An Apoptosis-Inducing Peptidic Heptad That Efficiently Clusters Death Receptor†5.

Multivalent ligands of death receptors hold particular promise as tumor cell-specific therapeutic agents because they induce an apoptotic cascade in cancerous cells. Herein, we present a modular approach to generate death receptor 5 (DR5) binding constructs comprising multiple copies of DR5 targeting peptide (DR5TP) covalently bound to biomolecular scaffolds of peptidic nature. This strategy allows for efficient oligomerization of synthetic DR5TP-derived peptides in different spatial orientations using a set of enzyme-promoted conjugations or recombinant production. Heptameric constructs based on a short (60-75 residues) scaffold of a C-terminal oligomerization domain of human C4b binding protein showed remarkable proapoptotic activity (EC50=3 nm) when DR5TP was ligated to its carboxy terminus. Our data support the notion that inter-ligand distance, relative spatial orientation and copy number of receptor-binding modules are key prerequisites for receptor activation and cell killing.

Engineering a Constrained Peptidic Scaffold towards Potent and Selective Furin Inhibitors.

We report the engineering of the monocyclic sunflower trypsin inhibitor (SFTI-1[1,14]) into a potent furin inhibitor. In a rational approach, we converted the native scaffold of this trypsin-like serine protease inhibitor into a subtilisin-like one by substitutions in the canonical and, particularly, in the substrate-binding loop. Although the substrate sequence for furin is Arg-X-Arg/Lys-Arg↓, the most potent inhibitor had a lysine at position P1. C-terminally truncated versions demonstrated the strongest activity, thus suggesting a lack of interaction between this motif and the surface of furin. This observation was further supported by molecular modeling. With an inhibition constant of 0.49 nm, the engineered peptide H-KRCKKSIPPICF-NH2 is a promising compound for further development of furin inhibitors aimed at controlling the activity of this protease in vitro and in vivo.

Locked by Design: A Conformationally Constrained Transglutaminase Tag Enables Efficient Site-Specific Conjugation.

Based on the crystal structure of a natural protein substrate for microbial transglutaminase, an enzyme that catalyzes protein crosslinking, a recognition motif for site-specific conjugation was rationally designed. Conformationally locked by an intramolecular disulfide bond, this structural mimic of a native conjugation site ensured efficient conjugation of a reporter cargo to the therapeutic monoclonal antibody cetuximab without erosion of its binding properties.

Potent inhibitors of human matriptase-1 based on the scaffold of sunflower trypsin inhibitor.

Sunflower trypsin inhibitor-1 (SFTI-1), a bicyclic tetradecapeptide, has become a versatile tool as a scaffold for the development of the inhibitors of therapeutically relevant serine proteases, among them matriptase and kallikreins. Herein, we report the rational design of potent monocyclic and bicyclic inhibitors of human matriptase-1. We found that the presence of positive charge and lack of bulky residues at the peptide N-terminus is required for the maintenance of inhibitory activity. Replacement of the N-terminal glycine residue by lysine allowed for the chemical conjugation with a fluorophor via the ε-amino group without significant loss of inhibitory activity. Head-to-tail and side-chain-to-tail cyclization resulted in potent inhibitors with comparable activities against matriptase-1. The most potent synthetic bicyclic inhibitor found in this study (Ki = 2.6 nM at pH 7.6) is a truncated version of SFTI-1 (cyclo-KRCTKSIPPRCH) lacking a C-terminal proline and aspartate residue. It combines an internal disulfide bond with a peptide macrocycle that is formed through side-chain-to-tail cyclization of the ε-amino group of an N-terminal lysine and a C-terminal proline.

Combinatorial tuning of peptidic drug candidates: high-affinity matriptase inhibitors through incremental structure-guided optimization.

Herein we report a convenient strategy for the development of novel, highly-potent peptidic inhibitors of the trypsin-like serine protease matriptase based on the monocyclic variant of the sunflower trypsin inihibitor-1 (SFTI-1[1,14]). We screened SFTI-1[1,14] variants possessing incremental modifications of the parent peptide for beneficial binding properties. This compound library comprising 6 peptides and 16 triazole-containing peptidomimetics was established via structure-guided rational design and synthesized using a divergent strategy employing "copper-click" chemistry. The most favorable amino acid substitutions were combined in one framework yielding potent SFTI-1-derived matriptase inhibitor-1 (SDMI-1) and the truncated dodecapeptide variant (SDMI-2) with single-digit nanomolar inhibition constants. In silico studies indicated that the improved matriptase affinity compared to the parent peptide is caused by the successful establishment of additional favorable proton donor-acceptor interactions between basic inhibitor side chains and acidic residues on the surface of the target enzyme. SDMI-1 and 2 are potent inhibitors of the pharmaceutically relevant protease matriptase at a near physiological pH and, thus, may find applications in therapy or diagnostics.

Between two worlds: a comparative study on in vitro and in silico inhibition of trypsin and matriptase by redox-stable SFTI-1 variants at near physiological pH.

A comparative study on in vitro and in silico inhibition of trypsin and matriptase by derivatives of the sunflower trypsin inhibitor-1 at near physiological pH is reported. Besides wild-type bicyclic SFTI-1, monocyclic variants possessing native cystine as well as redox-stable triazolyl side-chain macrocyclization motifs were studied for the first time in matriptase inhibition assays. Interestingly, monocyclic SFTI-1[1,14] demonstrated higher potency against this pharmacologically relevant protease compared to its bicyclic counterpart. Structural analysis of binding/inhibition of investigated SFTI-1 derivatives was performed using a combination of molecular dynamics simulations and docking experiments. In silico data were in good accordance with in vitro results, indicating the importance of the terminal inhibitor regions for the affinity towards matriptase. Presented work gives new perspectives for the optimization of the SFTI-1 framework towards in vivo applications.

Combinatorial optimization of cystine-knot peptides towards high-affinity inhibitors of human matriptase-1.

Cystine-knot miniproteins define a class of bioactive molecules with several thousand natural members. Their eponymous motif comprises a rigid structured core formed by six disulfide-connected cysteine residues, which accounts for its exceptional stability towards thermic or proteolytic degradation. Since they display a remarkable sequence tolerance within their disulfide-connected loops, these molecules are considered promising frameworks for peptide-based pharmaceuticals. Natural open-chain cystine-knot trypsin inhibitors of the MCoTI (Momordica cochinchinensis trypsin inhibitor) and SOTI (Spinacia oleracea trypsin inhibitor) families served as starting points for the generation of inhibitors of matriptase-1, a type II transmembrane serine protease with possible clinical relevance in cancer and arthritic therapy. Yeast surface-displayed libraries of miniproteins were used to select unique and potent matriptase-1 inhibitors. To this end, a knowledge-based library design was applied that makes use of detailed information on binding and folding behavior of cystine-knot peptides. Five inhibitor variants, four of the MCoTI family and one of the SOTI family, were identified, chemically synthesized and oxidatively folded towards the bioactive conformation. Enzyme assays revealed inhibition constants in the low nanomolar range for all candidates. One subnanomolar binder (Ki = 0.83 nM) with an inverted selectivity towards trypsin and matriptase-1 was identified.