A majority of industrial chemical processes are enabled by heterogeneous catalysis. In order to likewise utilize biocatalysts, enzymes can be effectively immobilized on suitable enzyme carrier resins.
The development, synthesis, and use of enzyme carrier resins has hugely benefited from previous knowledge about chromatography resins. Typical polymeric carriers offer a very wide selection of properties, which allows to find a tailored match to the enzyme and process. Commercial polymeric resins are likely the easiest and most convenient way of immobilizing enzymes. Entirely synthetic resins or such based on polymers of natural origin (agarose, chitosan, cellulose, albumin, etc.) are commerically available.
When working with an enzyme carrier, deciding on physical parameters is critical in order to choose the right specimen. Criteria to consider include:
In general, the carriers used for enzyme immobilization by adsorption can be divided into both organic and inorganic origins. The most common inorganic carriers are
The organic carriers, by contrast, include compounds of natural origin, such as
as well as synthetic compounds, mainly polymers such as
The advantage of these matrices is that they can readily be modified chemically to craft specific conditions for a given enzyme and its application. However, prior selection at this stage, further considerations should be made:
When deciding on a carrier to immobilize an enzyme on, one must consider
Given a fixed total mass of carrier, a smaller particle size provides a larger surface area for immobilization than larger particles. Given a fixed particle size, using porous particles allows for a larger surface area. However, given porous particles of a fixed total mass, smaller particles still provide a larger surface area for immobilization. The explanation stems from the fact that immobilized are unevenly distributed and concentrated primarily on the carrier particle's outer layers.
Consequences arising from the use of small particles are a more cumbersome separation from the reaction medium, reduced sedimentation rate, and increased backpressure caused by packed beds. The last phenomenon is particularly pronounced in packed bed column reactors but less severe in rotating bed reactors. The flow resistance through a packed bed depends on properties of the liquid phase, its velocity through the bed, the particle size, the void fraction and the depth of the bed.
These are the same parameters that one needs to consider when immobilizing or using immobilized enzymes, i.e., to maximize surface area and minimize diffusion resistance. A balance will have to be struck.
The recyclability and long-term operation of an immobilized enzyme is very dependent on enzyme stability as well as on process and storage conditions. However, sufficient mechanical stability is also a requirement. Materials that break under process conditions will result in fines and problems during the downstream processing.
The crux of the matter is that the mechanical strength of a particle is related to its size, morphology, and porosity. Thus, pore size, pore size distribution, and pore geometry directly influence the mechanical strength of the particle: The larger the pore, the more friable the material.
Notably, when working with an enzyme carrier, the pressure drop across a packed bed can be hundreds of bars, which affects the stability. But the pressure drop across a rotating bed reactor (RBR) is a fraction of a bar and the process conditions in an RBR are much gentler to the particles compared to a classical packed bed column. This allows for the use of both softer and more friable particles. Thus, with the use of RBR, reusing of immobilised emzymes is possible with more stability and mechanical strength.
Find examples of immobilized enzyme applications in the RBR:
Interested in the rotating bed reactor? - Let SpinChem help you understand how the rotating bed reactor technology can simplify and improve your process. Get in touch with us today to get started towards better, more efficient biocatalysis.
References:[1] S. Cantone, et al., Chem. Soc. Rev., 2013, 42(15), pp. 6262-6276.[2] A. Liese et al., Chem. Soc. Rev., 2013, 42(15), pp. 6236-6249.[3] N. Miletić et al., Macromol. Biosci., 2011, 11(11), pp. 1537-1543. (image credit)
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