Hydrogen ion titration and sodium ion activity of tropomyosin solutions

Mg2+-paracrystal formation of tropomyosin as a condensation phenomenon. Effects of pH, salt, temperature, and troponin binding

Tropomyosin (Tm) paracrystal formation induced by Mg2+ was studied by monitoring increases in light scattering. Paracrystals formed above a critical Tm concentration with lag phases in the time courses at pH 7.5 and 6.0, indicating that condensation polymerization processes are involved. The kinetic data at pH 7.5 reasonably fit a model in which nucleation and elongation are taken into account. The rate and extent of light scattering increased at low [Mg2+] and decreased at high [Mg2+] with a maximum at [Mg2+] = 15 mM, indicating different effects of Mg2+ in the two [Mg2+] ranges. The paracrystals were destabilized by increasing the salt concentration and decreasing the temperature. Mg2+ produces paracrystals at pH 6.0 and pH 7.5 by different kinetic mechanisms. Different Tm intermolecular interactions at the two pH values were indicated by studies of the excimer fluorescence of pyrene-labeled Tm and by effects of salt and temperature on the kinetics. At pH 6.0 Tm more readily formed paracrystals with decreased electrostatic effects. Effects of troponin on Mg2+-paracrystal formation of Tm at the two pH values correlated with the known differences in paracrystal structure when troponin is bound to Tm.

Effects of the state of the succinimido-ring on the fluorescence and structural properties of pyrene maleimide-labeled alpha alpha-tropomyosin

Rabbit skeletal alpha alpha-tropomyosin was labeled at Cys-190 with pyrene maleimide to form (S-[N-(1-pyrene)succinimido])2-tropomyosin (pyreneI-Tm). The product with cleaved succinimido-rings, pyreneII-Tm was also prepared by incubation of pyreneI-Tm at pH greater than 7.5. The pH dependence of the rate of cleavage indicated that hydrolysis rather than aminolysis was the more likely reaction. PyreneI-Tm exhibited a loss in helix content and end-to-end polymerization compared with unlabeled Tm, which increased upon formation of pyreneII-Tm. The cleavage resulted in increased interchain excited state excimer fluorescence originating from pyrene-pyrene interaction between the chains. Thus, increased pyrene-pyrene interaction at Cys 190 leads to an increase in unfolding, the effects of which appear to be transmitted to the ends of tropomyosin. The fluorescence properties of the two types of pyrene-succinimide adducts of dithiothreitol were very similar to the corresponding adducts of pyrene-Tm indicating excimer formation through ground state pyrene-pyrene interaction.