Enzymes stereoselective action

Although most anesthetics are achiral or are adininistered as racemic mixture, the anesthetic actions are stereoselective. This property can define a specific, rather than a nonspecific, site of action. Stereoselectivity is observed for such barbiturates as thiopental, pentobarbital, and secobarbital. The (3)-enantiomer is modestly more potent (56,57). Additionally, the volatile anesthetic isoflurane also shows stereoselectivity. The (3)-enantiomer is the more active (58). Further evidence that proteins might serve as appropriate targets for general anesthetics come from observations that anesthetics inhibit the activity of the enzyme luciferase. The potencies parallel the anesthetic activities closely (59,60). 

Lipoxygenases catalyse the regio-specific and stereoselective oxygenation of unsaturated fatty acids. The mammalian enzymes have been detected in human platelets, lung, kidney, testes and white blood cells. The leukotrienes, derived from the enzymatic action of the enzyme on arachidonic acid, have effects on neutrophil migration and aggregation, release of lysosomal enzymes, capillary permeability, induction of pain and smooth muscle contraction (Salmon, 1986). 

The stereoselective nature of drug action should not be unexpected in view of the fact that all structures with which those compounds interact, receptors and enzymes. 

Regio- or stereoselectivity of enzymes action is much easier to mimic than the enormous yields of reactions they catalyze (accelerations factors of 106-1012). A simple example of this kind is provided by chlorination ofanisol 180 that produces only p-chloroanisol 181 in the presence of a-cyclodextrin 13 while both 181 and 182 are formed without the latter factor [109]. By using suitably... 

Finally, because enzymes are usually stereoselective, one drug enantiomer is often more susceptible than the other to drug-metabolizing enzymes. As a result, the duration of action of one enantiomer may be quite different from that of the other. Similarly, drug transporters may be stereoselective. 

For preparative applications, the expensive and configurationally unstable donor 128 can be simply prepared in situ by the action of ribose 5-phosphate isomerase (EC 5.3.1.6) on D-ribose 5-phosphate (39). This technique was applied to the stereoselective synthesis of d-[1-13C] fructose 6-phosphate 38 from [13C] formaldehyde [376,377] which also included a second enzymatic isomerization of the D-arafrino-3-hexulose 6-phosphate intermediate 129 into the more stable 2-hexulose derivative 38. Notable are the conflicting demands for high substrate levels (necessary to shift the fully reversible multi-component equilibrium) versus the notorious enzyme inactivation that occurs at higher formaldehyde concentrations. 

The stereochemistry of the product of a reaction will be influenced by the structures of the reagent and substrate and the mechanisms by which they react. For example, the hydroxylation of but-2-ene by osmium tetroxide and water yields a racemate whilst bromination of the same compound with bromine produces a meso compound (Figure 10.5). Flowever, a stereoselective reaction is most likely to occur when steric hindrance at the reaction centre restricts the approach of the reagent to one direction (Figure 10.6). Furthermore, the action of both enzyme and non-enzyme catalysts may also be used to introduce specific stereochemical centres into a molecule. 

Alternatively, an A- or B-selective enzyme is used for the preparation of A or B labeled NADH. The action of the enzyme of unknown selectivity on this material is then examined. Viola et al. have described improved stereoselective methods for the preparation of the [4-2H]NADPH isomers [103]. For the A-labeled material,... 

Nature attempts to simplify its chemical environment and has selected L-amino acids as the natural form. This has the saving feature that only one set of enzymes with binding sites for L-amino acids must be produced. However, D-amino acids do arise in nature from spontaneous isomerization of L-amino acids (see above) or from a very few sites in nature (the action of penicillin involves a structure that contains D-alanine, see below). While the enzymes that act on L-amino acids will almost never interact with D-amino acids and vice versa, there are rare examples of enzymes that interact with both isomers. In the latter case, the enzyme binding site must be ambidextrous and have extra features that accommodate both isomers. In other words, enzymes naturally show stereoselectivity and avoidance of stereochemical selection is an exception. 

After Werner s death in 1919, there was little activity in the field of stereochemistry of coordination compounds. An exception is found in the work of Yuji Shibata, who had been one of Werner s students, and who continued with excellent stereochemical work when he returned to Japan. His work on the enzyme-like activity of cobalt complexes furnishes especially interesting examples of stereoselectivity and of the catalytic action of such compounds (6). T. P. McCutcheon and V. L. King, Americans who had done their theses on stereochemical topics under Werner s guidance, did not continue in that field. King, who actually performed the first resolution of an asymmetric complex ( 7), went into industrial work. McCutcheon became a member of the faculty at the University of Pennsylvania and did research on complex compounds, but not on their stereochemistry. 

The utility of this methodology is illustrated by the stereoselective synthesis of ( + )-cer-ulenin (759), an antifungal antibiotic first isolated from the culture filtrate of Cephalosporium caerulens. Its ability to inhibit lipid biosynthesis in Escherichia coli by irreversibly binding P-keto-acyl-carrier protein synthetase, the enzyme responsible for the chain lengthening reaction in fatty acid synthesis, has attracted interest in its mechanism of action. D-Tartaric acid... 

Addition of methyl groups to either one or both of the ethylene carbons results in chiral molecules. Muscarinic receptors (see below) display stereoselectivity for the enantiomers of methacholine. The S-(+)-enantiomer is equipotent with acetylcholine, and the R-(-)-enantiomer is approximately 20-fold less potent. Acetylcholinesterase hydrolyzes the S-(+)-isomer much slower (approximately half the rate) than acetylcholine. The R-(-)-isomer is not hydrolyzed by AChE and even acts as a weak competitive inhibitor of the enzyme. This stability toward AChE hydrolysis as well as the AChE inhibitory effect of the R-(-)-enantiomer may explain why racemic methacholine produces a longer duration of action than acetylcholine. The nicotinic receptor and AChE exhibit little stereoselectivity for the optical isomers of acetyl-a-methylcholine. 

Stereospecific Michael addition reactions also may be catalyzed by hydrolytic enzymes (Scheme 2.205). When ot-trifluoromethyl propenoic acid was subjected to the action of various proteases, lipases and esterases in the presence of a nucleophile (NuH), such as water, amines, and thiols, chiral propanoic acids were obtained in moderate optical purity [1513]. The reaction mechanism probably involves the formation of an acyl enzyme intermediate (Sect. 2.1.1, Scheme 2.1). Being an activated derivative, the latter is more electrophilic than the free carboxylate and undergoes an asymmetric Michael addition by the nucleophile, directed by the chiral environment of the enzyme. In contrast to these observations made with crude hydrolase preparations, the rational design of a Michaelase from a lipase-scaffold gave disappointingly low stereoselectivities [1514-1517]. 

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