Conformational change that accompanies pepsinogen activation observed in real time by fluorescence energy transfer

Pig pepsinogen has been reacted with N-carboxymethylisatoic anhydride to form N-carboxymethyl-anthraniloyl-(CMA-) pepsinogen, derivatized at Lysp18, Lysp23, Lysp27, Lysp30, and Lys320. Conformational change associated with activation was detected by following energy transfer from tryptophan residues of the pepsin moiety, excited at 295 nm, to CMA groups, monitored by emission above 415 nm. Efficiency of this energy transfer is a measure of conformational change. For this zymogen derivative the change in efficiency occurs with a first order rate constant of 0.041 s-1 at pH 2.4, 22 degrees, which equals the rate at which, following acidification, alkali-stable potential activity becomes alkali-labile. For the native zymogen the rate of this conversion had been shown to be identical to the rate of cleavage of the scissile bond of pepsinogen. Therefore, the correspondence in this derivative of the rates of conversion to alkali lability and change in energy transfer demonstrates that a conformational change accompanies the peptide bond cleavage of activation.