Titration of substrate C to PRD25N. Portions of HSQC spectra are shown for the different substrate C-to-protein concentration ratios (A) 0:1, (B) 0.5:1, (C) 1:1, (D) 2:1, (E) 4:1, and (F) 8:1 (protein concentration, 0.3mM; subscripts indicate bound and free forms).