Synthesis of functional dipeptide carnosine from nonprotected amino acids using carnosinase-displaying yeast cells

Identification and structure-based engineering of a dipeptidase CpPepD from Clostridium perfringens for the synthesis of l-carnosine

l-Carnosine (l-Car), an endogenous dipeptide presents in muscle and brain tissues of various vertebrates, has a wide range of application values. The enzymatic preparation of l-Car is a promising synthetic method because it avoids the protection and deprotection steps. In the present study, a dipeptidase gene (CpPepD) from Clostridium perfringens with high l-Car synthetic activity was cloned and characterized. In an effort to improve the performance of this enzyme, we carried out site saturation mutagenesis using CpPepD as the template. By the o-phthalaldehyde (OPA)-derived high throughput screening method, mutant A171S was obtained with 2.2-fold enhanced synthetic activity. The enzymatic properties of CpPepD and mutant A171S were investigated. Under the optimized conditions, 63.94 mM (14.46 g L-1) or 67.02 mM (15.16 g L-1) l-Car was produced at the substrate concentrations of 6 M β-Ala and 0.2 M l-His using wild-type or mutant A171S enzyme, respectively. Although the mutation enhanced the enzyme activity, the reaction equilibrium was barely affected.

Characterization of a new L-carnosine synthase mined from deep-sea sediment metagenome

L-Carnosine is a natural biologically active dipeptide with critical physiological functions, such as antioxidant, antiglycation, and cytoplasmic buffering properties. Direct enzymatic synthesis is a promising way for L-carnosine production. In this study, a new aminopeptidase (gene_236976) with synthetic activity toward L-carnosine was identified by a metagenome mining approach from deep-sea sediment and functionally expressed in Escherichia coli. The enzyme shared a low identity of 14.3% with reported L-carnosine dipeptidase (SmPepD) from Serratia marcescens. β-Alanine methyl ester was proven to be the best substrate for the synthesis, and no ATP was needed for the enzymatic reaction. The enzyme activity was increased by structure-guided rational design. Only the mutant of G310 site gave positive results, and G310A mutant showed the best performance among the site-direct saturation mutagenesis, indicating that the additional CH₃ group of mutant G310A was the main factor affecting the enzymatic activity. The engineered enzyme produced about 10 mM L-carnosine was produced from substrates of 50 mM β-alanine methyl ester and 50 mM L-histidine, under a tentatively optimized condition. This study enriched the enzyme resources for developing the microbial synthesis process of L-carnosine production.

Generation of Arming Yeasts with Active Proteins and Peptides via Cell Surface Display System: Cell Surface Engineering, Bio-Arming Technology

The cell surface display system in yeast enables the innovative strategy for improving cellular functions in a wide range of applications such as biofuel production, bioremediation, synthesis of valuable chemicals, recovery of rare metal ions, development of biosensors, and high-throughput screening of protein/peptide library. Display of enzymes for polysaccharide degradation enables the construction of metabolically engineered whole-cell biocatalyst owing to the accessibility of the displayed enzymes to high-molecular-weight polysaccharides. In addition, along with fluorescence-based activity evaluation, fluorescence-activated cell sorting (FACS), and yeast cell chip, the cell surface display system is an effective molecular tool for high-throughput screening of mutated protein/peptide library. In this article, we describe the methods for cell surface display of proteins/peptides of interest on yeast, evaluation of display efficiency, and harvesting of the displayed proteins/peptides from cell surface.

A green-by-design bioprocess forl-carnosine production integrating enzymatic synthesis with membrane separation

This simple and clean bioprocess enables the economically attractive and environmentally benign production of the bioactive dipeptidel-Car.

Establishment of cell surface engineering and its development

Cell surface display of proteins/peptides has been established based on mechanisms of localizing proteins to the cell surface. In contrast to conventional intracellular and extracellular (secretion) expression systems, this method, generally called an arming technology, is particularly effective when using yeasts as a host, because the control of protein folding that is often required for the preparation of proteins can be natural. This technology can be employed for basic and applied research purposes. In this review, I describe various strategies for the construction of engineered yeasts and provide an outline of the diverse applications of this technology to industrial processes such as the production of biofuels and chemicals, as well as bioremediation and health-related processes. Furthermore, this technology is suitable for novel protein engineering and directed evolution through high-throughput screening, because proteins/peptides displayed on the cell surface can be directly analyzed using intact cells without concentration and purification. Functional proteins/peptides with improved or novel functions can be created using this beneficial, powerful, and promising technique.

Display of heterologous proteins on the Saccharomyces cerevisiae surface display system using a single constitutive expression vector

In this study, we constructed a novel and simple yeast surface display system with a single expression vector. The newly established system uses a bidirectional expression vector carrying the AGA1 gene driven by the PGK1 promoter in one direction and the AGA2-expression cassette driven by the TEF1 promoter in the reverse direction, and uses the geneticin, a G418-resistant gene, as the selection marker for transformants. Because all the display elements are put into one expression vector, the new system is much simpler to use, and there is no need for any genetic modification of the host strains; therefore, the new system can be used in wild type as well as laboratory strains of Saccharomyces cerevisiae. The display efficiency of heterologous proteins using the new system has been confirmed by displaying enhanced green fluorescent protein and Eimeria tenella (a chicken protozoan parasite) microneme protein2 (EtMic2) on several S. cerevisiae strains. We also tested the new system with an aga2 mutant strain of S. cerevisiae. The results indicate that the native expressed Aga2 protein has no effect on the display efficiency of heterologous proteins.

Generation of arming yeasts with active proteins and peptides via cell surface display system: cell surface engineering, bio-arming technology

The cell surface display system in yeast enables the innovative strategy for improving cellular functions in a wide range of applications such as biofuel production, bioremediation, synthesis of valuable chemicals, recovery of rare metal ions, development of biosensors, and high-throughput screening of proteins/peptides library. Display of enzymes for polysaccharide degradation enables the construction of metabolically engineered whole-cell biocatalyst owing to the accessibility of the displayed enzymes to high-molecular-weight polysaccharides. In addition, along with fluorescence-based activity evaluation, fluorescence-activated cell sorting (FACS), and yeast cell chip, the cell surface display system is an effective molecular tool for high-throughput screening of mutated proteins/peptides library. In this article, we describe the methods for cell surface display of proteins/peptides of interest on yeast, evaluation of display efficiency, and harvesting of the displayed proteins/peptides from cell surface.

Arming Technology in Yeast-Novel Strategy for Whole-cell Biocatalyst and Protein Engineering

Cell surface display of proteins/peptides, in contrast to the conventional intracellular expression, has many attractive features. This arming technology is especially effective when yeasts are used as a host, because eukaryotic modifications that are often required for functional use can be added to the surface-displayed proteins/peptides. A part of various cell wall or plasma membrane proteins can be genetically fused to the proteins/peptides of interest to be displayed. This technology, leading to the generation of so-called "arming technology", can be employed for basic and applied research purposes. In this article, we describe various strategies for the construction of arming yeasts, and outline the diverse applications of this technology to industrial processes such as biofuel and chemical productions, pollutant removal, and health-related processes, including oral vaccines. In addition, arming technology is suitable for protein engineering and directed evolution through high-throughput screening that is made possible by the feature that proteins/peptides displayed on cell surface can be directly analyzed using intact cells without concentration and purification. Actually, novel proteins/peptides with improved or developed functions have been created, and development of diagnostic/therapeutic antibodies are likely to benefit from this powerful approach.

Development of surface-engineered yeast cells displaying phytochelatin synthase and their application to cadmium biosensors by the combined use of pyrene-excimer fluorescence

The development of simple, portable, inexpensive, and rapid analytical methods for detecting and monitoring toxic heavy metals are important for the safety and security of humans and their environment. Herein, we describe the application of phytochelatin (PC) synthase, which plays a critical role in heavy metal responses in higher plants and green algae, in a novel fluorescent sensing platform for cadmium (Cd). We first created surface-engineered yeast cells on which the PC synthase from Arabidopsis (AtPCS1) was displayed with retention of enzymatic activity. The general concept for the sensor is based on the Cd level-dependent synthesis of PC2 from glutathiones by AtPCS1-displaying yeast cells, followed by simple discriminative detection of PC2 via sensing of excimer fluorescence of thiol-labeling pyrene probes. The intensity of excimer fluorescence increased in the presence of Cd up to 1.0 μM in an approximately dose-dependent manner. This novel biosensor achieved a detection limit of as low as 0.2 μM (22.5 μg/L) for Cd. Although its use may be limited by the fact that Cu and Pb can induce cross-reaction, the proposed simple biosensor holds promise as a method useful for cost-effective screening of Cd contamination in environmental and food samples. The AtPCS1-displaying yeast cells also might be attractive tools for dissection of the catalytic mechanisms of PCS.

Development of a novel rDNA based plasmid for enhanced cell surface display on Yarrowia lipolytica

In this study, a novel rDNA based plasmid was developed for display of heterologous proteins on the cell surface of Yarrowia lipolytica using the C-terminal end of the glycosylphosphatidylinositol (GPI) anchored Y. lipolytica cell wall protein 1 (YlCWP1). mCherry was used as a model protein to assess the efficiency of the constructed plasmid. Y. lipolytica transformants harbouring the expression cassettes showed a purple colour phenotype on selective YNB-casamino plates as compared to control cells indicating that mCherry was displayed on the cells. Expression of mCherry on cells of Y. lipolytica was confirmed by both fluorescent microscopy and flow cytometry. Furthermore, SDS-PAGE analysis and matrix-assisted laser desorption/ionization (MALDI)-time-of (TOF)-mass spectrometry (MS) peptide mass fingerprinting (PMF) confirmed that the protein cleaved from the yeast cells using enterokinase was mCherry. Efficient cleavage of mCherry reported in this work offers an alternative purification method for displayed heterologous proteins on Y. lipolytica cells using the plasmid constructed in this study. The developed displaying system offers great potential for industrial production and purification of heterologous proteins at low cost.

Cell surface engineering of yeast for applications in white biotechnology

Cell surface engineering is a promising strategy for the molecular breeding of whole-cell biocatalysts. By using this strategy, yeasts can be constructed by the cell surface display of functional proteins; these yeasts are referred to as arming yeasts. Because reactions using arming yeasts as whole-cell biocatalysts occur on the cell surface, materials that cannot enter the cell can be used as reaction substrates. Numerous arming yeasts have therefore been constructed for a wide range of uses such as biofuel production, synthesis of valuable chemicals, adsorption or degradation of environmental pollutants, recovery of rare metal ions, and biosensors. Here, we review the science of yeast cell surface modification as well as current applications and future opportunities.