Sequence-dependent cleavage of albumins with palladium(II) complexes: role of serine residue in controlling the high regioselectivity of protein cleavage

Bovine serum, pig serum, and chicken egg albumins were incubated at 60 degreesC for 2 days or at 50 degreesC for 6 days with the pyridine (py) complex trans-[Pd(py)(2)(H2O)(2)](2+), the ethylenediamine (en) complex cis-[Pd(en)(H2O)(2)](2+), the 1,4-diazacycloheptane (dach) complex cis-[Pd(dach)(H2O)(2)](2+), and the 1,5-dithiacyclooctan-3-ol (dtco-3-OH) complex cis-[Pd(dtco-3-OH)(H2O)(2)](2+) at pH 4.5 +/- 0.2. The mole ratio of the complex and the protein was 8:1 or 16:1. Except for cis-[Pd(dtco-3-OH)(H2O)(2)](2+), which was unreactive, the remaining three palladium(H) complexes consistently cleaved bovine albumin at the sites Glu57-Ser58, Gly85-Asp86, Leu103-Ser104, and Lys285-Ser286; pig albumin at the site Lys285-Ser286; and chicken albumin at the sites Ala6-Ser7, Ala220-Ser221, Gly237-Thr238, and Met239-Ser240. With all the three active Pd(II) complexes and all the three proteins, each site of major cleavage is a peptide bond followed by two residues, the first of which is serine or threonine and the second of which is histidine or methionine. The latter of the two residues serves as an anchor for the Pd(H) complex, The former of the two residues, with its alcoholic side chain, seems to guide the cleavage of the peptide bond immediately preceding it. To explain this guidance, we optimized the geometry of the relevant tripeptide complex [Pd(OH)(Ac-Gly-SerH(-1)-MetH(-1)-NMe-kappa(3)N(S),N-M,S](-) by MM+ and ZINDO/1 calculations. A mechanism involving the nucleophilic attack of the hydroxylic group of serine on the carbonyl carbon atom of glycine resembles the catalytic mechanism of serine proteases. (C) 2002 Elsevier Science B.V. All rights reserved.