title: Exploring the active site of tripeptidyl-peptidase II through studies of pH dependence of reaction kinetics.
abstract: Tripeptidyl-peptidase II (TPP II) is a subtilisin-like serine protease which forms a large enzyme complex (>4MDa). It is considered a potential drug target due to its involvement in specific physiological processes. However, information is scarce concerning the kinetic characteristics of TPP II and its active site features, which are important for design of efficient inhibitors. To amend this, we probed the active site by determining the pH dependence of TPP II catalysis. Access to pure enzyme is a prerequisite for kinetic investigations and herein we introduce the first efficient purification system for heterologously expressed mammalian TPP II. The pH dependence of kinetic parameters for hydrolysis of two different chromogenic substrates, Ala-Ala-Phe-pNA and Ala-Ala-Ala-pNA, was determined for murine, human and Drosophila melanogaster TPP II as well as mutant variants thereof. The investigation demonstrated that TPP II, in contrast to subtilisin, has a bell-shaped pH dependence of k(cat)(app)/K(M) probably due to deprotonation of the N-terminal amino group of the substrate at higher pH. Since both the K(M) and k(cat)(app) are lower for cleavage of AAA-pNA than for AAF-pNA we propose that the former can bind non-productively to the active site of the enzyme, a phenomenon previously observed with some substrates for subtilisin. Two mutant variants, H267A and D387G, showed bell-shaped pH-dependence of k(cat)(app), possibly due to an impaired protonation of the leaving group. This work reveals previously unknown differences between TPP II orthologues and subtilisin as well as features that might be conserved within the entire family of subtilisin-like serine peptidases.
authors.affiliation: Department of Medical Biochemistry and Microbiology, Uppsala University, Uppsala, Sweden. sandra.eklund@imbim.uu.se
authors.forename: Sandra
authors.initials: S
authors.lastname: Eklund
authors.forename: Ann-Christin
authors.initials: AC
authors.lastname: Lindås
authors.forename: Emil
authors.initials: E
authors.lastname: Hamnevik
authors.forename: Mikael
authors.initials: M
authors.lastname: Widersten
authors.forename: Birgitta
authors.initials: B
authors.lastname: Tomkinson
mesh_terms: D000595:Amino Acid Sequence; D019943:Amino Acid Substitution; D000626:Aminopeptidases / Q000096:biosynthesis / Q000737:chemistry* / Q000235:genetics; D000818:Animals; D020134:Catalytic Domain; D017124:Conserved Sequence; D004152:Dipeptidyl-Peptidases and Tripeptidyl-Peptidases / Q000096:biosynthesis / Q000737:chemistry* / Q000235:genetics; D004330:Drosophila / Q000201:enzymology; D029721:Drosophila Proteins / Q000096:biosynthesis / Q000737:chemistry* / Q000235:genetics; D004926:Escherichia coli; D006801:Humans; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D051379:Mice; D008958:Models, Molecular; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D010455:Peptides / Q000737:chemistry; D011485:Protein Binding; D059748:Proteolysis; D011994:Recombinant Proteins / Q000096:biosynthesis / Q000737:chemistry / Q000235:genetics; D012697:Serine Endopeptidases / Q000096:biosynthesis / Q000737:chemistry* / Q000235:genetics; D040681:Structural Homology, Protein; D013379:Substrate Specificity; D013381:Subtilisins / Q000737:chemistry
chemical_list: D029721:Drosophila Proteins; D010455:Peptides; D011994:Recombinant Proteins; D000626:Aminopeptidases; D004152:Dipeptidyl-Peptidases and Tripeptidyl-Peptidases; C048001:tripeptidyl-peptidase 2; D012697:Serine Endopeptidases; D013381:Subtilisins