Soft interaction and excluded volume effect compete as polyethylene glycols modulate enzyme activity.

Polyethylene glycols (PEGs) can either preferentially bind to biomolecules or exert excluded volume effect depending upon their chain length and concentration. We have studied the effect of ethylene glycol (EG) and PEGs of different chain lengths (Mn 400 and 4000) on the enzyme efficiency of hen-egg-white lysozyme (HEWL) on Micrococcus lysodeikticus (M. Lys.) cell. The activity shows a bell-like profile as the turnover number increases from ~1.3 × 105 s-1 M-1 in water to ~1.7 × 105 s-1 M-1 in presence of 2% PEG-400 beyond which it decreases to ~0.7 × 105 s-1 M-1 at 20% PEG-400. Solvent polarity, excluded volume effect, soft nonspecific interactions and structural flexibility are found to be the competing factors which govern the overall enzyme activity as evidenced from circular dichroism (CD) and fluorescence measurements. Thermal unfolding temperature (Tm) of HEWL also shows a bell-shaped profile with PEG concentration which establishes possible correlation with its activity. We also observe a minimum in the activation energy barrier for the catalysis at low osmolyte concentrations. The maximum in the enzyme efficiency has been explained on the basis of an optimization between excluded volume effect and soft interaction among the protein and the cosolutes.