Computer-Guided Surface Engineering for Enzyme Improvement

Non-covalent binding tags for batch and flow biocatalysis

Enzyme immobilization offers considerable advantage for biocatalysis in batch and continuous flow reactions. However, many currently available immobilization methods require that the surface of the carrier is chemically modified to allow site specific interactions with their cognate enzymes, which requires specific processing steps and incurs associated costs. Two carriers (cellulose and silica) were investigated here, initially using fluorescent proteins as models to study binding, followed by assessment of industrially relevant enzyme performance (transaminases and an imine reductase/glucose oxidoreductase fusion). Two previously described binding tags, the 17 amino acid long silica-binding peptide from the Bacillus cereus CotB protein and the cellulose binding domain from the Clostridium thermocellum, were fused to a range of proteins without impairing their heterologous expression. When fused to a fluorescent protein both tags conferred high avidity specific binding with their respective carriers (low nanomolar K_d values). The CotB peptide (CotB1p) induced protein aggregation in the transaminase and imine reductase/glucose oxidoreductase fusions when incubated with the silica carrier. The Clostridium thermocellum cellulose binding domain (CBDclos) allowed immobilization of all the proteins tested, but immobilization led to loss of enzymatic activity in the transaminases (< 2-fold) and imine reductase/glucose oxidoreductase fusion (> 80%). A transaminase-CBDclos fusion was then successfully used to demonstrate the application of the binding tag in repetitive batch and a continuous-flow reactor.

Design of fusion enzymes for biocatalytic applications in aqueous and non-aqueous media

Biocatalytic cascades play a fundamental role in sustainable chemical synthesis. Fusion enzymes are one of the powerful toolboxes to enable the tailored combination of multiple enzymes for efficient cooperative cascades. Especially, this approach offers a substantial potential for the practical application of cofactor-dependent oxidoreductases by forming cofactor self-sufficient cascades. Adequate cofactor recycling while keeping the oxidized/reduced cofactor in a confined microenvironment benefits from the fusion fashion and makes the use of oxidoreductases in harsh non-aqueous media practical. In this mini-review, we have summarized the application of various fusion enzymes in aqueous and non-aqueous media with a focus on the discussion of linker design within oxidoreductases. The design and properties of the reported linkers have been reviewed in detail. Besides, the substrate loadings in these studies have been listed to showcase one of the key limitations (low solubility of hydrophobic substrates) of aqueous biocatalysis when it comes to efficiency and economic feasibility. Therefore, a straightforward strategy of applying non-aqueous media has been briefly discussed while the potential of using the fusion oxidoreductase of interest in organic media was highlighted.

State-of-the-Art Biocatalysis

The use of enzyme-mediated reactions has transcended ancient food production to the laboratory synthesis of complex molecules. This evolution has been accelerated by developments in sequencing and DNA synthesis technology, bioinformatic and protein engineering tools, and the increasingly interdisciplinary nature of scientific research. Biocatalysis has become an indispensable tool applied in academic and industrial spheres, enabling synthetic strategies that leverage the exquisite selectivity of enzymes to access target molecules. In this Outlook, we outline the technological advances that have led to the field's current state. Integration of biocatalysis into mainstream synthetic chemistry hinges on increased access to well-characterized enzymes and the permeation of biocatalysis into retrosynthetic logic. Ultimately, we anticipate that biocatalysis is poised to enable the synthesis of increasingly complex molecules at new levels of efficiency and throughput.

Tuning of adsorption of enzymes to polymer

In the past years, several serine hydrolases such as cutinases, esterases and lipases have shown the ability to degrade not only natural polymers but also synthetic polyesters, even aromatic representatives like polyethylene terephthalate (PET). Hence, cutinases and related ester hydrolases have become very important to be applied in the biocatalytic plastic recycling as green alternative to chemical recycling as well as to the functionalization of polyester surfaces in order to change superficial properties like hydrophobicity or hydrophilicity. Sorption characteristics of the enzymes to the polymers have turned out to be a crucial process for efficient polymer hydrolysis. Hence, special attention was paid on tuning the sorption of the enzymes to the hydrophobic polymers. Engineering of the enzyme surface, fusion of hydrophobic substrate-binding domains or truncation of domains hindering the access of the polymer to the enzyme has led to significant improvement of sorption processes and consequently increased activity on the bulky substrate. Finally, the combination of engineering approaches has proved that they can bring additional advantages in improving the enzyme activity when used in a synergistic manner.

The hydrophobicity of an amino acid residue in a flexible loop of KP-43 protease alters activity toward a macromolecule substrate

Abstract

KP-43, a 43-kDa alkaline serine protease, is resistant to chemical oxidants and surfactants, making it suitable for use in laundry detergents. An amino acid residue at position 195, in a unique flexible loop that binds a Ca²⁺ ion, dramatically affects the proteolytic activity and thermal stability of KP-43. In the present study, we obtained 20 variants with substitutions at position 195 and investigated how these residues affect hydrolytic activity toward a macromolecular substrate (casein) and a synthetic tetra-peptide (AAPL). At pH 10, the variant with the highest caseinolytic activity, Tyr195Gln, exhibited 4.4-fold higher activity than the variant with the lowest caseinolytic activity, Tyr195Trp. A significant negative correlation was observed between the hydrophobicity of the residue at position 195 and caseinolytic activity at pH 8–10. At pH 7, the correlation became weak; at pH 6, the correlation reversed to positive. Unlike casein, in the case of hydrolysis of AAPL, no correlation was observed at pH 10 or pH 6. Because the amino acid residue at position 195 is located on the protein surface and considered sufficiently far from the active cleft, the variation in caseinolytic activity between the 20 variants was attributed to changes in interaction efficiency with different states of casein at different pH values. To improve the enzymatic activity, we propose substituting amino acid residues on the protein surface to change the efficiency of interaction with the macromolecular substrates.

Key points

• A single amino acid residue on the protein surface markedly changed enzyme activity.

• The hydrophobicity of the amino acid residue and enzyme activity had a correlation.

• The key amino acid residue for substrate recognition exists on the protein surface.

Electronic supplementary material

The online version of this article (10.1007/s00253-020-10826-2) contains supplementary material, which is available to authorized users.

Effects of His-tag on Catalytic Activity and Enantioselectivity of Recombinant Transaminases

Recombinant proteins were often expressed with His-tag to simplify the purification process. Among them, transaminase was mostly expressed with fusion tags and widely used in the production of numerous amino moieties. However, the existence of the His-tag has been reported to affect various properties of different recombinant enzymes, while the effect on transaminase was rarely studied. In this paper, we investigated the effect of His-tag on transaminase based on the various activities of 4-aminobutyrate-2-oxoglutarate transaminase (GabT) when it was expressed in vector pETDuet-1. We found that His-tag did not affect the enantioselectivity, but decreased the catalytic activity to different extents according to its existence and location. Native GabT maintained the highest catalytic activity; GabT with C-terminal His-tag showed slightly lower activity than native GabT but about 2.2-fold higher than GabT with N-terminal His-tag. Besides, other fusion tags like T7-tag and S-tag inserted between N-His-tag and GabT can relieve the decreasing effect of His-tag on GabT activity. Furthermore, whole cell catalytic activity of several transaminases was improved by deleting the N-terminal His-tag. This study provided a strategy for the efficient expression of recombinant transaminase with improved catalytic activity and might attract attention to the effect of His-tag on other enzymatic properties.

Protein engineering: the potential of remote mutations

Engineered proteins, especially enzymes, are now commonly used in many industries owing to their catalytic power, specific binding of ligands, and properties as materials and food additives. As the number of potential uses for engineered proteins has increased, the interest in engineering or designing proteins to have greater stability, activity and specificity has increased in turn. With any rational engineering or design pursuit, the success of these endeavours relies on our fundamental understanding of the systems themselves; in the case of proteins, their structure-dynamics-function relationships. Proteins are most commonly rationally engineered by targeting the residues that we understand to be functionally important, such as enzyme active sites or ligand-binding sites. This means that the majority of the protein, i.e. regions remote from the active- or ligand-binding site, is often ignored. However, there is a growing body of literature that reports on, and rationalises, the successful engineering of proteins at remote sites. This minireview will discuss the current state of the art in protein engineering, with a particular focus on engineering regions that are remote from active- or ligand-binding sites. As the use of protein technologies expands, exploiting the potential improvements made possible through modifying remote regions will become vital if we are to realise the full potential of protein engineering and design.