A number of simple oligopeptides have been recently developed as minimalistic catalysts for mimicking the activity and selectivity of natural proteases. Although the arrangement of amino acid residues in natural enzymes provides a strategy for designing artificial enzymes, creating catalysts with efficient binding and catalytic activity is still challenging. In this study, we used the polyproline scaffold and designed a series of 13-residue peptides with a catalytic dyad or triad incorporated to serve as artificial enzymes. Their catalytic efficiency on ester hydrolysis was evaluated by ultraviolet–visible spectroscopy using the p -nitrophenyl acetate assay, and their secondary structures were also characterized by circular dichroism spectroscopy. The results indicate that a well-formed polyproline II structure may result in a much higher catalytic efficiency. This is the first report to show that a functional dyad or triad engineered into a polyproline helix framework can enhance the catalytic activity on ester hydrolysis. Our study has also revealed the necessity of maintaining an ordered structure and a well-organized catalytic site for effective biocatalysts.

Cis−trans isomerization of proline is involved in various biological processes, such as protein folding, cell signaling, and ion-channel gating. Polyproline is a useful system for better understanding proline isomerization because it exists predominantly as two forms, all-cis polyproline I (PPI) and all-trans polyproline II (PPII) helices. The stability of PPI and PPII can be modulated by various effects, including aromatic−proline interactions, terminal charges, and stereoelectronic effects. Here, we used a series of oligoproline peptides in which positively charged or negatively charged amino acids were incorporated into the termini to investigate their effects on polyproline conformation. Circular dichroism measurements show that a cationic residue at the C-terminus or an anionic residue at the N-terminus increases the stability of a PPII helix; in particular, the C-terminal cationic residues impose an enormous impact on PPII stability. The electrostatic attractions between a cationic sidechain and the C-terminal carboxylate exhibit a greater effect than those between an anionic sidechain and the Nterminal ammonium on the conversion of PPII to PPI, suggesting that the stabilization effect of electrostatic interactions on PPII is directional. In contrast, incorporating a cationic residue seems more favorable than adding an anionic residue into the N-terminus because the cationic residue can stabilize PPI. Moreover, the predicted dipole moments from optimized oligopeptide models reveal that the macrodipole of the peptides with a cationic residue at the Cterminus exhibits the opposite direction to that of other peptides in the PPI conformation, suggesting that such a dipole distortion may cause these peptides to disfavor PPI helices. Together, we have found that the introduction of terminal electrostatic interactions can have a significant effect on PPII stability, providing useful information to the design of polyprolinebased scaffolds for biomedical applications.

Human Pin1 is involved in cancer developments and has been a pharmaceutical target. Thus, finding a high-affinity inhibitor of Pin1 has become an attractive topic. The WW domain of human Pin1 can recognize the phosphoserine/phosphothreonine-proline (pS/pT-P) motifs, while its PPIase domain catalyzes the cis/trans isomerization of prolyl bonds to regulate the cell cycle. Here we incorporated a series of 4-substituted proline derivatives into the phosphopeptides and investigated their affinities for the WW domain of Pin1 to develop better inhibitors of Pin1. On the basis of the ligand Myt1-T412 [PPA(pT)PP], we synthesized several phosphopeptides in which the proline residue in the pT-P motif was replaced with various 4-substituted proline derivatives. Isothermal titration calorimetry and fluorescence anisotropy analyses show that the replacement of proline with (2S,4R)-4-fluoroproline increases the binding affinity of the peptide. Circular dichroism measurements suggest that a more PPII-like structure of phosphopeptides makes them bind to the WW domain more tightly. Chemical shift perturbation experiments also indicate that (2S,4R)-4-fluoroproline interacts with Trp34 of the WW domain in the binding site. Results of molecular modeling further suggested that a strong C-H···π interaction induced by (2S,4R)-4-fluoroproline is important in enhancing the affinity of the peptide for the WW domain. The results of this study provide new valuable information for designing and developing effective inhibitors of human Pin1.

A polyproline fragment containing a catalytic dyad of His–His or Ser–His was coupled with a self-assembling peptide MAX1 to design new hydrolases (H2H5 and H2S5) for catalyzing ester hydrolysis. Circular dichroism measurements indicated that the peptides change their conformation from random coils to β-sheets when pH increases from 5 to 10. IR spectra also displayed the vibration modes corresponding to their β-structures at pH 9.0. Transmission electron microscopy (TEM) and atomic force microscopy (AFM) measurements showed that in solution, the designed peptides self-assemble into network fibrils having a significantly increased catalytic efficiency on ester hydrolysis. On p-nitrophenyl acetate (p-NPA) substrate, the designed peptides exhibit high catalytic efficiency at pH 9.0 (k cat/K M = 12.1 M–1 s–1 for H2H5, 13.3 M–1 s–1 for H2S5), and their efficiency is even better at pH 10.0 (k cat/K M = 24.3 M–1 s–1 for H2H5, 99.4 M–1 s–1 for H2S5). Additionally, H2H5 and H2S5 also display good activity on catalyzing the hydrolysis of p-nitrophenyl-(2-phenyl)-propanoate (p-NPP) and p-nitrophenyl methoxyacetate (p-NPMA). Combining the polyproline-based catalytic scaffold with a self-assembling peptide generates an efficient hydrolase, providing a new design for effective artificial enzymes.

The low-wavenumber Raman spectra in combination with theoretical calculations via solid-state density functional theory (DFT)-D3 are displayed. The vibrational structures and interaction with solvent of poly-l-proline and the oligoproline P12 series are identified.