Advances in the stereoselective synthesis of antifungal agents and ...

Chapter 2 52This equation can be further simplified because the maximum velocitywill occur when the enzyme is saturated and [ES]=[E] 0 . V MAX can then bedefined as K 2 [E] 0 . Substitution into eq.8 leads to the Michaelis-Mentenequation 9:v =V MAX [S]Km + [S]Equation 9: Michaelis-Menten equationThe Michaelis-Menten equation 9 expresses the rate equation for theenzyme catalyzed transformed of one single substrate.2.2.3 SpecificitySelectivity constitutes the major synthetic value of enzyme catalysts.Because enzymes are large chiral molecules with a unique structure at theactive site, they can be highly selective for certain types of substratestructures and reactions. Useful types of enzyme catalyzed reactionsinclude the chemoselective reaction of defined functional groups in a givenmolecule. The reactions are:RegioselectiveEnantioselective with racemic substratesEnantioselective with prochiral substratesDiasteroselectiveAll such selective reactions occur because during the reaction theprochiral or chiral reactants form diasteromeric enzyme transition statecomplexes that differ in transition state energy (G#).2.2.3.1. Kinetic resolution of a racemateIn the kinetic resolution of a racemate the two enantiomeric substratesare competing for the same active site of the enzyme E. If A and B are twoenantiomers, the reaction scheme is as follows (Scheme 2.9):