Asymmetric Citric Acid 249 



least a three point interaction between the substrate and the 

 enzyme. In order to illustrate the formation of citric acid 

 we shall assume for convenience that the oxaloacetate inter- 

 acts with the enzyme through the two carboxyl groups, which 

 are in the plane of the paper. Furthermore, we shall assume 

 that the "activated acetate" interacts with the enzyme, which 

 has a configuration such that it can only react when it 

 approaches the carbonyl group of the oxaloacetate from 



HOo5cH2CO + (-CH2COX) 



COOH 



HOOCCHpCOHCH^COOH 



Pigeon Liver 

 Extract I. 



COOH 



HOOCCOCH2CH2COOH 



1 



Pigeon Liver 

 Extract II. 



KltoOi^ 



CO2 + HOOCCH2CH2COOH 



100^ 05? 



Fig. 4. Distribution of label from oxaloacetate in citrate and 

 a-ketoglutarate (Lorber et al., 1950). 



the rear. In this way, the citric acid derived from labelled 

 CO2 would have the configuration shown in the diagram. If 

 it were derived from a labelled Cg fragment, it would have the 

 antipodal configuration, since it would be a non-superim- 

 posable mirror image of the first. This can be clearly seen 

 by examination of molecular models. 



In order to consider the steps leading from citrate to a- 

 ketoglutarate let us suppose that the asymmetric interaction 

 of enzyme and substrate occurs at three points: with the 

 tertiary carboxyl group at a, a primary carboxyl group at 



