Bioreduction process engineering

The most established of bioreduction processes focuses on carbon-oxygen double bond reduction. For example, alcohol dehydrogenases (ADHs), which can even catalyze the reduction of sterically demanding substrates such as 2-hydroxy ketones, can be used effectively [8]. The mild conditions of the enzymatic reaction mean that 

In many cases, the substrates for reaction are poorly water-soluble, meaning it is necessary to consider organic solvent or two-liquid-phase systems to ensure the concentration of substrate (and hence product) in the reactor is sufficient. Two-liquid-phase systems may also be used for product removal as discussed later. There are relatively few reports of solvent effects on the various bioreduction enzymes, but a recent publication found ene-reductases to be robust in several aqueous-organic two-phase systems [9]. 

Despite all these advantages, implementation into a process still demands other considerations, and specifically for bioreductions, the follo ving challenges need to be addressed  

Cofactor regeneration. The first and most important challenge is the requirement that the enzymes have for NAD(P)H, in order that the reductases can work effectively. The expense of nicotinamide cofactors (even though the cost has come down in recent years) necessitates that they are regenerated rather than used in stoichiometric amounts for any scalable reaction. Many strategies exist to solve this problem, as discussed later in this text, but selecting the optimal one is not necessarily straightforward. 

Limited substrate and product transport into and out of whole-cell biocatalysts. A third problem, which also concerns those reductions carried out in whole-cell format, is that the cell membrane can act as a barrier for diffusion of substrate into the cell and product out of the cell, leading to low specific reaction rates (rate per mass of cells). Both cell permeabilization and use of transporters or alternatively the use of isolated enzymes are potential strategies to overcome this challenge. 

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