Biocatalysis, barriers

The potential environmental and economic advantages that biocatalysts promise certainly warrant the necessary investments required to overcome the energy barriers of reducing biocatalysis to practice in the synthesis of both commodity and fine chemicals. The last quarter-century has brought extensive advancement is the field of biocatalysis and the next quarter-century promises to yield further advances. 

The whole-cell biocatalysis approach is typically used when a specific biotransformation requires multiple enzymes or when it is difficult to isolate the enzyme. A whole-cell system has an advantage over isolated enzymes in that it is not necessary to recycle the cofactors (nonprotein components involved in enzyme catalysis). In addition, it can carry out selective synthesis using cheap and abundant raw materials such as cornstarches. However, whole-cell systems require expensive equipment and tedious work-up because of large volumes, and have low productivity. More importantly, uncontrolled metabolic processes may result in undesirable side reactions during cell growth. The accumulation of these undesirable products as well as desirable products may be toxic to the cell, and these products can be difficult to separate from the rest of the cell culture. Another drawback to whole-cell systems is that the cell membrane may act as a mass transport barrier between the substrates and the enzymes. 

The exciting technical opportunities in biocatalysis are tempered by the major barriers to commercialization which still exist. Most notably, these include low stability of an expensive catalyst, and the high separation and capital costs associated with low concentrations of reactants and products. 

Reaction Paths and Energy Barriers in Catalysis and Biocatalysis... 

Enzymes have been naturally tailored to perform under physiological conditions. However, biocatalysis refers to the use of enzymes as process catalysts under artificial conditions (in vitro), so that a major challenge in biocatalysis is to transform these physiological catalysts into process catalysts able to perform under the usually tough reaction conditions of an industrial process. Enzyme catalysts (biocatalysts), as any catalyst, act by reducing the energy barrier of the biochemical reactions, without being altered as a consequence of the reaction they promote. However, enzymes display quite distinct properties when compared with chemical catalysts most of these properties are a consequence of their complex molecular stracture and will be analyzed in section 1.2. Potentials and drawbacks of enzymes as process catalysts are summarized in Table 1.1. 

Biocatalysis in organic solvents has unique advantages compared to traditional aqueous enzymology/fermentation. Often times in nonaqueous media enzymes exhibit properties drastically different from those displayed in aqueous buffers. These novel properties are given in Table 4.3. In addition to those mentioned in Table 4.3, the solubility of hydrophobic substrates and/or products increases in organic solvents, which diminishes diffusional barriers for bioconversions, and thus speeds up the reactions and improves the potential for direct applications in industrial chemical processes. Once organic solvent becomes a reaction medium, there cannot be contamination, which thus precludes release of proteolytic enzymes by microbes and favors the direct application of the process in an industrial setting. Most proteins (enzymes) inherently function in an aqueous environment, and hence their behavior in nonaqueous solvents is completely different due to the loss in the three-dimensional structure. Thus, only polar solvents... 

Looking at the history of biocatalysis as a tool for synthetic organic chemists, many of the barriers in obtaining sufficient amounts of active protein, a sufficiently wide diversity of biocatalysts, and enzymes that have robust performance have now been solved. Many of the advanced tools used to study and evolve proteins can be applied to the development of biocatalysts, making targeted advances in enzyme performance in months that would have been impossible or too impractical even a few years ago. An example is shown in Figure 15.2. 

As an alternative to chemical conversion, bioconversirai offers superior chemo-, regio-, and enantioselectivity, using nontoxic biocatalysts at ambient temperature and pressure while producing biodegradable waste (Thomas et al. 2002). However, biocatalysis requires the availability of a specific enzyme for each individual reaction. The same is true for FDCA, where indeed the lack of suitable enzymes was long considered the major technical barrier for the development of an efficient bioprocess (Werpy et al. 2004). 

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