Enzymes reacting with both enantiomeric forms of a substrate

The racemase enzymes are also of interest in this connection. Both enantiomers function as substrates and presumably bind to the same active site. 

It is worthwhile to appreciate that there is a definite free energy change associated with racemization (defined as conversion of either 100% R or 100% S to 50% R, 50% S). Since at equilibrium, equal amounts must be present the equilibrium constant is 1. Hence the standard free energy change (AG°) is 0. This, however, refers to the conversion of 1 mole R to 1 mole S in other words, to inversion not racemization. 

For the actual free energy change of racemization (AG) it can be shown that AG= -RTIn 2 

there is a driving force for racemization of about —400 cal/mol at 25°C. The entropy term for racemization is positive—the system becomes more random. 

The racemase enzymes present an interesting situation in terms of chiral recognition. In terms of the classification discussed later, they lack both substrate and product selectivity. The simplest kinetic equations that could be written are as follows (D and L represent the two enantiomers of a substrate, E the enzyme)  

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