| Selective cleavage of proteins is a common procedure in many biochemical applications ranging from standard protein sequencing to novel methods in proteomics and bioengineering. Few enzymes and synthetic reagents are available for this important task, but new chemical reagents with improved efficiency and adjustable selectivity are highly desired. Complexes of palladium(II) and platinum(II), two chemically similar transition-metal ions, are new reagents for selective cleavage of peptides and proteins. The cleavage by Pd(II) complexes, such as [Pd(H2O)4]2+ or cis-[Pd(en)(H 2O)2]2+, consistently occurs in weakly acidic aqueous solutions at the amide bond involving the N-terminus of the residue preceding histidine and methionine residues, i.e., the X-Y bond in the sequence segments X-Y-Met-Z and X-Y-His-Z, where X, Y, and Z are any non-coordinating residues. As pH is raised to neutral, the cleavage becomes sequence-specific---only the X-Pro bond in X-Pro-Met-Z and X-Pro-His-Z sequences is cleaved because of the unique interplay between the anchoring residue and the proline residue preceding it. The cleavage by Pt(II) complexes, such as cis-[Pt(en)(H 2O)2]2+, occurs exclusively at the peptide bond involving the C-terminus of methionine residues, i.e. the Met-Z bond.;In studies with peptide substrates, we explain the exceptional proteolytic selectivity of Pd(II) and Pt(II) complexes by identifying the hydrolytically-active modes in which these metal ions bind to the side chains of the anchoring residues and to the polypeptide backbone. The selectivity of cleavage originates in the selectivity of the coordination---under the reaction conditions, both methionine and histidine residues can bind to the Pd(II) reagents, whereas only methionine residue can bind to the Pt(II) reagent. The mechanism of cleavage originates in the modes of coordination---the anchored metal ion can approach the scissile peptide bond and activate it toward hydrolysis. The studies with protein substrates confirmed the cleavage pattern observed with peptides, and demonstrated that the Pd(II) and Pt(II) complexes are well-suited for biochemical applications. The ability of these complexes to cleave proteins at relatively few sites, with explicable selectivity and good yields, bodes well for their growing use in biochemical and bioanalytical practice. |
