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The life sciences are central to a growing number of industrial sectors. Figure 3.1 shows areas where biotechnology has already had an impact, or is likely to become important in the next few decades. "Red biotech" encompasses the pharmaceutical sector, an extremely well-developed biotechnology area because

Biotechnology - major sectors

New pharma compound: materials, devices Control systems, gene th

Biodegradation End-of-pipe solution Integrated processes

New pharma compound: materials, devices Control systems, gene th

Biofuels Bioelectricity

Biofuels Bioelectricity

Energy

Agriculture

Environment

Chemistry

Energy

Agriculture

Agro-efficiency lood quality

Agrochemicals

New materials New compounds New prntrrar*

Green Chemistry

Agro-efficiency lood quality

Agrochemicals

FIGURE 3.1 The many shades of biotechnology. White or industrial biotechnology is often seen as close to green chemistry, with similar goals such as long-term sustainability, minimization of toxic by-products, and a reduction of solvent use.

Pharma

Green Chemistry of its importance in generating successful new drugs, antibiotics, and vaccines. "Green biotech" has also had a major impact due to the development of new disease- and insect-resistant crops. This area has raised significant concern, especially in Western Europe, where genetically modified crops are generally not accepted, in contrast to the United States, Canada, Brazil, Argentina, and, lately, China as well, where such crops contribute a sizable portion of total corn, soya, and cotton production.

There are two areas that have become quite popular in the last few years. "White biotechnology" deals with the production of chemicals via biocatalytic processes, an area that is now seen as a significant contributor to new processes in the chemical industry.* "Black biotechnology" focuses on biofuels, which are produced from biomass. Major examples include bioethanol from sugar cane (Brazil, Australia) and corn starch (United States) and biodiesel from plant oils (Brazil and Germany; developing in the United States).

3.1.1 White Biotechnology

The development of industrially interesting bioconversions will in many cases involve reactions of organic compounds, most of which are not soluble in aqueous media. At the same time, the enzyme systems or whole cells that will be developed for such biocatalytic reactions will typically function optimally in aqueous media. Thus, it is necessary to develop (a) solutions that allow the

* (McKinsey 2004), see http://www.mckinsey.com/clientservice/chemicals/pdf/BioVision_Booklet. final.pdf and http://www.mckinsey.com/clientservice/chemicals/potentialprofit.asp.

combination of biocatalysts that function best in aqueous environments and (b) reaction substrates and products that dissolve best in apolar solvents. The simplest approach is to combine such media in a single reactor. Apolar solvents and aqueous media are usually poorly or not at all miscible. However, when they are mixed vigorously, emulsions are formed, much like those produced when olive oil and vinegar are mixed to produce a salad dressing. An interesting question is whether enzymes and microorganisms can function in apolar media and emulsions.

3.1.2 Biotransformations in the Presence of Solvents

Many solvents are chemically inert with respect to biological systems. Their mode of action is due not to chemical effects of the individual molecules, but to their bulk-solvent properties, which may or may not affect a particular macromolecular assembly. Thus, biocatalysis need not be restricted to aqueous systems (Klibanov 2000), and it is not surprising that there are enzymes that catalyze reactions of nonwater-soluble compounds. These compounds occur in nature, and many of these are therefore likely to be synthesized and degraded by some living systems. In addition, the past decades of membrane research have demonstrated that many enzymes operate within or near biomembranes and are therefore exposed (to a large extent) to an apolar rather than an aqueous environment.

Klibanov and his coworkers at MIT have capitalized on these ideas and shown that there are enzymes that are perfectly capable of functioning in bulk organic solvents that contain only a few percent water (Zaks 1985). Such enzymes may or may not be soluble in organic solvents, depending on the nature of the enzyme surface. Arnold and her coworkers have shown that it is possible to modify an enzyme surface by substituting apolar amino acids for polar surface residues, such that the enzyme becomes soluble and more active in a bulk organic solvent (Chen and Arnold 1993). Therefore, there is no intrinsic reason why enzymatic reactions should be restricted to water-soluble compounds or should occur in aqueous environments only (Dordick 1988).

Clearly then, some enzymes operate on apolar compounds, and some enzymes function in apolar environments. Indeed, the development of enzymatic reactions that occur in purely nonaqueous media is conceivable. Ultimately, bioconversions of apolar organic compounds may be based on biocatalysis with inexpensive enzymes, tailored to specific reactions, with high selectivity and productivities, long useful lifetimes, and easy introduction into existing biore-actor and downstream processing systems. In time, such biocatalysis will blend with chemical catalysis, the only distinguishable characteristic perhaps being the nature of the catalyst.

For complex enzyme systems and for enzymes that require energy input via cosubstrates and cofactors, this situation is not likely to be attained soon. Here, whole cells will continue to be the mainstay of bioprocesses. Such cells will have to meet several key requirements: solvent tolerance, high selectivity and productivity of relevant intracellular enzymes, cofactor generation where needed, and prolonged cell survival and enzyme function in large-scale bioprocesses.

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