Ruth Maas

Autodisplay of enzymes—Molecular basis and perspectives

To display an enzyme on the surface of a living cell is an important step forward towards a broader use of biocatalysts. Enzymes immobilized on surfaces appeared to be more stable compared to free molecules. It is possible by standard techniques to let the bacterial cell (e.g. Escherichia coli) decorate its surface with the enzyme and produce it on high amounts with a minimum of costs and equipment. Moreover, these cells can be recovered and reused in several subsequent process cycles. Among other systems, autodisplay has some extra features that could overcome limitations in the industrial applications of enzymes. One major advantage of autodisplay is the motility of the anchoring domain. Enzyme subunits exposed at the cell surface having affinity to each other will spontaneously form dimers or multimers. Using autodisplay enzymes with prosthetic groups can be displayed, expanding the application of surface display to the industrial important P450 enzymes. Finally, up to 10⁵-10⁶ enzyme molecules can be displayed on a single cell. In the present review, we summarize recent achievements in the autodisplay of enzymes with particular attention to industrial needs and process development. Applications that will provide sustainable solutions towards a bio-based industry are discussed.

Esterase Autodisplay: Enzyme Engineering and Whole-Cell Activity Determination in Microplates with pH Sensors

ABSTRACT Among the GDSL family of serine esterases/lipases is a group of bacterial enzymes that posses C-terminal extensions involved in outer membrane anchoring or translocation. ApeE from Salmonella enterica serovar Typhimurium, a member of this group, has been expressed in Escherichia coli and was resistant to protease digestion when the protease was added to whole cells, indicating a periplasmic localization. The five consensus blocks conserved within all GDSL esterases were identified in ApeE by multiple sequence alignment and separated from the C-terminal extension. The DNA sequence spanning the four invariant residues Ser, Gly, Asn, and His, and hence representing the catalytic domains of ApeE, was amplified by PCR and fused in frame to the transport domains of the autodisplay system. The resulting artificial esterase, called EsjA, was overexpressed in the cell envelope of E. coli and was shown to be active by the use of α-naphthyl acetate (α-NA) as a substrate in an in-gel activity stain after sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Surface exposure of EsjA was indicated by its accessibility to protease added to whole cells. The esterase activity of whole cells displaying EsjA was determined by a pH agar assay and by the use of microplates with integrated pH-dependent optical sensors. α-NA, α-naphthyl butyrate, and α-naphthyl caproate were used as substrates, and it turned out that the substrate preferences of artificial EsjA were altered in comparison to original ApeE. Our results indicate that autodisplay of esterase in combination with pH sensor microplates can provide a new platform technology for the screening of tailor-made hydrolase activities.

Autodisplay of Nitrilase from Alcaligenes faecalis in E. coli Yields a Whole Cell Biocatalyst for the Synthesis of Enantiomerically Pure (R)‐Mandelic Acid

In the present study, a whole cell biocatalyst for the synthesis of (R)‐mandelic acid from mandelonitrile was constructed. For this purpose, nitrilase from Alcaligenes faecalis subsp. faecalis ATCC 8750 was displayed on the surface of Escherichia coli by using Autodisplay. Localization of the nitrilase in the cell envelope of E. coli was monitored by SDS‐PAGE and surface exposure was verified by its accessibility to externally added protease. The whole cell biocatalyst converted up to 2.6 mM of (R)‐mandelic acid under optimum conditions at pH 7.5 and 45 °C within 24 h (1 mL culture, OD578=10). By using chiral HPLC, the ee value of the product was determined to be >99 %. The surface displayed nitrilase showed an apparent Km value of 3.6 mM and an apparent Vmax value of 1 nmol min−1 mL−1 when a bacterial suspension of OD578 3 was used. Substrate inhibition by benzaldehyde was similar to that of the free enzyme. The whole‐cell biocatalyst retained 55 % of its initial (R)‐mandelic acid production after 5 cycles of repeated use, and could be stored at −70 °C for 180 d without a substantial loss of activity. In addition the whole cell biocatalyst converted 9.3 mM phenylacetonitrile within 16 h.

Development of a Whole Cell Biocatalyst for the Efficient Prenylation of Indole Derivatives by Autodisplay of the Aromatic Prenyltransferase FgaPT2

The following study depicts the development of a whole cell biocatalyst for the prenylation of indole derivatives. For this purpose the prenyltransferase FgaPT2 from Aspergillus fumigatus was displayed on the surface of Escherichia coli cells by using Autodisplay. The presence of the prenyltransferase in the outer membrane was detected by using SDS‐PAGE and Western Blot after the proteins of the outer membrane were isolated. The orientation of the prenyltransferase towards the outside of the cells was investigated by accessibility testing with externally added proteases. The FgaPT2 whole cell biocatalyst converted up to 250 μM of indole‐3‐propionic acid, approximately 25 % of the substrate used in the assay (100 μL sample, OD578=40). Another indole substrate, L‐β‐homotryptophan was also investigated and a conversion of 13 % was determined. By optimizing the assay conditions the conversion rate could be raised to approximately 30 % of indole‐3‐propionic acid during a 24 h incubation time at 20 °C. The whole cell biocatalyst endured a storage period of one month at 8 °C without any detectable loss in activity. Reusability was confirmed by recycling the biocatalyst. After three cycles of consecutive use, the whole cell biocatalyst retained a conversion rate of 46 % of indole‐3‐propionic acid and 23 % of L‐β‐homotryptophan after the third cycle.

Lignocellulases: a review of emerging and developing enzymes, systems, and practices

The highly acclaimed prospect of renewable lignocellulosic biocommodities as obvious replacement of their fossil-based counterparts is burgeoning within the last few years. However, the use of the abundant lignocellulosic biomass provided by nature to produce value-added products, especially bioethanol, still faces significant challenges. One of the crucial challenging factors is in association with the expression levels, stability, and cost-effectiveness of the cellulose-degrading enzymes (cellulases). Interestingly, several recommendable endeavors in the bid to curb these challenges are in pursuance. However, the existing body of literature has not well provided the updated roadmap of the advancement and key players spearheading the current success. Moreover, the description of enzyme systems and emerging paradigms with high prospects, for example, the cell-surface display system has been ill-captured in the literature. This review focuses on the lignocellulosic biocommodity pathway, with emphasis on cellulase and hemicellulase systems. The paradigm shift towards cell-surface display system and its emerging recommendable developments have also been discussed. The attempts in supplementing cellulase with other enzymes, accessory proteins, and chemical additives have also been discussed. Moreover, some of the prominent and influential discoveries in the cellulase fraternity have been discussed.Graphical abstractThe roadmap of cellulose-degrading enzymes

Co-expression of active human cytochrome P450 1A2 and cytochrome P450 reductase on the cell surface of Escherichia coli

BackgroundHuman cytochrome P450 (CYP) enzymes mediate the first step in the breakdown of most drugs and are strongly involved in drug–drug interactions, drug clearance and activation of prodrugs. Their biocatalytic behavior is a key parameter during drug development which requires preparative synthesis of CYP related drug metabolites. However, recombinant expression of CYP enzymes is a challenging bottleneck for drug metabolite biosynthesis. Therefore, we developed a novel approach by displaying human cytochrome P450 1A2 (CYP1A2) and cytochrome P450 reductase (CPR) on the surface of Escherichia coli.ResultsTo present human CYP1A2 and CPR on the surface, we employed autodisplay. Both enzymes were displayed on the surface which was demonstrated by protease and antibody accessibility tests. CPR activity was first confirmed with the protein substrate cytochrome c. Cells co-expressing CYP1A2 and CPR were capable of catalyzing the conversion of the known CYP1A2 substrates 7-ethoxyresorufin, phenacetin and the artificial substrate luciferin-MultiCYP, which would not have been possible without interaction of both enzymes. Biocatalytic activity was strongly influenced by the composition of the growth medium. Addition of 5-aminolevulinic acid was necessary to obtain a fully active whole cell biocatalyst and was superior to the addition of heme.ConclusionWe demonstrated that CYP1A2 and CPR can be co-expressed catalytically active on the cell surface of E. coli. It is a promising step towards pharmaceutical applications such as the synthesis of drug metabolites.

A simplified process design for P450 driven hydroxylation based on surface displayed enzymes.

New production routes for fine and bulk chemicals are important to establish further sustainable processes in industry. Besides the identification of new biocatalysts and new production routes the optimization of existing processes in regard to an improved utilization of the catalysts are needed. In this paper we describe the successful expression of P450BM3 on the surface of E. coli cells with the Autodisplay system. The successful hydroxylation of palmitic acid by using surface‐displayed P450BM3 was shown. Besides optimization of surface protein expression, several cofactor regeneration systems were compared and evaluated. Afterwards, the development of a suitable process for the biocatalytic hydroxylation of fatty acids based on the re‐use of the catalysts after a simple centrifugation was investigated. It was shown that the catalyst can be used for several times without any loss in activity. By using surface‐displayed P450s in combination with an enzymatic cofactor regeneration system a total turnover number of up to 54,700 could be reached, to the knowledge of the authors the highest value reported for a P450 monooxygenase to date. Further optimizations of the described reaction system can have an enormous impact on the process design for more sustainable bioprocesses. Biotechnol. Bioeng. 2016;113: 1225–1233.

Autoantibodies to αS1-Casein Are Induced by Breast-Feeding

Background The generation of antibodies is impaired in newborns due to an immature immune system and reduced exposure to pathogens due to maternally derived antibodies and placental functions. During nursing, the immune system of newborns is challenged with multiple milk-derived proteins. Amongst them, caseins are the main constituent. In particular, human αS1-casein (CSN1S1) was recently shown to possess immunomodulatory properties. We were thus interested to determine if auto-antibodies to CSN1S1 are induced by breast-feeding and may be sustained into adulthood. Methods 62 sera of healthy adult individuals who were (n = 37) or were not (n = 25) breast-fed against human CSN1S1 were investigated by a new SD (surface display)-ELISA. For cross-checking, these sera were tested for anti Epstein-Barr virus (EBV) antibodies by a commercial ELISA. Results IgG-antibodies were predominantly detected in individuals who had been nursed. At a cut-off value of 0.4, the SD-ELISA identified individuals with a history of having been breast-fed with a sensitivity of 80% and a specificity of 92%. Under these conditions, 35 out of 37 sera from healthy donors, who where breast-fed, reacted positively but only 5 sera of the 25 donors who were not breast-fed. The duration of breast-feeding was of no consequence to the antibody reaction as some healthy donors were only short term breast-fed (5 days minimum until 6 weeks maximum), but exhibited significant serum reaction against human CSN1S1 nonetheless. Conclusion We postulate that human CSN1S1 is an autoantigen. The antigenicity is orally determined, caused by breast-feeding, and sustained into adulthood.