Structure-activity relationships for inhibition of papain by peptide Michael acceptors

Two series of peptidyl Michael acceptors, N-Ac-L-Phe-NHCH2CH = CH-E with different electron withdrawing groups (E = CO2CH3, 1a; SO2CH3, 1b; CO2H, 1c; CN, 1d; CONH2, 1e; and C6H4-p-NO2, 1f) and R-NHCH2CH = CHCOOCH3 with different recognition and binding groups (R = N-Ac-D-Phe, 2a; N-Ac-L-Leu, 3a; N-Ac-L-Met, 4a; PhCH2CH2CO, 5a; PhCO, 6a), were synthesized and evaluated as inactivators against papain. It was found that the inhibition of papain by peptidyl Michael acceptors is a general phenomenon and that the intrinsic chemical reactivity of the E group in the Michael acceptors has a direct effect on the kinetics of the inactivation process as reflected in k2/Ki. At pH 6.2, the reactivity of papain toward the Michael acceptors is about 283,000-fold higher than the reactivity of the model thiol 3-mercaptopropionate. This large increase in reactivity is attributable to at least 2 factors; one is the low apparent pKa of Cys-25 of papain, and the other is the recruitment of catalytic power by specific enzyme-substrate interactions. The unexpectedly high reactivity of 1c (E = COOH) was rationalized by proposing a direct interaction of the acid group with His-159 in the active site of papain. The unexpected inactivity of 1f (E = C6H4-p-NO2) as a Michael acceptor and its very powerful competitive inhibition of papain were rationalized by molecular graphics which showed the nitrophenyl moiety rotated out of conjugation with the olefin and interacting instead with the hydrophobic S1' region of papain. A plot of log (k2/Ki) for 1a-6a vs log (kcat/Km) for analogous R-Gly-p-NA substrates was linear (r = 0.98) with slope of 0.83, suggesting that binding energy from specific enzyme-ligand interactions can be used to drive the self-inactivation reaction to almost the same extent as it is used to drive catalysis.