Water Effects on Selectivity

When water molecules interact with an enzyme, it is natural that conformational changes can occur, which in turn can cause changes in the selectivity of the enzyme. Since enantioselectivity of enzymes is of major importance for many applications, it is a common task to investigate how to choose reaction conditions providing the maximal enantioselectivity. As might be expected, because water can interact with enzymes in many ways, it is difficult to generalize the effects. In some studies of lipase-catalyzed esterification reactions, no effects of water activity on enantioselectivity were observed [30]. In a similar study, no effects were observed in most cases, while the enantioselectivity of one lipase-catalyzed reaction decreased 

In the competition between the two pathways leading to the R and S enantiomers, the enthalpy effect dominated, which resulted in the formation of an excess of the S enantiomer [33]. 

Another type of important selectivity is that between hydrolysis and transferase reactions (transesterification, transglycosylation, etc.) catalyzed by hydrolases. In this case, water can act both as a reactant and as a substance that modifies the properties of the enzyme. Effects of water as a reactant can be expected to be governed by the concentration or activity of water, as with other substrates. The effects of water as an enzyme modifier are considerably more difficult to predict. 

The most straightforward way to quantify the competition between the transferase reaction and hydrolysis is to measure the initial ratio of these two reactions. Intuitively, one would assume the transferase/hydrolysis ratio to decrease with increasing water activity because of the effect of water as a reactant This is often the case when lipases are used as catalysts [34—36]. However, in reactions catalyzed by glycosidases and proteases the transferase/hydrolysis ratio can either increase or decrease with increasing water activity [37, 38]. 

The competition between transferase and hydrolysis reactions can be described in terms of nucleophile (acceptor) selectivities of the enzymes, and selectivity constants can be defined. These constants are meant to quantify the intrinsic selectivity of the enzymes. Selectivity constants in combination with the concentrations (or thermodynamic activities) of the competing nucleophiles give the transferase/hydrolysis ratio. The selectivity constants are defined as follows [38, 39]  

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