of synthetic biology continues to define itself, it has merged concepts from many related areas of research: molecular biology, genetics, bioengineering, and chemistry. At the 2013 Spring American Chemical Society National Meeting in New Orleans, LA, this mixture was manifested in a wealth of sessions emphasizing the use of modern synthetic biological approaches to solve many of today's biggest chemical problems. As a result of the field's diverse yet pervasive nature, synthetic biology concepts were present in several of the conferences many divisions, including Biological Chemistry, Biochemical Technology, Cellulose and Renewable Materials, and several others. Here we offer a snapshot of some of the exciting research discussed in the dedicated synthetic biology sessions throughout the week.

High level production of tyrosinase in recombinant Escherichia coli

BACKGROUND

Tyrosinase is a bifunctional enzyme that catalyzes both the hydroxylation of monophenols to o-diphenols (monophenolase activity) and the subsequent oxidation of the diphenols to o-quinones (diphenolase activity). Due to the potential applications of tyrosinase in biotechnology, in particular in biocatalysis and for biosensors, it is desirable to develop a suitable low-cost process for efficient production of this enzyme. So far, the best production yield reported for tyrosinase was about 1 g L(-1), which was achieved by cultivating the filamentous fungus Trichoderma reesei for 6 days.

RESULTS

In this work, tyrosinase from Verrucomicrobium spinosum was expressed in Escherichia coli and its production was studied in both batch and fed-batch cultivations. Effects of various key cultivation parameters on tyrosinase production were first examined in batch cultures to identify optimal conditions. It was found that a culture temperature of 32 °C and induction at the late growth stage were favorable, leading to a highest tyrosinase activity of 0.76 U mL(-1). The fed-batch process was performed by using an exponential feeding strategy to achieve high cell density. With the fed-batch process, a final biomass concentration of 37 g L(-1) (based on optical density) and a tyrosinase activity of 13 U mL(-1) were obtained in 28 hours, leading to a yield of active tyrosinase of about 3 g L(-1). The highest overall volumetric productivity of 103 mg of active tyrosinase per liter and hour (corresponding to 464 mU L(-1) h(-1)) was determined, which is approximately 15 times higher than that obtained in batch cultures.

CONCLUSIONS

We have successfully expressed and produced gram quantities per liter of active tyrosinase in recombinant E. coli by optimizing the expression conditions and fed-batch cultivation strategy. Exponential feed of substrate helped to prolong the exponential phase of growth, to reduce the fermentation time and thus the cost. A specific tyrosinase production rate of 103 mg L(-1) h(-1) and a maximum volumetric activity of 464 mU L(-1) h(-1) were achieved in this study. These levels have not been reported previously.

Protein heteroconjugation by the peroxidase-catalyzed tyrosine coupling reaction

Combining different proteins can integrate the functions of each protein to produce novel protein conjugates with wider ranges of applications. We have previously introduced a peptide containing tyrosine residues (Y-tag) at the C-terminus of Escherichia coli alkaline phosphatase (BAP). The tyrosine residues in the Y-tag were efficiently recognized by horseradish peroxidase (HRP) and were site-specifically cross-linked with each other to yield BAP homoconjugates. In this study, the HRP-catalyzed tyrosine coupling reaction was used for protein heteroconjugation. Streptavidin (SA) was selected as the conjugation partner for BAP. The Y-tag (GGGGY) was genetically introduced to the C-terminus of SA. Prior to heteroconjugation, the reactivity of the Y-tagged SA was examined. The Y-tagged SA cross-linked to form an SA homoconjugate upon HRP treatment, whereas wild-type SA remained essentially intact. In the heteroconjugation reaction of BAP and SA, the Y-tagged BAP and SA were efficiently cross-linked with each other upon HRP treatment. The functions of the BAP-SA conjugates were evaluated by measuring the BAP enzymatic activity on a biotin-coated plate. The BAP-SA conjugate tethered to the plate showed BAP enzymatic activity, indicating that both BAP and SA retained their functions following heteroconjugation. The BAP-SA conjugate prepared from both Y-tagged BAP and SA showed the highest enzymatic activity on the biotin-coated plates. This result illustrates the advantage of the protein conjugation reaction in which multiple numbers of proteins can be conjugated at the same time.

Bacterial tyrosinases: old enzymes with new relevance to biotechnology

Tyrosinases are copper-containing dioxygen activating enzymes found in many species of bacteria and are usually associated with melanin production. These proteins have a strong preference for phenolic and diphenolic substrates and are somewhat limited in their reaction scope, always producing an activated quinone as product. Despite this fact they have potential in several biotechnological applications, including the production of novel mixed melanins, protein cross-linking, phenolic biosensors, production of l-DOPA, phenol and dye removal and biocatalysis. Although most studies have used Streptomyces sp. enzymes, there are several other examples of these proteins that are also of potential interest. For instance a solvent tolerant enzyme has been described, as well as an enzyme with both tyrosinase and laccase activities, enzymes with altered substrate preferences, an enzyme produced as an inactive zymogen as well as examples which do not require auxiliary proteins for copper insertion (unlike the Streptomyces sp. enzymes which do require such a protein). This article will summarise the reports on the biotechnological applications of bacterial tyrosinases as well as the current information available on the different types of this enzyme.

Biofabrication with Biopolymers and Enzymes: Potential for Constructing Scaffolds from Soft Matter

Purpose

Regenerative medicine will benefit from technologies capable of fabricating soft matter to have appropriate architectures and that provide the necessary physical, chemical and biological cues to recruit cells and guide their development. The goal of this report is to review an emerging set of biofabrication techniques and suggest how these techniques could be applied for the fabrication of scaffolds for tissue engineering.

Methods

Electrical potentials are applied to submerged electrodes to perform cathodic and anodic reactions that direct stimuli-responsive film-forming polysaccharides to assemble into hydrogel films. Standard methods are used to microfabricate electrode surfaces to allow the electrical signals to be applied with spatial and temporal control. The enzymes mushroom tyrosinase and microbial transglutaminase are used to catalyze macromolecular grafting and crosslinking of proteins.

Results

Electrodeposition of the polysaccharides chitosan and alginate allow hydrogel films to be formed in response to localized electrical signals. Co-deposition of various components (e.g., proteins, vesicles and cells), and subsequent electrochemical processing allow the physical, chemical and biological activities of these films to be tailored. Enzymatic processing allows for the generation of stimuli-responsive protein conjugates that can also be directed to assemble in response to imposed electrical signals. Further, enzyme-catalyzed crosslinking of gelatin allows replica molding of soft matter to create hydrogel films with topological structure.

Conclusions

Biofabrication with biological materials and mechanisms provides new approaches for soft matter construction. These methods may enable the formation of tissue engineering scaffolds with appropriate architectures, assembled cells, and spatially organized physical, chemical and biological cues.

Cross-linking and immobilisation of different proteins with recombinant Verrucomicrobium spinosum tyrosinase

This paper reports on the cross-linking and immobilisation of various proteins by the recombinant tyrosinase from Verrucomicrobium spinosum (Vs-tyrosinase). In general it is found that Vs-tyrosinase can readily cross-link proteins with a low degree of complexity, such as casein, but that the enzyme cannot readily cross-link well folded protein substrates such as lysozyme, myoglobin, cytochrome c or Candida antarctica lipase B (CALB). However, the inclusion of phenolic compounds (phenol or caffeic acid) to reaction mixtures of these proteins can greatly enhance the levels of cross-linking. For example it is possible to prepare cross-linked aggregates of industrially applicable enzymes such as CALB by simply incubating it with Vs-tyrosinase and phenol. The resulting aggregates can be collected by centrifugation and retain high levels of activity and may find applications in biocatalysis.

Biological nanofactories facilitate spatially selective capture and manipulation of quorum sensing bacteria in a bioMEMS device

The emergence of bacteria that evade antibiotics has accelerated research on alternative approaches that do not target cell viability. One such approach targets cell-cell communication networks mediated by small molecule signaling. In this report, we assemble biological nanofactories within a bioMEMS device to capture and manipulate the behavior of quorum sensing (QS) bacteria as a step toward modifying small molecule signaling. Biological nanofactories are bio-inspired nanoscale constructs which can include modules with different functionalities, such as cell targeting, molecular sensing, product synthesis, and ultimately self-destruction. The biological nanofactories reported here consist of targeting, sensing, synthesis and, importantly, assembly modules. A bacteria-specific antibody constitutes the targeting module while a genetically engineered fusion protein contains the sensing, synthesis and assembly modules. The nanofactories are assembled on chitosan electrodeposited within a microchannel of the bioMEMS device; they capture QS bacteria in a spatially selective manner and locally synthesize and deliver the "universal" small signaling molecule autoinducer-2 (AI-2) at the captured cell surface. The nanofactory based AI-2 delivery is demonstrated to alter the progression of the native AI-2 based QS response of the captured bacteria. Prospects are envisioned for utilizing our technique as a test-bed for understanding the AI-2 based QS response of bacteria as a means for developing the next generation of antimicrobials.

Role of the C-terminal extension in a bacterial tyrosinase

The well studied bacterial tyrosinases from the Streptomyces sp. bacteria are distinguishable from their eukaryotic counterparts by the absence of a C-terminal extension. In the present study, we report that the tyrosinase from the bacterium Verrucomicrobium spinosum also has such a C-terminal extension, thus making it distinct from the Streptomyces enzymes. The entire tyrosinase gene from V. spinosum codes for a 57 kDa protein (full-length unprocessed form), which has a twin arginine translocase type signal peptide, the two copper-binding motifs typical of the tyrosinase protein family and the aforementioned C-terminal extension. We expressed various mutants of the recombinant enzyme in Escherichia coli and found that removal of the C-terminal extension by genetic engineering or limited trypsin digest of the pro-form results in a more active enzyme (i.e. 30-100-fold increase in monophenolase and diphenolase activities). Further studies also revealed the importance of a phenylalanine residue in this C-terminal domain. These results demonstrate that the V. spinosum tyrosinase is a new example of this interesting family of enzymes. In addition, we show that this enzyme can be readily overproduced and purified and that it will prove useful in furthering the understanding of these enzymes, as well as their biotechnological application.

In situ generation of pH gradients in microfluidic devices for biofabrication of freestanding, semi-permeable chitosan membranes

We report the in situ generation of pH gradients in microfluidic devices for biofabrication of freestanding, semi-permeable chitosan membranes. The pH-stimuli-responsive polysaccharide chitosan was enlisted to form a freestanding hydrophilic membrane structure in microfluidic networks where pH gradients are generated at the converging interface between a slightly acidic chitosan solution and a slightly basic buffer solution. A simple and effective pumping strategy was devised to realize a stable flow interface thereby generating a stable, well-controlled and localized pH gradient. Chitosan molecules were deprotonated at the flow interface, causing gelation and solidification of a freestanding chitosan membrane from a nucleation point at the junction of two converging flow streams to an anchoring point where the two flow streams diverge to two output channels. The fabricated chitosan membranes were about 30-60 microm thick and uniform throughout the flow interface inside the microchannels. A T-shaped membrane formed by sequentially fabricating orthogonal membranes demonstrates flexibility of the assembly process. The membranes are permeable to aqueous solutions and are removed by mildly acidic solutions. Permeability tests suggested that the membrane pore size was a few nanometres, i.e., the size range of antibodies. Building on the widely reported use of chitosan as a soft interconnect for biological components and microfabricated devices and the broad applications of membrane functionalities in microsystems, we believe that the facile, rapid biofabrication of freestanding chitosan membranes can be applied to many biochemical, bioanalytical, biosensing applications and cellular studies.

Orthogonal enzymatic reactions for the assembly of proteins at electrode addresses

The ability to interface proteins to device surfaces is important for a range of applications. Here, we enlist the unique capabilities of enzymes and biologically derived polymers to assemble target proteins to electrode addresses. First, the stimuli-responsive aminopolysaccharide chitosan is directed to assemble at the electrode address in response to electrode-imposed signals. The electrodeposited chitosan film serves as the biodevice interface for subsequent protein assembly. Next, tyrosinase is used to catalyze grafting of a protein or peptide tether to the chitosan film. Finally, microbial transglutaminase (mTG) catalyzes the assembly of target proteins to the tether. mTG covalently links proteins through their glutamine (Gln) and lysine (Lys) residues. Since Gln and Lys residues of globular proteins are often inaccessible to mTG, we engineered our target proteins to have fusion tags with added Gln or Lys residues. This assembly method employs the electrical signal to confer spatial selectivity (during chitosan electrodeposition) and employs the enzymes to confer chemical selectivity (i.e., amino acid residue selectivity). Further, this method is mild, since no reactive reagents or protection steps are required, and all steps are performed in aqueous solution. These results demonstrate the potential for employing biological materials and mechanisms to biofabricate the biodevice interface.

Programmable assembly of a metabolic pathway enzyme in a pre-packaged reusable bioMEMS device

We report a biofunctionalization strategy for the assembly of catalytically active enzymes within a completely packaged bioMEMS device, through the programmed generation of electrical signals at spatially and temporally defined sites. The enzyme of a bacterial metabolic pathway, S-adenosylhomocysteine nucleosidase (Pfs), is genetically fused with a pentatyrosine "pro-tag" at its C-terminus. Signal responsive assembly is based on covalent conjugation of Pfs to the aminopolysaccharide, chitosan, upon biochemical activation of the pro-tag, followed by electrodeposition of the enzyme-chitosan conjugate onto readily addressable sites in microfluidic channels. Compared to traditional physical entrapment and surface immobilization approaches in microfluidic environments, our signal-guided electrochemical assembly is unique in that the enzymes are assembled under mild aqueous conditions with spatial and temporal programmability and orientational control. Significantly, the chitosan-mediated enzyme assembly can be reversed, making the bioMEMS reusable for repeated assembly and catalytic activity. Additionally, the assembled enzymes retain catalytic activity over multiple days, demonstrating enhanced enzyme stability. We envision that this assembly strategy can be applied to rebuild metabolic pathways in microfluidic environments for antimicrobial drug discovery.

Chitosan: a soft interconnect for hierarchical assembly of nano-scale components

Traditional microfabrication has tremendous capabilities for imparting order to hard materials (e.g., silicon wafers) over a range of length scales. However, conventional microfabrication does not provide the means to assemble pre-formed nano-scale components into higher-ordered structures. We believe the aminopolysaccharide chitosan possesses a unique set of properties that enable it to serve as a length-scale interconnect for the hierarchical assembly of nano-scale components into macro-scale systems. The primary amines (atomic length scale) of the glucosamine repeating units (molecular length scale) provide sites to connect pre-formed or self-assembled nano-scale components to the polysaccharide backbone (macromolecular length scale). Connections to the backbone can be formed by exploiting the electrostatic, nucleophilic, or metal-binding capabilities of the glucosamine residues. Chitosan's film-forming properties provide the means for assembly at micron-to-centimetre lengths (supramolecular length scales). In addition to interconnecting length scales, chitosan's capabilities may also be uniquely-suited as a soft component-hard device interconnect. In particular, chitosan's film formation can be induced under mild aqueous conditions in response to localized electrical signals that can be imposed from microfabricated surfaces. This capability allows chitosan to assemble soft nano-scale components (e.g., proteins, vesicles, and virus particles) at specific electrode addresses on chips and in microfluidic devices. Thus, we envision the potential that chitosan may emerge as an integral material for soft matter (bio)fabrication.

Magnetic nanofactories: Localized synthesis and delivery of quorum-sensing signaling molecule autoinducer-2 to bacterial cell surfaces

Magnetic 'nanofactories', for localized manufacture and signal-guided delivery of small molecules to targeted cell surfaces, are demonstrated. They recruit nearby raw materials for synthesis, employ magnetic mobility for capture and localization of target cells, and deliver molecules to cells triggering their native phenotypic response, but with user-specified control. Our nanofactories, which synthesize and deliver the "universal" bacterial quorum-sensing signal molecule, autoinducer AI-2, to the surface of Escherichia coli, are assembled by first co-precipitating nanoparticles of iron salts and the biopolymer chitosan. E. coli AI-2 synthases, Pfs and LuxS, constructed with enzymatically activatable "pro-tags", are then covalently tethered onto the chitosan. These enzymes synthesize AI-2 from metabolite S-adenosylhomocysteine. Chitosan serves as a molecular scaffold and provides cell capture ability; magnetite provides stimuli responsiveness. These magnetic nanofactories are shown to modulate the natural progression of quorum-sensing activity. New prospects for small molecule delivery, based on localized synthesis, are envisioned.

Biopolymer-based materials: the nanoscale components and their hierarchical assembly

Protein and nucleic acid biopolymers are well appreciated for their high-performance capabilities for molecular recognition, catalysis and information storage. Increasingly, these biopolymers are being examined for materials applications. Less tractable are polysaccharides and polymers of phenols, which, despite being nature's most abundant macromolecules, remain largely ignored for advanced materials applications. In our opinion, it seems certain that biology will contribute two major capabilities for materials biofabrication - the means to generate biopolymeric components with nanoscale precision, and the mechanisms for the hierarchical assembly of nanocomponents. These capabilities will enable unprecedented control of materials structure and provide exciting opportunities at the convergence of molecular biology and macromolecular science.

Protein assembly onto patterned microfabricated devices through enzymatic activation of fusion pro-tag

We report a versatile approach for covalent surface-assembly of proteins onto selected electrode patterns of pre-fabricated devices. Our approach is based on electro-assembly of the aminopolysaccharide chitosan scaffold as a stable thin film onto patterned conductive surfaces of the device, which is followed by covalent assembly of the target protein onto the scaffold surface upon enzymatic activation of the protein's "pro-tag." For our demonstration, the model target protein is green fluorescent protein (GFP) genetically fused with a pentatyrosine pro-tag at its C-terminus, which assembles onto both two-dimensional chips and within fully packaged microfluidic devices in situ and under flow. Our surface-assembly approach enables spatial selectivity and orientational control under mild experimental conditions. We believe that our integrated approach harnessing genetic manipulation, in situ enzymatic activation, and electro-assembly makes it advantageous for a wide variety of bioMEMS and biosensing applications that require facile "biofunctionalization" of microfabricated devices.