Study of equilibration of the system involving two alternative, enzymically active complementing structures simultaneously formed from two overlapping fragments of staphylococcal nuclease.

Quantitative complementation of two overlapping fragments of staphylococcal nuclease, Nuclease-(1-126) (residues 1 to 126) and Nuclease-T-(50-149) (residues 50 to 149), simultaneously forms in 1 min, two alternative, enzymically active ordered structures (types I and II) resembling nuclease (149 residues) (Taniuchi, H., and Anfinsen, C.B. (1971) J. Biol. Chem. 246, 2291-2301). We determined the ratio of type I to type II complex formed from the two fragments as a function of time, temperature, and the presence or absence of the ligands thymidine 3',5'-diphosphate and calcium ion. The ratio of type I to type II complex was determined on the basis of the quantities of their derived complexes obtained after each experiment by removing the redundant amino acid sequences by limited digestion with trypsin in the presence of ligands. The quantity of the derived complexes was estimated by quantitative determination of the component fragments separated by gel filtration. The ratio of type I to type II complex formed in 2 min after mixing the two fragments was approximately 0.3 and appears to be independent of temperature and the presence or absence of ligands. The equilibrium of the system of type I and II complexes is attained through unfolding and folding. The ratios of type I to type II complex at the apparent equilibrium state of the system at 6 and 23 degrees were approximately 1.1 and 2.4, respectively. The observations indicate that the rate of unfolding of type II complex is greater than that of type I complex at 6 degrees and increases more than that of type I complex with increasing temperature. Thus, the change of the complementing structure from type I complex with increasing temperature. Thus, the change of the complementing structure from type I to type II causes a decrease in the activation free energy, an increase in the activation enthalpy, and thereby an increase in the activation entropy of unfolding. Since the unfolded states with which type I and II complexes are in equilibrium are the same, the distribution of the population of type I and II complexes at the equilibrium state will be determined on the basis of the respective decreases in Gibbs standard free energy from the unfolded state to type I and II complexes. On this basis type I complex has a lower energy by deltaG0 = -0.05 and -0.51 kcal mol-1 at 6 and 23 degrees, respectively, than type II complex. Nevertheless, at the initial complementation the population of type I complex formed is approximately one-third that of type II complex at both 6 and 23 degrees. That is, the probability (rate) of folding is not related to the decrease in energy from the unfolded to the folded state. Using van't Hoff's equation deltaH = 7.5 kcal mol-1 and then deltaS degrees = 27 cal deg-1 mol-1 from type II to type I complex.