Ween urea concentrations 0?.5 M and then it switches to be the slower one, hence, what is referred to as the main phase in Figure 5 is represented by both kobs1 and kobs2. doi:10.1371/journal.pone.0050055.gFolding of a Circularly Permuted PDZ DomainFolding of a Circularly Permuted PDZ DomainFigure 4. Double jump experiments of cpSAP97 PDZ2 and pwtSAP97 PDZ2. A. Plot of the amplitudes for the two observed rate constants in an interrupted refolding experiment for cpSAP97 PDZ2. See panel D for examples of raw data. B. Plot of the amplitudes for the two observed rate constants in an interrupted refolding experiment for pwtSAP97 PDZ2. The data is from a GSK864 site previous publication [21]. C. The experimental data from Figure 4A together with simulated traces for the square model. The simulation was done using Copasi [29] and using the rate constants in Table S2. The initial distribution of the D states, 72 D and 28 Dcis-P, was calculated from the ratio of D and Dcis-P at equilibrium in the interrupted unfolding experiment. The excellent fit illustrates that the square model can explain our experimental data. D. Examples of experimental traces from interrupted refolding of cpSAP97 PDZ2 after various delay times. The traces were fitted to a double exponential curve (black) with shared rate constants and kinetic amplitudes plotted versus delay time (panel A). E. Plot of the amplitudes for the two observed rate constants in an interrupted unfolding experiment for cpSAP97 PDZ2. F. Plot of the amplitudes for the two observed rate constants in an interrupted unfolding experiment for pwtSAP97 PDZ2. The delay time plotted on the x-axis is the incubation time of the first mix. For example, in an interrupted refolding experiment it is the time the protein is MedChemExpress GSK962040 allowed to refold before unfolding is initiated by the second mixing event. doi:10.1371/journal.pone.0050055.gpreviously reported for proline isomerization [35]. These results together with the proposed square-folding scheme for the cpSAP97 PDZ2 suggest that the proline cis-trans isomerization is the likely cause for the slow kinetic phase. While addition of Pro cis-trans isomerases has been employed to confirm Pro phases, results from these experiments may sometimes be inconclusive due to, for example, the specificity of the enzyme and isomerization of non-Pro peptide bonds [36]. Given the complexity of the observed kinetics for cpSAP97 PDZ2, we chose not to perform such experiments.sized that there is a proline phase also in the folding of pwtSAP97 PDZ2 that previously escaped detection, since an interrupted unfolding 1407003 experiment was not included in the previous analysis [21]. To compare the pwt- and cpSAP97 PDZ2 folding pathways, we therefore did an interrupted unfolding experiment for 15857111 pwtSAP97 PDZ2. This experiment clearly confirmed that pwtSAP97 PDZ2, similarly to cpSAP97 PDZ2, has two distinct states at high urea (Figure 4F). Thus, the simplest folding scheme for the pwtSAP97 PDZ2 would also be a square model (Figure 5).The Canonical pwtSAP97 PDZ2 also Folds According to a Square Reaction SchemeResults from the interrupted refolding experiments for pwtSAP97 PDZ2 [21] (replotted in Figure 4B) and cpSAP97 PDZ2 initially appear to be different due to the lack of obvious transition for pwtSAP97 PDZ2 that corresponds to the main folding phase. However, a possible explanation for this are the similar rates between Dcis-P to D and D to N, respectively. In fact, since we argue that the transition between D a.Ween urea concentrations 0?.5 M and then it switches to be the slower one, hence, what is referred to as the main phase in Figure 5 is represented by both kobs1 and kobs2. doi:10.1371/journal.pone.0050055.gFolding of a Circularly Permuted PDZ DomainFolding of a Circularly Permuted PDZ DomainFigure 4. Double jump experiments of cpSAP97 PDZ2 and pwtSAP97 PDZ2. A. Plot of the amplitudes for the two observed rate constants in an interrupted refolding experiment for cpSAP97 PDZ2. See panel D for examples of raw data. B. Plot of the amplitudes for the two observed rate constants in an interrupted refolding experiment for pwtSAP97 PDZ2. The data is from a previous publication [21]. C. The experimental data from Figure 4A together with simulated traces for the square model. The simulation was done using Copasi [29] and using the rate constants in Table S2. The initial distribution of the D states, 72 D and 28 Dcis-P, was calculated from the ratio of D and Dcis-P at equilibrium in the interrupted unfolding experiment. The excellent fit illustrates that the square model can explain our experimental data. D. Examples of experimental traces from interrupted refolding of cpSAP97 PDZ2 after various delay times. The traces were fitted to a double exponential curve (black) with shared rate constants and kinetic amplitudes plotted versus delay time (panel A). E. Plot of the amplitudes for the two observed rate constants in an interrupted unfolding experiment for cpSAP97 PDZ2. F. Plot of the amplitudes for the two observed rate constants in an interrupted unfolding experiment for pwtSAP97 PDZ2. The delay time plotted on the x-axis is the incubation time of the first mix. For example, in an interrupted refolding experiment it is the time the protein is allowed to refold before unfolding is initiated by the second mixing event. doi:10.1371/journal.pone.0050055.gpreviously reported for proline isomerization [35]. These results together with the proposed square-folding scheme for the cpSAP97 PDZ2 suggest that the proline cis-trans isomerization is the likely cause for the slow kinetic phase. While addition of Pro cis-trans isomerases has been employed to confirm Pro phases, results from these experiments may sometimes be inconclusive due to, for example, the specificity of the enzyme and isomerization of non-Pro peptide bonds [36]. Given the complexity of the observed kinetics for cpSAP97 PDZ2, we chose not to perform such experiments.sized that there is a proline phase also in the folding of pwtSAP97 PDZ2 that previously escaped detection, since an interrupted unfolding 1407003 experiment was not included in the previous analysis [21]. To compare the pwt- and cpSAP97 PDZ2 folding pathways, we therefore did an interrupted unfolding experiment for 15857111 pwtSAP97 PDZ2. This experiment clearly confirmed that pwtSAP97 PDZ2, similarly to cpSAP97 PDZ2, has two distinct states at high urea (Figure 4F). Thus, the simplest folding scheme for the pwtSAP97 PDZ2 would also be a square model (Figure 5).The Canonical pwtSAP97 PDZ2 also Folds According to a Square Reaction SchemeResults from the interrupted refolding experiments for pwtSAP97 PDZ2 [21] (replotted in Figure 4B) and cpSAP97 PDZ2 initially appear to be different due to the lack of obvious transition for pwtSAP97 PDZ2 that corresponds to the main folding phase. However, a possible explanation for this are the similar rates between Dcis-P to D and D to N, respectively. In fact, since we argue that the transition between D a.
