Original scientific paper

Selective Labeling of the Enzyme Papain with Chloro (terpyridine)platinum(II)

Sharon L. Pinnow ; Department of Chemistry, Iowa State University, Ames, Iowa 50011, U.S.A.
Herb M. Brothers II ; Department of Chemistry, Iowa State University, Ames, Iowa 50011, U.S.A.
Nenad M. Kostić ; Department of Chemistry, Iowa State University, Ames, Iowa 50011, U.S.A.

Abstract

The complex [Pt(trpy)Cl]+ reacts with imidazole, histidine, and the tripeptide Gly-His-Gly to yield complexes in which the chloride ligand is replaced by the imidazole ring; [Pt(trpy)Cl]+ reacts with 2-mercaptoethanol, cysteine, and the tripeptide glutathione to yield complexes in which the chloride ligand is replaced by the thiolate group. When the enzyme papain is treated with an equimolar amount of [Pt(trpy)Cl]+, only Cys 25 at the active site is tagged with a Pt(trpy)2+ group; when papain is treated with an excess of [Pt(trpy)Cl] + , both Cys 25 and His 81 are so tagged. When this diplatinated enzyme is treated with a large excess of bromide anion, only the tag from His 81 is removed. The rate constants for the displacement of the chloride ligand in [Pt(trpy)Cl]+ with glutathione, papain, and histidine at pH 5.0 are 30.2, 21.3, and 0.274 M-1 s-1, respectively. These values indicate that the sulfhydryl group is more nucleophilic than the imidazole group and that Cys 25 at the active site of papain is somewhat shielded from external attack by the [Pt(trpy)Cl]+ complex. When the diplatinated papain is treated with a nucleophile stronger than histidine but weaker them cysteine, the Pt(trpy)2+ tag is selectively removed from His 81.

Keywords

Hrčak ID:

137332

URI

https://hrcak.srce.hr/137332

Publication date:

31.12.1991.

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