In Vitro and In Planta Cyclization of Target Peptides Using an Asparaginyl Endopeptidase from Oldenlandia affinis

Reversed Proteolysis—Proteases as Peptide Ligases

Historically, ligase activity by proteases was theoretically derived due to their catalyst nature, and it was experimentally observed as early as around 1900. Initially, the digestive proteases, such as pepsin, chymotrypsin, and trypsin were employed to perform in vitro syntheses of small peptides. Protease-catalyzed ligation is more efficient than peptide bond hydrolysis in organic solvents, representing control of the thermodynamic equilibrium. Peptide esters readily form acyl intermediates with serine and cysteine proteases, followed by peptide bond synthesis at the N-terminus of another residue. This type of reaction is under kinetic control, favoring aminolysis over hydrolysis. Although only a few natural peptide ligases are known, such as ubiquitin ligases, sortases, and legumains, the principle of proteases as general catalysts could be adapted to engineer some proteases accordingly. In particular, the serine proteases subtilisin and trypsin were converted to efficient ligases, which are known as subtiligase and trypsiligase. Together with sortases and legumains, they turned out to be very useful in linking peptides and proteins with a great variety of molecules, including biomarkers, sugars or building blocks with non-natural amino acids. Thus, these engineered enzymes are a promising branch for academic research and for pharmaceutical progress.

Cyclotides: From Structure to Function

This Review explores the class of plant-derived macrocyclic peptides called cyclotides. We include an account of their discovery, characterization, and distribution in the plant kingdom as well as a detailed analysis of their sequences and structures, biosynthesis and chemical synthesis, biological functions, and applications. These macrocyclic peptides are around 30 amino acids in size and are characterized by their head-to-tail cyclic backbone and cystine knot motif, which render them to be exceptionally stable, with resistance to thermal or enzymatic degradation. Routes to their chemical synthesis have been developed over the past two decades, and this capability has facilitated a wide range of mutagenesis and structure-activity relationship studies. In turn, these studies have both led to an increased understanding of their mechanisms of action as well as facilitated a range of applications in agriculture and medicine, as ecofriendly crop protection agents, and as drug leads or scaffolds for pharmaceutical design. Our overall objective in this Review is to provide readers with a comprehensive overview of cyclotides that we hope will stimulate further work on this fascinating family of peptides.