Stereospecificity of enzyme

A final word needs to be said about the supposedly unique features of enzymes, namely, their ability to produce enantiomerically pure products. This is not the place to speculate about the stereospecificity of enzymes, a problem that has been discussed elegantly by Comforth (15). It cannot be denied that the high (>99.9%) enantiomeric purity achieved by enzymes may be uniquely useful in the case of liquid products. However, when crystalline products are obtained in an asymmetric synthesis and the e.e. exceeds 80%, crystallization to enantiomeric purity without excessive loss of material is routinely achieved. 

Virtually all biological reactions are stereospecific. This generalization applies not only to the enzyme-catalyzed reactions of intermediary metabolism, but also to the processes of nucleic acid synthesis and to the process of translation, in which the amino acids are linked in specific sequence to form the peptide chains of the enzymes. This review will be restricted mainly to some of the more elementary aspects of the stereospecificity of enzyme reactions, particularly to those features of chirality which have been worked out with the help of isotopes. 

The characteristic stereospecificity of enzymes has been exploited in the design of an electrochemical cell for the conversion of L-lactate to D-lactate (Scheme 23)117. Enzymatic oxidation of L-lactate by L-lactate dehydrogenase affords pyruvate. Pyruvate is then reduced electrochemically to racemic lactate. A second enzymatic oxidation of the latter by L-lactate dehydrogenase selectively converts L-lactate to pyruvate, leaving D-lactate behind. The ingenious feature of this system is the fact that pyruvate can be re-reduced... 

During the last two decades, the mechanisms of many enzymic processes have been established, and model systems have been developed that effectively mimic their action. In particular, the roles of thiamin, NAD, pyridoxal, flavins, Bl2, ferridoxin, and metals in many enzymic processes now are understood. Model systems have been developed to imitate the action of decarboxylases and esterases, to imitate the action of enzymes in binding their substrates, and to approach the stereospecificity of enzymes. Our laboratory recently has found phosphorylating agents that release monomeric methyl metaphosphate, which in turn carries out phosphorylation reactions, including some at carbonyl oxygen atoms, that suggest the actions of ATP. The ideas of biomimetic chemistry are illustrated briefly in terms of the processes mentioned above. 

Finally, if enzymes can be stabilized in the presence of organic solvents by immobilization, an area that appears especially promising is the synthesis of organic flavor compounds. This is likely to require regeneration of expensive cosubstrates, such as the nicotinamide coenzymes, and such processes remain to be optimized. However, the stereospecificity of enzyme-catalyzed reactions and the high... 

Concurrently with the development of research dealing with the stereochemical course of substitution at chiral phosphorus centers, the question of the stereospecificities of enzymes for metal-nucleotide complexes has also been addressed. The methodologies developed for the two types of studies differed initially, but significant overlap has appeared. The applications of the two approaches have unmasked the complete stereochemical courses of several phosphotransferase reactions. 

Chiral methyl chiral lactic acid (5). This labeled molecule, useful for study of stereospecificity of enzymic reactions, has been prepared in a way that allows for synthesis of all 12 possible isomers. One key step is the stereospecific debromination of 1, accomplished by conversion to the vinyl-palladium cr-complex 2 followed by cleavage with CF3COOT to give the tritium-labeled 3. The next step is the catalytic deuteration of 3, accomplished with a rhodium(I) catalyst complexed with the ligands norbornadiene and (R)-l,2-bis(diphenylphosphino)propane. This reaction gives 4 with an optical purity of 81%. The product is hydolyzed to 5, which is obtained optically pure by cr3rstallization. 

Catalytic antibodies - These interesting molecules are antibodies with a very specific binding site to the transition state of an enzymatic reaction. The resulting molecules, called abzymes, act like antibodies. In some cases, abzymes can speed up reaction rates as much as lO -fold over the uncatalyzed reaction. The stereospecificity of enzymes (including abzymes) may provide a tremendous aid to the synthesis of stereospecific compounds in organic chemistry. 

Communicated in writing) Dr. Eley s review of enzyme catalysis made only brief reference to the very marked stereospecificity of enzyme reactions. Adsorption of substrates on enzymic proteins cannot account satisfactorily for this unless there is at least a two-point chemisorption throughout the whole of the reaction process. 

Type V CSPs are protein phases. Because of the well established chemo- and stereospecificity of enzymes, a large number of experimentalists have adapted proteins in one form or another as stationary phases for chiral separations. The intermolecular forces responsible for analyte binding to these biopolymers are the same as for most other CSPs but the size and complexity of proteins makes them difficult to study computationally. One would think that with approximately 400 entries in the Brookhaven Protein Databank to select from, separation scientists would have used one of these proteins as a chiral selector and then use those atomic coordinates to carry out molecular modeling studies. Only one example has appeared in the literature where information from the PDB has been used to serve as a beginning point for molecular modeling of a protein CSP. In all other examples the CSP is viewed as having an unknown structure and Quantitative Structure-Enantioselective Retention Relationships (QSERRs) have been carried out. 

Stereospecificity of enzyme reactions , in Progress in Stereochemistry (Butterworths Scientific Publications, London), 1954, p. 318. 

The stereospecificity of enzymes in both of these types (axial-equatorial and diaxial) of /3-eliminative degradation has already been indicated for the degradation of dermatan sulfate, only enzymes that had... 

Popjak, G. Stereospecificity of Enzyme Reactions. In Boyer, P. D., ed.. The Enzymes, 3rd ed., Vol. 2, Kinetics and Mechanism. New York Academic Press, 1970. [A review of stereochemical aspects of the citric acid cycle.]... 

The next phase of research in asymmetric synthesis may be characterized by more systematic attempts to develop reactions approaching the absolute stereospecificity of enzyme systems. 

Battersby and his group have recently reported the details of an extensive study of biogenetic interrelations among tetrahydroprotoberberines, benzophenanthridines, rhoeadines, and phthalideisoquinolines. A key step in this study of the stereospecificity of enzyme reactions was the use of horse liver alcohol dehydrogenase to prepare the complementary R- and 5-benzyl alcohols shown. These tritiated materials were needed for the synthesis of chirally labeled scoulerine. 

The enantiomorph of this deuteroamine, prepared by decarboxylation of a-deuteroglutamate in water, does not exchange deuterium with the solvent, under the same conditions. (a-Deuteroglutamate is prepared by enzyme-catalysed racemisation of d- or L-glutamate in deuterium oxide). Additional support for the stereospecificity of enzyme-catalysed decarboxylation of amino acids comes from experiments in which tyrosine was decarboxylated in DgO to give R-a-... 

---