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Overlapping binding sites for trypsin and papain on a Kunitz‐type proteinase inhibitor from Prosopis juliflora
Author(s) -
Franco Octávio L.,
Grossi de Sá Maria F.,
Sales Maurício P.,
Mello Luciane V.,
Oliveira Adeliana S.,
Rigden Daniel J.
Publication year - 2002
Publication title -
proteins: structure, function, and bioinformatics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.699
H-Index - 191
eISSN - 1097-0134
pISSN - 0887-3585
DOI - 10.1002/prot.10228
Subject(s) - papain , trypsin , biochemistry , enzyme , trypsin inhibitor , chemistry , kunitz sti protease inhibitor , cysteine , binding site , biology
Proteinase inhibitors are among the most promising candidates for expression by transgenic plants and consequent protection against insect predation. However, some insects can respond to the threat of the proteinase inhibitor by the production of enzymes insensitive to inhibition. Inhibitors combining more than one favorable activity are therefore strongly favored. Recently, a known small Kunitz trypsin inhibitor from Prosopis juliflora (PTPKI) has been shown to possess unexpected potent cysteine proteinase inhibitory activity. Here we show, by enzyme assay and gel filtration, that, unlike other Kunitz inhibitors with dual activities, this inhibitor is incapable of simultaneous inhibition of trypsin and papain. These data are most readily interpreted by proposing overlapping binding sites for the two enzymes. Molecular modeling and docking experiments favor an interaction mode in which the same inhibitor loop that interacts in a canonical fashion with trypsin can also bind into the papain catalytic site cleft. Unusual residue substitutions at the proposed interface can explain the relative rarity of twin trypsin/papain inhibition. Other changes seem responsible for the relative low affinity of PTPKI for trypsin. The predicted coincidence of trypsin and papain binding sites, once confirmed, would facilitate the search, by phage display for example, for mutants highly active against both proteinases. Proteins 2002;49:335–341. © 2002 Wiley‐Liss, Inc.