Biocatalysis for Industrial Application

Catalysis by enzymes is an attractive approach in green chemistry, enabling the synthesis of products without generation of hazardous materials. Enzymes are favourable for use in industry as they attain high reaction rates and can be enantioselective and chemoselective. However, a number of limitations regarding the use of enzymes exist, particularly concerning their stability, cost, reuse and purification from reaction media and products. Additionally, enzymes can typically, only be used under mild conditions; aqueous solutions, moderate temperature and pH, adding to the difficulty of their use. Enzymes suffer from a lack of long-term stability, both operationally and in storage. They can become denatured when exposed to high temperatures or even lose catalytic activity at room temperature. The inability to recover and reuse soluble enzymes after a reaction also hampers their widespread use in industry.

Proteins of thermophilic origin are of great interest in the biopharmaceutical industry due to their increased stability towards temperature and harsh reaction conditions, alleviating some of the above described difficulties. A focus on the engineering and production of such proteins allows for their use in place of their mesophilic counterparts.

Immobilization of enzymes on solid supports is an approach applied to overcome the outlined drawbacks, leading to increased activity and stability as well as ease of recovery from reaction media and potential reuse. An attractive technique is the design of enzymatic reactors.

A THERMOPHILIC ALDEHYDE DEHYDROGENASE

Found in all kingdoms of life, aldehyde dehydrogenases (ALDH) (EC;1.2.1.3) constitute a large family of NAD(P)⁺ -dependent enzymes with a molecular mass of ca. 50–60 kDa. They are often not substrate specific and catalyse the oxidation of aldehydes to carboxylic acids.

ALDHs have the potential to act as a biocatalytic route for carboxylic acid synthesis. Traditional methods for the oxidation of aldehydes to carboxylic acids is no longer sustainable due to the use of stoichiometric amounts of transition metal oxidants, salts or Ag₂O in combination with sodium cyanide, besides complicated reaction protocols. Investigation of ALDHs in biocatalysis has only been marginally investigated, but their exquisite chemoselectivity and broad substrate scope makes them attractive candidates. Additionally, there is a considerable demand for more stable and better performing catalysts, to which an efficient solution is the use of thermophilic enzymes.

Investigation of the ALDH_Tt from Thermus thermophilus demonstrated catalytic activity most optimally at 50 °C for an array of aldehyde substrates. The substrate scope provides a range of interesting potential carboxylic acid products, most notably terephthalic acid and p-toluic acid, which are used as a precursor in polyethylene terephthalate (PET) plastic manufacture and as an intermediate in terephthalic acid and polymer synthesis, respectively.

>>> Study of ALDH from Thermus thermophilus—Expression, Purification and Characterisation of the Non-Substrate Specific, Thermophilic Enzyme Displaying Both Dehydrogenase and Esterase Activity

>>> Insights into Aldehyde Dehydrogenase Enzymes: A Structural Perspective