Regiospecificity, enzymes

This hypothesis is well in line with the finding that the formation of 9 by condensation of 6 and 8 can proceed in the absence of a catalyst under mild conditions. Specifically, the reaction occurs with appreciable velocity at room temperature in aqueous solution of neutral pH with reactant concentrations in the low mmolP range. The regiospecificity of the uncatalyzed reaction is significantly lower as compared to the strictly regiospecific enzyme-catalyzed reaction. Most likely, the spontaneous reaction is subject to partitioning... 

S. Muemmler, M. Rueffer and M.H. Zenk, S-Adenosylmethionine (S)-Scoulerine-9-0-methyltransferase, a highly stereo- and regiospecific enzyme in tetrahydroberberine bioynthesis. Plant Cell Reports 4 (1985), 36-39. 

Biosynthesis glycerol-3-phosphate is converted to triacylglycerol through lysophosphatidic diCid, phosphatidic acid, and 1,2-diacylglycerol. Acylation at each glycerol hydroxyl is effected by acyl-coenzyme-A or acyl-ACP in the presence of regiospecific enzymes. 

The enzymatic transformation of natural products is by for file most attractive option. In this approach, it can be envisaged that sterols, which are relatively abundant, may be selectively modified to produce desired products. Hie diversity of enzyme activities, their reaction specificity, regiospecificity and stereospedfidty are all features which could contribute to carrying out the desired changes. This does not mean, however, that transformations using enzyme systems are simple. Nevertheless, biotransformations have become of vital importance in the production of steroids. 

It has been shown that it is possible to compel regiospecific para substitution by enclosing the substrate molecules in a cavity from which only the para position projects. Anisole was chlorinated in solutions containing a cyclodextrin, a molecule in which the anisole is almost entirely enclosed (see Fig. 3.4). With a high enough concentration of cyclodextrin, it was possible to achieve a para/ortho ratio of 21.6 (in the absence of the cyclodextrin the ratio was only 1.48). This behavior is a model for the regioselectivity found in the action of enzymes. 

While the uncatalyzed reaction requires extremely high temperatures and proceeds nonspecifically, the enzymatic reaction proceeds regiospecifically and stereospecifically under physiological conditions of temperature and pressure. The P450 enzyme utilizes its iron-porphyrin centers as metal-containing... 

First-pass clearance can be tracked to gut-stability or metabolism by phase I and then either direct clearance or clearance of the metabolite by phase It enzymes or biliary, renal or plasma clearance. Metabolite stability by phase I enzymes include inhibition, induction, regiospecificity, lability or affinity toward several cytochrome... 

Heme oxygenase catalyzes the regiospecific oxidation of heme to bili-verdin (Fig. 6) and has been isolated and purified from several sources 247-251). In mammals, the enzyme is membrane-boimd, a complication... 

Scheme IV.71) shows the involvement of two chlorinating steps first, the chlorination of compound 179 at position 7 into 180, and second, the chlorination of 4-(2-amino-3-chlorophenyl)pyrrole (183) at position 3, forming 184. Two chlorinating enzymes responsible for these regiospecific chlorina-tions have been identified (85BBA181). 

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