A critical consideration in the development of biocatalytic systems is the form in which the enzyme or enzyme system is going to be used. There are two general approaches. One is to use isolated enzymes. If these are inexpensive, they can be used as disposable biocatalysts, as is the case for glucose isomerase,10 which is the key biocatalyst in the production of high-fructose corn syrups from starch, or the lipases and proteases that are present in detergents. Alternatively, if enzymes are expensive to produce, they can be immobilized and used repeatedly by recovering the enzyme particles after each use.
The second general approach is to use whole cells that contain the enzyme or enzymes used in the biocatalytic process.11 The use of whole cells has the added advantage that coenzyme-dependent enzymes can be used because it is possible to regenerate the relevant coenzyme, through metabolism of the whole cells. This, of course, requires that the whole cells are not only physically intact but also meta-bolically active. Since coenzymes are often involved in building new molecules, industrial biocatalysis typically uses whole-cell systems.
Much of industrial chemistry takes place in organic solvents, or involves apolar compounds. Biocatalysis, in contrast, typically involves aqueous environments. Nevertheless, enzymes and microorganisms do in fact encounter apolar environments in Nature. Every cell is surrounded by at least one cell membrane, and more complex eukaryotic cells contain large amounts of intracellular membrane systems. These membranes consist of lipid bilayers into which many proteins are inserted: present estimates, based on genomic information, are that about one-third of all proteins are membrane proteins, many of which are so-called intrinsic proteins that are intimately threaded through the apolar bilayer. These proteins are essentially dissolved in, and function partly within, an apolar phase.
The notion that enzymes might well function in apolar solvents has been explored in detail and confirmed by Klibanov and his followers during the past two decades.12,13 Similarly, many microorganisms have been found to grow well in the presence of bulk solvents.14-16 This has permitted the development of biotransformation systems in which water-insoluble compounds are dissolved in apolar phases, to be converted to products by whole cells present in an aqueous phase, the products generally then dissolving again in the apolar phase.16-20
In short, microbial cells can be employed as very effective reactors for the conversion of substrates to products, operating in mixed aqueous-apolar systems, optimized for the best space-time yields attainable at lowest cost.
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