Influence of Leaving-Group Electronic Effect on alpha-Chymotrypsin: Catalytic Constants of Specific Substrates.
Rate constants and binding constants for the alpha-chymotrypsin-catalyzed hydrolysis of N-acetyltyrosine, tryptophan, and phenylalanine anilides are presented. Both k(cat) and K(m) are independent of electronic effects in the substrate over a range of 9.8 orders of magnitude (as measured by pK of the leaving group). Similarly, K(m) is independent of charge and orientation about the alpha-carbon for various substrates and pseudo-substrates. These results are not consistent with the pretransition state protonation hypothesis; instead, they are discussed in terms of a tetrahedral intermediate that is thermodynamically less stable than the Michaelis complex.
Department of Chemistry, Northwestern University, Evanston, Illinois 60201.
M
M
Philipp
R M
RM
Pollack
M L
ML
Bender