Dirk Holtmann

Enzymatic reductions for the chemist

When challenged by a difficult reduction reaction, a chemist should always also consider biocatalysis in synthesis planning. The inherent selectivity of enzymes has been known for many decades now and the practical applicability of biocatalysis has undergone dramatic improvements rendering it a true alternative to established chemocatalysis. In this contribution recent developments in the field of enzymatic reduction using whole cells and isolated enzymes are reviewed.

Methanol-based industrial biotechnology: current status and future perspectives of methylotrophic bacteria

Methanol is one of the building blocks in the chemical industry and can be synthesized either from petrochemical or renewable resources, such as biogas. Bioprocess technology with methylotrophic bacteria is well established, as illustrated by large-scale single-cell protein production in the past. During recent years, the first genomes of methylotrophs have been sequenced and significant progress in elucidating their metabolism has been made. In addition, the tool set for genetic engineering of methylotrophic bacteria has expanded greatly and strategies to produce fine and bulk chemicals with methylotrophs have been described. This review highlights the potential of these bacteria for the development of economically competitive bioprocesses based on methanol as an alternative carbon source, bringing together biological, technical and economic considerations.

How Green is Biocatalysis? To Calculate is To Know

Green chemistry aims to minimize the environmental hazards of chemical processes and their products. Ever since its introduction by Anastas, this principle has inspired researchers to critically rethink chemistry in view of its potential impact on the environment. Especially, the 12 Principles of Green Chemistry have been an inspiring guideline for this process. Biocatalysis is widely considered as one of the key technologies fulfilling the 12 principles and thereby being green chemistry per se. The reader will recognize stereotypical boilerplate statements such as “enzymes as renewable and biodegradable catalysts”, “working under environmentally benign conditions (temperature, pH, etc.)”, and “operating in water as an environmentally benign solvent” frequently found in the introductory passages to biocatalysis publications. Indeed, these are important parameters that can make biocatalysis environmentally more acceptable than “classical” chemical methods, provided the advantages are not (over)compensated by the disadvantages. Unfortunately, the latter are discussed to a much lesser extent. We also noted a certain tendency to pick a few of the 12 principles to underline the greenness of the method published, which certainly is in contrast to the envisioned use of the 12 principles as a cohesive system. We believe that the time is now to transition from just claiming environmental benefits of (bio)catalysis to quantifying the environmental impact. Indeed, the potential of biocatalysis as a tool to make chemical processes greener has been demonstrated by a limited number of studies. These studies mostly comprise the life-cycle assessment (LCA) of the processes. Unfortunately, LCAs are still rather complex and work intensive. Therefore, LCAs are appreciated by industry to evaluate existing processes (and to use the positive result as a selling argument), whereas research-oriented academic groups generally do not possess the expertise, resources, and interest to perform LCAs. As a result, sustainability issues are often addressed in the phase of manuscript preparation as described above. We believe that on the long term, careless, qualitative use of the term green chemistry will discredit the concept. Therefore, with this contribution we wish to promote the use of simple metrics to assess the environmental footprint of a given method in a semi-quantitative way. To calculate is to know (better). Sheldon proposed the E factor (environmental factor) to assess the greenness of a given reaction. The E factor denotes the amount of waste generated per product equivalent [Eq. (1)] .

Electroactive bacteria—molecular mechanisms and genetic tools

In nature, different bacteria have evolved strategies to transfer electrons far beyond the cell surface. This electron transfer enables the use of these bacteria in bioelectrochemical systems (BES), such as microbial fuel cells (MFCs) and microbial electrosynthesis (MES). The main feature of electroactive bacteria (EAB) in these applications is the ability to transfer electrons from the microbial cell to an electrode or vice versa instead of the natural redox partner. In general, the application of electroactive organisms in BES offers the opportunity to develop efficient and sustainable processes for the production of energy as well as bulk and fine chemicals, respectively. This review describes and compares key microbiological features of different EAB. Furthermore, it focuses on achievements and future prospects of genetic manipulation for efficient strain development.

Reactor concepts for bioelectrochemical syntheses and energy conversion

In bioelectrochemical systems (BESs) at least one electrode reaction is catalyzed by microorganisms or isolated enzymes. One of the existing challenges for BESs is shifting the technology towards industrial use and engineering reactor systems at adequate scales. Due to the fact that most BESs are usually deployed in the production of large-volume but low-value products (e.g., energy, fuels, and bulk chemicals), investment and operating costs must be minimized. Recent advances in reactor concepts for different BESs, in particular biofuel cells and electrosynthesis, are summarized in this review including electrode development and first applications on a technical scale. A better understanding of the impact of reactor components on the performance of the reaction system is an important step towards commercialization of BESs.

Immobilization of histidine-tagged proteins on electrodes.

The development of new enzyme immobilization techniques that do not affect catalytic activity or conformation of a protein is an important research task in biotechnology including biosensor applications and heterogeneous reaction systems. One of the most promising approaches for controlled protein immobilization is based on the immobilized metal ion affinity chromatography (IMAC) principle originally developed for protein purification. Here we describe the current status and future perspectives of immobilization of His-tagged proteins on electrode surfaces. Recombinant proteins comprising histidine-tags or histidine rich native proteins have a strong affinity to transition metal ions. For metal ion immobilization at the electrode surface different matrices can be used such as self-assembled monolayers or conductive polymers. This specific technique allows a reversible immobilization of histidine-tagged proteins at electrodes in a defined orientation which is an important prerequisite for efficient electron transfer between the electrode and the biomolecule. Any application requiring immobilized biocatalysts on electrodes can make use of this immobilization approach, making future biosensors and biocatalytic technologies more sensitive, simpler, reusable and less expensive while only requiring mild enzyme modifications.

Enantioselective Oxidation of Aldehydes Catalyzed by Alcohol Dehydrogenase

Teaching old dogs new tricks: Alcohol dehydrogenases (ADHs) may be established redox biocatalysts but they still are good for a few surprises. ADHs can be used to oxidize aldehydes, and this was demonstrated by the oxidative dynamic kinetic resolution of profens. In the presence of a suitable cofactor regeneration system, this reaction can occur with high selectivity.

Gas diffusion electrode as novel reaction system for an electro-enzymatic process with chloroperoxidase

The versatile enzyme chloroperoxidase was used in a new reaction system, based on a gas diffusion electrode, for enzymatic chlorinations, sulfoxidations and oxidations. This is the first report on the combination of hydrogen peroxide production at a GDE with an enzymatic reaction.

Enzymatic halogenation of the phenolic monoterpenes thymol and carvacrol with chloroperoxidase

The conversion of the phenolic monoterpenes thymol and carvacrol into antimicrobials by (electro)-chemoenzymatic halogenation was investigated using a chloroperoxidase (CPO) catalyzed process. The CPO catalysed process enables for the first time the biotechnological production of chlorothymol, chlorocarvacrol and bromothymol as well as a dichlorothymol with high conversion rates, total turnover numbers and space time yields of up to 90%, 164000 and 4.6 mM h−1, respectively.

Specific oxyfunctionalisations catalysed by peroxygenases: opportunities, challenges and solutions

Peroxygenases enable H2O2-driven oxyfunctionalisation of organic compounds such as selective epoxidation, hydroxylation and heteroatom oxygenation. Therefore, peroxygenases are potentially very useful tools for the organic chemist. This contribution reviews the current state of the art in peroxygenases, their availability, engineering and reaction scope. Current challenges and possible solutions are critically discussed.