Enzymes chiral methylene group

Use of Enzymes in Determining the Stereochemistry at a Chiral Methylene Group... 

Hsu s group in Taiwan have developed a procedure for the synthesis of (Y)-2-amino-4-phenylbutanoic acid, the phenylalanine homologue with one additional methylene group. Hydantoinase and L-A-carbamoylase genes have been cloned from different Bacillus species and overexpressed in E. coli. Both the R- and the 5-enantiomers were cleaved by the hydantoinase, but only the 5-form of the A-carbamoyl amino acid was hydrolyzed by the second enzyme. The reactions could be run in a single pot, with successive addition of the two enzymes, and were successful in the sense of giving a product of high chiral purity. However, the yield was... 

S,35)-3-hydroxy[2- H], Hj]butyryl-CoA as substrate. It was cleaved by the enzyme and the resulting radioactive acetate examined for its chirality by standard procedures. The results demonstrated that the cleavage reaction (and presumably the condensation reaction) proceeds with inversion of the methylene group that becomes the methyl group on cleavage [21]. 

Figure 2.9 Oxidation of an alcohol with NAD, and its reverse reaction, reduction of an aldehyde or ketone. B represents some basic group on the enzyme. The subscripts R and S are a means of distinguishing between the two hydrogen atoms of the methylene group, see the section on chirality at the end of this chapter...

The chiral [ H. H. Hlmethyl group has been an essential experimental tool for evaluating stereochemical trends among enzyme-catalyzed reactions involving the interconversion of methyl and methylene functions (-CH3 -CH2-)... 

Because biological reactions involve chiral enzymes, enantiotopic groups and faces typically show different reactivity. For example, the two methylene hydrogens in ethanol are enantiotopic. Enzymes that oxidize ethanol, called alcohol dehydrogenases, selectively remove the pro-/ hydrogen. This can be demonstrated by using a deuterated analog of ethanol in the reaction. 

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