Carbon biotransformation reactions

The level and activity of specific enzymes involved in biotransformation can differ depending on the species, strain, age, and sex of the test animal. For example, cats cannot carry out glucuronidation reactions, newborn rats have relatively low cytochrome P450 activity, and male rats are more sensitive to carbon tetrachloride toxicity than female rats. These differences are important to consider when interpreting the results from toxicological studies. The observation that age, sex, and genetics can significantly influence biotransformation reactions in animals raises the question of whether these characteristics also affect the biotransformation capacity of humans. 

A number of excellent reviews with comprehensive coverage on the literature of biooxidations have appeared in journals and books l 8). In this chapter we will only try to highlight some of these biotransformation reactions, in particular hydroxyla-tion of non-activated carbon atoms and double-bond epoxidation reactions. 

Medium As discussed in Subheading 2., the most important factor m this respect is probably the medium. There are numerous reports of the way in which the nutrient source affects the secondary metabolism. Such factors include carbon, phosphorus and nitrogen source, levels, and ratios inorganics present m medium levels of aeration/oxygenation the physical state of the medium (liquid or solid). The diversity of metabolism elicited by a range of media should be considered insofar as the biotransformation reactions are a product of secondary metabolism. 

The optical purity (enantiomeric excess, e.e.) of the biotransformation reaction product, 2-hydroxypropiophenone, was found to be greater than 98%. The absolute configuration of the carbinol carbon of 2-hydroxypropiophenone produced by A. calcoaceticus S) was the same as in the predominant enantiomer produced by P. putida. 

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). 

As mentioned in the introductory part, stereochemical course of the conversion of isocitric acid to a-ketoglutaric acid in TCA cycle is completely enantiose-lective although the reaction does not form an asymmetric carbon in the usual metabolic path. If such type of oxidative decarboxylation can be applied to synthetic compounds, it is expected that an entirely new type of asymmetric biotransformation will be developed. 

In some cases, microorganisms can transform a contaminant, but they are not able to use this compound as a source of energy or carbon. This biotransformation is often called co-metabolism. In co-metabolism, the transformation of the compound is an incidental reaction catalyzed by enzymes, which are involved in the normal microbial metabolism.33 A well-known example of co-metabolism is the degradation of (TCE) by methanotrophic bacteria, a group of bacteria that use methane as their source of carbon and energy. When metabolizing methane, methanotrophs produce the enzyme methane monooxygenase, which catalyzes the oxidation of TCE and other chlorinated aliphatics under aerobic conditions.34 In addition to methane, toluene and phenol have been used as primary substrates to stimulate the aerobic co-metabolism of chlorinated solvents. 

Despite the diverse range of documented enzyme-catalyzed reactions, there are only certain types of transformations that have thus far emerged as synthetically useful. These reactions are the hydrolysis of esters, reduction/oxidation reactions, and the formation of carbon-carbon bonds. The first part of this chapter gives a brief overview by describing some examples of various biotransformations that can easily be handled and accessed by synthetic organic chemists. These processes are now attracting more and more attention from nonspecialists of enzymes. 

In the arena of carbon-carbon bond-forming reactions, obviously a central feature in synthetic organic chemistry, the number of nonbiocatalytic methods in regular use far outweighs the small portfolio of biotransformations that can be considered to be available for general employment. 

A biotransformation, as defined by Straathof et al., ° is a process that describes a reaction or a set of simultaneous reactions in which a pre-formed precursor molecule is converted using enzymes and/or whole cells, or combinations thereof, either free or immobilised . Fermentation processes, with de novo product formation from a carbon and energy source, such as glucose via primary metabolism, are outside the scope of this chapter and book unless employed in conjunction with a biotransformation. 

Metabolic pathways of chloroform biotransformation are shown in Figure 2-3. Metabolism studies indicated that chloroform was, in part, exhaled from the lungs or was converted by oxidative dehydrochlorination of its carbon-hydrogen bond to form phosgene (Pohl et al. 1981 Stevens and Anders 1981). This reaction was mediated by cytochrome P-450 and was observed in the liver and kidneys (Ade et al. 1994 Branchfiower et al. 1984 Smith et al. 1984). In renal cortex microsomes of... 

Oxidative reactions at carbon predominate in the biotransformation of cyclic amiiies, and an important consequence of this is often the cleavage of the carbon-nitrogen bond. For example, A-dealkylation of N- alkyl substituted pyrrolidine (or piperidine, morpholine, etc.) involves an initial oxidative attack at the a- alkyl carbon atom to yield an N hydroxyalkyl derivative (carbinolamine), which is then metabolized to a secondary amine and the corresponding aldehyde. The metabolic conversion of nicotine to nornicotine (30 see Scheme 3) probably involves this mechanism, although the iminium ion (31) has also been suggested as an intermediate in the biotransformation (76JMC1168). Carbinolamines are unstable intermediates and have been identified only in a few cases, e.g. A-hydroxymethylcarbazole... 

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