Chemical modification causes similar change in dependence on water activity of chymotrypsin hydration and catalysis in hexane

Amino groups in alpha-chymotrypsin were reacted with pyromellitic anhydride, introducing 17 to 32 additional carboxyl groups. This modification causes a major change in the water adsorption isotherm of the lyophilized protein powder. Little water is bound by the modified enzyme at water activity (a(w)) below 0.35, but it shows increased water binding at a(w) over 0.5. This correlates with a similar change in the a(w) dependence of the catalytic activity of the enzyme powder suspended in hexane, with a much steeper increase in activity of the modified chymotrypsin.