title: Mechanism of action of papain: aryldehydroalanines as spectroscopic probes of acyl enzyme formation.
abstract: The alpha,beta-unsaturated aromatic amino acids phenyldehydroalanine (PDA) and styryldehydroalanine (SDA) were synthesized and used as sensitive spectroscopic probes to study the acylation of papain by specific substrates and inhibitors. The spectral changes observed upon acylation of the enzyme with peptides containing these amino acids are large red shifts of the absorption maxima (lambda max) of the chromophores. The magnitudes of the absorption shifts were 49 nm (from 277 to 326 nm) for PDA peptide and 59 nm (from 318 to 377 nm) for SDA peptides. The following specific substrates were synthesized: Ac-Phe-Phe-PDA-OEt, Ac-Phe-PDA-NH2, Ala-Ala-Phe-SDA-OME, Ala-Ala-Phe-SDA-NH2, Lys-Ala-(o-benzyl)tyrosyl-SDA-OMe, and Lys-Ala-(o-benzyl)-tyrosyl-SDA-NH2. Similarly, the specific competitive inhibitors Ac-Phe-PDA (Ki = 5.3 X 10(-6) M), Z-Phe-SDA (Ki = 5.6 X 10(-5) M), Ala-Ala-Phe-SDA (Ki = 2.9 X 10(-5) M), and Lys-Ala-(o-benzyl)tyrosyl-SDA (Ki = 1.1 X 10(-5) M) were prepared. An additional chromophore was used to follow the noncovalent association of an inhibitor or substrate with papain, independently from the acylation or deacylation steps. This chromophore, which was introduced into the peptides at position P2, IS p-(p"-dimethylaminophenylazo) phenylalanine (DAP). The light absorption spectrum of DAP is dependent on its environment. The substrates Ala-Ala-DAP-SDA-OMe and Ala-Ala-DAP-SDA-NH2 and the competitive inhibitor Ala-Ala-DAP-SDA (Ki = 2.5 X 10(-6) M) were prepared. The noncovalent binding of these peptides to the active site of papain was followed either by the increase in the absorption at 480 nm or the decrease at 550 nm. With these petides the acylation and deacylation reactions could be followed simultaneously at 377 nm. The extent of acyl enzyme formation was found to decrease in a sigmoidal way with increasing pH, with a transition point around pH 5.5.
mesh_terms: D000409:Alanine / Q000031:analogs & derivatives* / Q000138:chemical synthesis; D007700:Kinetics; D010206:Papain / Q000378:metabolism*; D010649:Phenylalanine / Q000031:analogs & derivatives; D013053:Spectrophotometry; D013329:Structure-Activity Relationship; D013379:Substrate Specificity
chemical_list: D010649:Phenylalanine; C018632:phenyldehydroalanine; C018668:styryldehydroalanine; D010206:Papain; D000409:Alanine