A New Cyclase Family Catalyzing Head-to-Tail Macrolactamization of Non-ribosomal Peptides

Penicillin-binding protein-type thioesterases: An emerging family of non-ribosomal peptide cyclases with biocatalytic potentials

Macrocyclization of peptides reduces conformational flexibilities, potentially leading to improved drug-like properties, such as target specificities and metabolic stabilities. As chemical methodologies often allow side reactions like epimerization and oligomerization, keen attention has been directed toward enzymatic peptide cyclization using peptide cyclases from specialized metabolic pathways. Penicillin-binding protein-type thioesterases (PBP-type TEs) are a recently identified family of peptide cyclases involved in the biosynthesis of non-ribosomal peptides in actinobacteria. PBP-type TEs have undergone intensive investigation due to their outstanding potential as biocatalysts. This review summarizes the rapidly growing knowledge on PBP-type TEs, with special emphasis on their functions, scopes, and structures, and efforts towards their biocatalytic applications.

Chemoenzymatic tandem cyclization for the facile synthesis of bicyclic peptides

Bicyclic peptides exhibit improved metabolic stabilities and target specificities when compared to their linear or mono-cyclic counterparts; however, efficient and straightforward synthesis remains challenging due to their intricate architectures. Here, we present a highly selective and operationally simple one-pot chemoenzymatic tandem cyclization approach to synthesize bicyclic peptides with small to medium ring sizes. Penicillin-binding protein-type thioesterases (PBP-type TEs) efficiently cyclized azide/alkyne-containing peptides in a head-to-tail manner. Successive copper (I)-catalyzed azide-alkyne cycloaddition generated bicyclic peptides in one-pot, thus omitting the purification of monocyclic intermediates. This chemoenzymatic strategy enabled the facile synthesis of bicyclic peptides bearing hexa-, octa-, and undecapeptidyl head-to-tail cyclic scaffolds.

Streamlined Chemoenzymatic Synthesis of Cyclic Peptides by Non-ribosomal Peptide Cyclases

Macrocyclization improves the pharmaceutical properties of peptides; however, regio- and chemoselective intramolecular cyclizations remain challenging. Here we developed a streamlined chemoenzymatic approach to synthesize cyclic peptides by exploiting non-ribosomal peptide (NRP) cyclases. Linear peptides linked to the resin through a C-terminal diol ester functionality are synthesized on a solid support, to circumvent the installation of leaving groups to the peptidic substrates in the liquid phase which often triggers undesirable epimerization. Cleavage of the resin-bound peptides yielded the diol esters with sufficient purity to be readily cyclized in a head-to-tail manner by SurE, a representative penicillin-binding protein-type thioesterase (PBP-type TE). Explorations of homologous wild-type enzymes as well as rational protein engineering have broadened the scope of the enzymatic macrolactamization. This method will potentially accelerate the exploitation of NRP cyclases as biocatalysts.

Chemo-Enzymatic Synthesis of Non-ribosomal Macrolactams by a Penicillin-Binding Protein-Type Thioesterase

Penicillin-binding protein-type thioesterases (PBP-type TEs) are an emerging family of non-ribosomal peptide cyclases. PBP-type TEs exhibit distinct substrate scopes from the well-exploited ribosomal peptide cyclases and traditional non-ribosomal peptide cyclases. Their unique properties, as well as their stand-alone nature, highlight PBP-type TEs as valuable candidates for development as biocatalysts for peptide macrocyclization. Here in this chapter, we describe the scheme for the chemoenzymatic synthesis of non-ribosomal macrolactam by SurE, a representative member of PBP-type TEs.

PenA, a penicillin-binding protein-type thioesterase specialized for small peptide cyclization

Penicillin-binding protein-type thioesterases (PBP-type TEs) are a recently identified group of peptide cyclases that catalyze head-to-tail macrolactamization of nonribosomal peptides. PenA, a new member of this group, is involved in the biosyntheses of cyclic pentapeptides. In this study, we demonstrated the enzymatic activity of PenA in vitro, and analyzed its substrate scope with a series of synthetic substrates. A comparison of the reaction profiles between PenA and SurE, a representative PBP-type TE, showed that PenA is more specialized for small peptide cyclization. A computational model provided a possible structural rationale for the altered specificity for substrate chain lengths.

One Pathway, Two Cyclic Non-Ribosomal Pentapeptides: Heterologous Expression of BE-18257 Antibiotics and Pentaminomycins from Streptomyces cacaoi CA-170360

The strain Streptomyces cacaoi CA-170360 produces the cyclic pentapeptides pentaminomycins A-H and BE-18257 A-C, two families of cyclopeptides synthesized by two non-ribosomal peptide synthetases encoded in tandem within the same biosynthetic gene cluster. In this work, we have cloned and confirmed the heterologous expression of this biosynthetic gene cluster, demonstrating that each of the non-ribosomal peptide synthetases present in the cluster is involved in the biosynthesis of each group of cyclopeptides. In addition, we discuss the involvement of a stand-alone enzyme belonging to the Penicillin Binding Protein family in the release and macrocyclization of the peptides.

Peptide Cyclization at High Concentration

The emergence of cyclic peptides as pharmaceuticals has led to an eruption of new methodologies for macrocyclization. However, the cyclization of peptides at high concentrations presents a challenge due to the production of side products like dimers and oligomers. This factor is more pronounced with the cyclization of peptides composed of fewer than seven amino acids, thus has created a need for a new synthetic strategy. Herein, we will elucidate a new chemoselective method termed ‘CyClick’ that works in an exclusively intramolecular fashion preventing the formation of commonly occurring side products such as dimers and oligomers, even at relatively high concentration.

1 Introduction

2 Known Methodologies

3 Novel CyClick Chemistry

4 Conclusion and Outlook