Attractive protein-polymer interactions markedly alter the effect of macromolecular crowding on protein association equilibria

Abstract

The dependence of the fluorescence of catalase upon the concentration of added superoxide dismutase (SOD) indicates that SOD binds to saturable sites on catalase. The affinity of SOD for these sites varies with temperature, and with the concentration of each of three nominally inert polymeric additives--dextran 70, Ficoll 70, and polyethylene glycol 2000. At room temperature (25.0 degrees C) and higher, the addition of high concentrations of polymer is found to significantly enhance the affinity of SOD for catalase, but with decreasing temperature the enhancing effect of polymer addition diminishes, and at 8.0 degrees C, addition of polymer has little or no effect on the affinity of SOD for catalase. The results presented here provide the first experimental evidence for the existence of competition between a repulsive excluded volume interaction between protein and polymer, which tends to enhance association of dilute protein, and an attractive interaction between protein and polymer, which tends to inhibit protein association. The net effect of high concentrations of polymer upon protein associations depends upon the relative strength of these two types of interactions at the temperature of measurement, and may vary significantly between different proteins and/or polymers.

Figure 1

Dependence of catalase fluorescence intensity at 25.0°C on the concentration of added SOD. (Circles) No additive. (Solid curve) Best-fit of Eqs. 2 and 3, with F₀ = 478, F_sat = 157, and log₁₀K_a = 4.71. (Squares) 200 g/L Ficoll 70 added. (Dotted curve) Best-fit of Eqs. 2 and 3, with F₀ = 565, F_sat = 192, and log₁₀K_a = 5.93. The concentration of catalase in the cuvette was 0.09 μM.

Figure 2

Dependence of ln Γ on additive concentration. Error bars correspond to ±2 SE of estimate (95% confidence limits). Lines plotted in each panel corresponding to a polymeric additive represent the best-fits of special case 1 (dashed) and special case 2 (dotted) of the molecular model introduced below to the full set of data representing both concentration and temperature dependence. (The two lines calculated for PEG superimpose.) The lines are calculated using parameter values specified in the section describing the molecular model and in Table 2.

Figure 3

Dependence of ln Γ on inverse temperature (K⁻¹) at an additive concentration of 200 g/L. The dotted lines in each panel represent the best fit of Eq. 7, with the following parameter values. Dextran 70: ΔΔH/R = 7500 ± 1500 K; ΔΔS/R = 25.6 ± 5.0. Ficoll 70: ΔΔH/R = 11,000 ± 1500 K; ΔΔS/R = 39.3 ± 5.0. PEG 2000: ΔΔH/R = 10,100 ± 1400 K; ΔΔS/R = 35.3 ± 4.6. Indicated uncertainties represent ±1 SE of estimate.

Figure 4

Dependence of ln Γ on temperature (°C) at an additive concentration of 200 g/L. Lines plotted in each panel corresponding to a polymeric additive represent the best-fits of special case 1 (dashed) and special case 2 (dotted) of the molecular model introduced below to the full set of data representing both concentration and temperature dependence. (The two lines calculated for PEG superimpose.) The lines are calculated using parameter values specified in the section describing the molecular model and in Table 2.