NEW SELECTIVITY AND TURNOVER IN PEPTIDE HYDROLYSIS BY METAL COMPLEXES - A PALLADIUM(II) AQUA COMPLEX CATALYZES CLEAVAGE OF PEPTIDES NEXT TO THE HISTIDINE RESIDUE

摘要

This seems to be the first report that a transition-metal complex bonded to a histidine residue effects hydrolytic cleavage of a peptide next to this residue. Dipeptides AcHis-Aa in which the C-terminal amino acid designated Aa is Gly, Ala, Ser, Thr, Leu, Phe, and Tyr are completely hydrolyzed at 60 degrees C and 1.46 less than or equal to pD less than or equal to 2.61 in the presence of cis-[Pd(en)(H2O)(2)](2+). The reaction is conveniently monitored by H-1 NMR spectroscopy, and we report the kinetics. The reaction is unimolecular with respect to the palladium(II)-peptide complex. The cleavage is regioselective. In all the aforementioned dipeptides and in the tripeptide AcGly-His-Gly only the amide bond involving the carboxylic group of histidine is cleaved; the amide bond involving the amino group of histidine is not cleaved. When the carboxylic group of histidine is free, as in AcGly-His, cis-[Pd(en)(H2O)(2)](2+) does not effect hydrolysis. Lability of palladium(II) complexes and the acidic solution make possible a modest turnover in hydrolysis; the catalyst can cleave several equivalents of the dipeptide. The dipeptides AcHis-Aa, and also one product of their cleavage, AcHis, exist free and bound to the catalyst. They form similar palladium(II) complexes, five types of which are distinguishable by H-1 NMR spectroscopy. The other products of cleavage, the amino acids Aa, exist free and in chelate complexes cis-[Pd(en)(N,O-Aa)](+). Partial binding of the catalyst to the peptide and to its cleavage products gives rise to an extended and complex equilibrium. Increase in pH favors catalytically-inactive palladium(II)-peptide complexes, inhibits their conversion into catalytically-active complexes, and lowers the observed rate constant for hydrolysis. Because the equilibria are reversible, even the peptide bound in inactive complexes eventually becomes hydrolyzed. When palladium is removed as a diethyldithiocarbamate complex, the equilibria are abolished and only ethylenediamine, AcHis, and-Aa remain. The rate constant for cleavage decreases as the steric bulk of the amino acid Aa increases and as intrapeptide hydrogen bonds mediated by water restrict the access of the palladium(II) catalyst to the His-Aa bond. This hydrogen bonding is possible only when the amino acid Aa contains a hydroxyl group in a flexible side chain, as in Ser and Thr. The intrapeptide hydrogen bonding is impossible when a hydroxyl group is held relatively rigidly, as in Tyr, and, of course, when the hydroxyl group is absent, as in the other four amino acids. The kinetic effects of steric bulk and of specific hydrogen bonding may allow sequence selectivity in cleavage of peptides with palladium(II) complexes. This study points the way toward artificial metallopeptidases, coordination complexes with enzyme-like properties.