Thermodynamic nonideality of enzyme solutions supplemented with inert solutes: yeast hexokinase revisited.

Published experimental results on the activating effect of polyethylene glycol on the interaction of yeast hexokinase with glucose (R.P. Rand, N.L. Fuller, P. Butko, G. Francis and P. Nicholls, Biochemistry, 32 (1993) 5925) are reinterpreted in statistical-mechanical terms of excluded volume. Of particular interest is the ability of this standard treatment of thermodynamic nonideality to accommodate the observed non-exponential dependence of the activation upon osmotic pressure of the polyethylene glycol solution--a dependence which is not predicted by analyses based on the concept of osmotic stress that was invoked originally to account for the results.