Fases del Proceso Administrativo

1.5.1 I n t r o d u c t i o n .

Biotransformations can be defined as the conversion of one compound to another using a biological or natural catalyst or more specifically, the ‘selective enzymic modification of defined pure compounds into defined final products’ (Kieslich 1984, Meyer et al., 1997.) The starting material may be synthetic or a natural product. The biocatalyst can be an enzyme or a whole cell system. The enzyme system may involve an isolated enzyme, a crude enzyme mixture or an enzyme complex. These different forms of enzymes in turn can be freely suspended or immobilised onto a solid support. Unlike chemical reactions, the products and the reactants of a biotransformation are usually structurally similar (Lilly 1992). Biocatalysts bring a reaction to its equilibrium point at a greater rate than would otherwise be seen. Biotransformations tend to be performed at lower temperatures and pressures than the equivalent chemical reaction,

making them more user friendly (Stanbury at a!., 1995, Hanson 1995.)

One of the most important early biotransformations was the hydrolysis of

penicillin to 6 -aminopenicillanic acid (6 -APA) (Structure 1.7) which was

performed using amidase from E.coli or other suitable microorganisms. This

process led to the production of semi-synthetic penicillins such as methicillin (Structure 1.8) from 6 -APA (Davis at a!., 1989)

H

Me Me

Structure 1.7 6-Aminopenicillanic acid

Chapter 1. Introduction RNH Me Me CO R = MeO, OMe Structure 1.8 Methicillin.

Originally biotransformations were applied almost exclusively to systems in which the reactant was highly water soluble. Therefore, the majority of biotransformations were carried out in aqueous media. The exceptions to this were the steroidal bioconversions discovered in the 1950s. Peterson (1952) reported the biotransformation of progesterone to 11-a-Hydroxyprogesterone

using Rhizopus nigricans (Scheme 1). During the same period two other

bioconversions of progesterone were described. Shull (1953) reported the 11-

p-hydroxylation by Curvularia lunata. Dehydrogenation by Corynebacterium

simplex was described by Nobile in 1955 (Lilly 1994). Research in the 1980’s led to the biological conversion of reactants with low water solubility. Biocatalysts that are suitable for use in two phase (organic/aqueous) systems were reviewed by Woodley and Lilly (1994).

Biotransformations are becoming increasingly valuable tools for the

preparation of novel organic molecules (Jones 1986, Faber 1992, Roberts and Turner 1992.) Processes that involve a bioconversion step are especially useful for the production of large molecules which contain a high degree of functionality. Biocatalysts can perform transformations that might not be possible using traditional organic synthetic methods. The high specificity of enzymes can allow the conversion of reactants to products with predicted selectivities, such as regioselectivity, chemoselectivity and enantioselectivity. These products can be used as chiral synthons and combined with traditional organic chemistry for the production of pharmaceuticals and other fine

Chapter 1. Introduction

chemicals. The biotransformation process tends to be more specific than the equivalent chemical reaction. The process can enable the modification, addition or removal of functional groups on specific sites of complex molecules. The chemical reaction to perform the same process often requires cost intensive techniques and the use of protecting groups.

The biotransformation processes can be simplified as in Figure. 1.1

Organism

Fermentation

Purified Enzyme Cells

Immobilised Enzyme

Whole cells

Reactant(s) _{Biotransformation} > Product(s)

Figure 1.1 Bioconversion Processes (from Lilly 1994.)

1 .5 .2 Bio t r a n s f o r m a t io n s u s in g e n z y m e s

Enzymes are, due to their nature, highly specific catalysts. They are of great use in the synthesis of chiral synthons (Roberts et al., 1992.) For an enzymic biotransformation the enzyme must be removed from the cell and purified sufficiently to remove all other enzymes that are likely to lower the yield of the

Chapter 1. Introduction

bioconversion product. The product yield can be decreased by other enzymes that will cause side or degradation reactions.

1.5.3 B i o t r a n s f o r m a t i o n s u s in g w h o l e c e l l s .

The most commonly used biocatalysts are whole cells. This is due to the lack of need to purify the cells before use and the robust nature of the cells (Lilly 1977.) There are disadvantages connected with the use of whole cells in biotransformations. The size and densities of the cells make their recovery during downstream processing difficult without the use of separation techniques, such as centrifugation or filtration. Another major disadvantage concerning the use of whole cells is the possibility of metabolism of the product by other enzymes that are present in the cells (Faber 1992.) This can decrease the purity and yield of the final product, as the by-products of the biotransformation require removal during the downstream processing.

In some cases these problems can be overcome by the immobilisation of the whole cells onto a solid support, although this is not often a successful procedure. Immobilisation can allow easier recovery of the biocatalyst for the reactor, thus allowing reuse of the catalyst and can therefore make the process more economically viable. The immobilised catalyst can be retained in the reactor.