Biotransformation reactions types

Biotransformation Product Final product Indication Reaction type Microorganism Process Ref. 

The concept of microbial models of mammalian metabolism was elaborated by Smith and Rosazza for just such a purpose (27-32). In principle, this concept recognizes the fact that microorganisms catalyze the same types of metabolic reactions as do mammals (32), and they accomplish these by using essentially the same type of enzymes (29). Useful biotransformation reactions common to microbial and mammalian systems include all of the known Phase I and Phase II metabolic reactions implied, including aromatic hydroxylation (accompanied by the NIH shift), N- and O-dealkylations, and glucuronide and sulfate conjugations of phenol to name but a few (27-34). All of these reactions have value in studies with the alkaloids. 

Once a chemical is in systemic circulation, the next concern is how rapidly it is cleared from the body. Under the assumption of steady-state exposure, the clearance rate drives the steady-state concentration in the blood and other tissues, which in turn will help determine what types of specific molecular activity can be expected. Chemicals are processed through the liver, where a variety of biotransformation reactions occur, for instance, making the chemical more water soluble or tagging it for active transport. The chemical can then be actively or passively partitioned for excretion based largely on the physicochemical properties of the parent compound and the resulting metabolites. Whole animal pharmacokinetic studies can be carried out to determine partitioning, metabolic fate, and routes and extent of excretion, but these studies are extremely laborious and expensive, and are often difficult to extrapolate to humans. To complement these studies, and in some cases to replace them, physiologically based pharmacokinetic (PBPK) models can be constructed [32, 33]. These are typically compartment-based models that are parameterized for particular... 

Several databases of published biotransformations are commercially available, such as Molecular Design Ltd s Metabolite and the Accelerys Metabolism Database (formerly produced by Synopsis). The former is quite extensive, and contains in vivo and in vitro biotransformation summaries from the literature, while the latter has as its core information based on the U.K. Royal Society of Chemistry s Biotransformations series (Hawkins, 1988-1996) supplemented by additional data from the literature. Both systems are searchable by reaction type. The intelligent use of such databases provides much valuable information on likely metabolic profile. 

Between May 1993 and October 1994 an enzymatic racemic resolution step was developed as part of a new technical synthesis from the lab- up to the 200 kg-scale. The choice of the biotransformation in question was predominantly determined by the extremely easy and cheap access to the substrate, and, secondly, by the high selectivity and easy working of the biotransformation itself. The biotransformation was a hydrolysis, the most frequently seen reaction type in the syn-... 

According to the Warwick Biotransformation Club Database in 1987/88 [60], approximately 65% of all applications reported fell into the classes of esterolytic reactions (ester hydrolysis, synthesis or transesterification) (40%) and dehydrogenase reactions (25%). Next in importance were oxygenase-mediated reactions, peptide and oligosaccharide synthesis, which together comprise 24% of the total. Reports of enzymatic procedures for carbon-carbon bond formation were very few in number (2%). All other reaction types comprised less than 10% of the total. 

A great wealth of potential biocatalysts are stored in international type culture collections. We could logically assume, that a large number of strains in these collections were already tested by biotechnological industries in broad screening programs in the search for special biotransformations or reaction types. 

In principle, numerous reports have detailed the possibility to modify an enzyme to carry out a different type of reaction than that of its attributed function, and the possibility to modify the cofactor of the enzyme has been well explored [8,10]. Recently, the possibility to directly observe reactions, normally not catalyzed by an enzyme when choosing a modified substrate, has been reported under the concept of catalytic promiscuity [9], a phenomenon that is believed to be involved in the appearance of new enzyme functions during the course of evolution [23]. A recent example of catalytic promiscuity of possible interest for novel biotransformations concerns the discovery that mutation of the nucleophilic serine residue in the active site of Candida antarctica lipase B produces a mutant (SerlOSAla) capable of efficiently catalyzing the Michael addition of acetyl acetone to methyl vinyl ketone [24]. The oxyanion hole is believed to be complex and activate the carbonyl group of the electrophile, while the histidine nucleophile takes care of generating the acetyl acetonate anion by deprotonation of the carbon (Figure 3.5). 

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