Fluorescence-Quenched Solid Phase Combinatorial Libraries in the Characterization of Cysteine Protease Substrate Specificity

摘要

To map the substrate specificity of cysteine proteases, two combinatorial peptide libraries were synthesized and screened using the archetypal protease, papain. The use of PEGA resin as the solid support for library synthesis facilitated the application of an on-resin fluorescence-quenched assay. Results from the screening of library 2 indicated a preference for Pro or Val in the S_3 subsite and hydrophobic residues in S_2; the most prevalent residue not being Phe but Val. The S_1 subsite exhibited a dual specificity for both small, nonpolar residues, Ala or Gly, as well as larger, Gln, and charged residues, Arg. Small residues predominated in the S_1'-S_4' subsites. Active peptides from the libraries and variations thereof were resynthesized and their kinetics of hydrolysis by papain assessed in solution phase assays. Generally, there was a good correlation between the extent of substrate cleavage on solid phase and the k_(cat)/K_M's obtained in solution phase assays. Several good substrates for papain were obtained, the best substrates being Y(NO_2)PMPPLCTSMK(Abz) (k_(cat)/K_M=2109(mM s)~(-1), Y(NO_2)PYAVQSPQK(Abz) (k_(cat)/K_M=1524(mM s)~(-1), and Y(NO_2)PVLRQQRSK-(Abz) (k_(cat)/K_M=1450 (mM s)~(-1). These results were interpreted in structural terms by the use of molecular dynamics (MD). These MD calculations indicated two different modes for the binding of substrates in the narrow enzyme cleft.