Comparison of C40/82A and P27A C40/82A Barstar Mutants Using ¹⁹F NMR^†

Barstar, an inhibitor of the enzyme barnase, contains two phenylalanine residues, three tryptophan residues, and two proline residues. After incorporating either 2-¹⁹F-Phe, 4-¹⁹F-Phe, or 6-¹⁹F-Trp, the structural, dynamic, and folding properties of two mutants (C40/82A, a double mutant, and P27A C40/82A, a triple mutant) were studied by ¹⁹F NMR. Experiments were performed as a function of temperature and urea with the two mutants. We show that the consequences of the P27A mutation are extensive. The effect of the mutation is transmitted to distant residues (Phe56 and Trp53) as well as to a residue deeply buried in the hydrophobic core (Phe74). By incorporating 2-¹⁹F-Phe, it is shown that Phe56 undergoes a slow ring flipping on the NMR time scale in the triple mutant that is not observed in the double mutant. On the other hand, incorporating 4-¹⁹F-Phe shows that the P27A mutation has little effect along the C^β−C^γ axis of Phe56. Labeling with 4-¹⁹F-Phe shows, from line broadening, that Phe74 experiences more dynamic motion than does Phe56 in both the double and triple mutant. After incorporating 6-¹⁹F-Trp, it is found that, in the triple mutant, Trp53 shows conformational heterogeneity at low temperature while Trp44, which is close to the P27A mutation, does not. At 20 °C, residual native-like structure was detected around Trp53 at high concentrations of denaturant. Barstar is cold denatured in the presence of urea. For the double mutant at temperatures below 15 °C, and in the presence of 2.5−3.5 M urea, the resonance for Phe74 broadens, and two peaks are observed at 5 °C indicative of an exchange process. From line-shape analysis, assuming a two-site conformational exchange, the rate constants as a function of temperature can be extracted. An Eyring plot is linear at 0 M urea but deviates from linearity below 20 °C in the presence of 2.5 or 3.5 M urea. The data as a function of urea suggest sequential events in the unfolding process.