Human Tissue Kallikrein S1 Subsite Recognition of Non-Natural Basic Amino Acids

We explored the unique substrate specificity of the primary S1 subsite of human urinary kallikrein (hK1), which accepts both Phe and Arg, using internally quenched fluorescent peptides Abz-F-X-S-R-Q-EDDnp and Abz-G-F-S-P-F-X-S-S-R-P-Q-EDDnp [Abz is o-aminobenzoic acid; EDDnp is N-(2,4-dinitrophenyl)ethylenediamine], which were based on the human kininogen sequence at the C-terminal region of bradykinin. Position X, which in natural sequence stands for Arg, received the following synthetic basic non-natural amino acids: 4-(aminomethyl)phenylalanine (Amf), 4-guanidine phenylalanine (Gnf), 4-(aminomethyl)-N-isopropylphenylalanine (Iaf), Nim-(dimethyl)histidine [H(2Me)], 3-pyridylalanine (Pya), 4-piperidinylalanine (Ppa), 4-(aminomethyl)cyclohexylalanine (Ama), and 4-(aminocyclohexyl)alanine (Aca). Only Abz-F-Amf-S-R-Q-EDDnp and Abz-F-H(2Me)]-S-R-Q-EDDnp were efficiently hydrolyzed, and all others were resistant to hydrolysis. However, Abz-F-Ama-S-R-Q-EDDnp inhibited hK1 with a Ki of 50 nM with high specificity compared to human plasma kallikrein, thrombin, plasmin, and trypsin. The Abz-G-F-S-P-F-X-S-S-R-P-Q-EDDnp series were more susceptible to hK1, although the peptides with Gnf, Pya, and Ama were resistant to it. Unexpectedly, the peptides in which X is His, Lys, H(2Me), Amf, Iaf, Ppa, and Aca were cleaved at amino or at carboxyl sites of these amino acids, indicating that the S1 subsite has significant preference for basic residues. Human plasma kallikrein did not hydrolyze any peptide of this series except the natural sequence where X is Arg. In conclusion, the S1 subsite of hK1 accepts amino acids with combined basic and aromatic side chain, although for the S1-P1 interaction the preference is for aliphatic and basic side chains.