Handbook of Solvents - George Wypych - ChemTech - Ventech!

874 Tsuneo Yamane
Very trace amount of water or nearly anhydrous state renders practically no enzymatic
reaction. In this context, it should be reminded that commercially available enzyme preparations,
or the enzymes even after lyophilization or other drying procedures, contain some
water bound to the enzyme proteins. Whereas, excess water in the reaction system results in
hydrolysis of the substrate, which is often unfavorable side reaction, giving rise to lower
yield of product. Thus, there exist usually the optimal water content for each enzymatic reaction
of concern.
Water molecules in the microaqueous system exist in three different states: 1) water
bound to the enzyme protein, 2) water dissolved freely in the solvent (plus dissolved substrate),
and 3) water bound to impurities existing in the enzyme preparation or bound to the
support materials if immobilized enzyme particles are used. Therefore, the following equation
with respect to water holds:
Total water = (Water bound to the enzyme) + (Water dissolved in the solvent) [14.4.3.1]
+ (Water bound to the immobilization support or to impurities of the enzyme preparation)
Water affecting most of the catalytic activity of the enzyme is the one bound to the enzyme
protein. 6 From the above equation, it can be well understood that the effect of water
varies depending on the amount of enzyme used and/or its purity, kind of solvent, and nature
of immobilization support, etc. as far as the total water content is used as the sole variable.
Also, it is often asked what is the minimal water content sufficient for enzymatic
activity? It should be recognized that a relation
between the degree of hydration of the
enzyme and its catalytic activity changes
continuously. There exist a thermodynamic
isotherm-type equilibrium between the protein-bound
water and freely dissolved water,
and its relationship is quite different
between water-miscible and water-insoluble
solvents. 4
A parameter better than the water
content, water activity, a w, was proposed to
generalize the degree of hydration of a
biocatalyst in organic media. 7 a w is a thermodynamic
parameter which determines
how much water is bound to the enzyme
and in turn decides the catalytic activity to a
large extent among different kinds of the
organic solvents. a w is especially useful
when water-insoluble organic solvent is
used because the precise water content is
hard to be measured due to its low solubil-
Figure 14.4.3.1 Activity of Lipozyme catalyst as a function
of water activity in a range of solvents. [Adapted, by
permission, from R.H. Valivety, P.J. Halling and A.R.
Macrae, Biochim. Biophys. Acta, 1118, 221 (1992)].
ity. It was shown that profiles between a w
and the reaction rate were similar when the
same reaction was carried out in different
solvents at varying water contents. 8 In dif-