This review provides a comprehensive update of the advances in discovery, biosynthesis, and engineering of ribosomally-synthesized and post-translationally modified peptides (RiPPs).

Regioselective modification of amino acids within the context of a peptide is common to a number of biosynthetic pathways and many such products have potential as therapeutics. The ATP dependent enzyme LynD heterocyclizes multiple cysteine residues to thiazolines within a peptide substrate. The enzyme requires the substrate to have conserved N-terminal leader for full activity. Catalysis is almost insensitive to immediately flanking residues in the substrate suggesting recognition occurs distant from the active site. Nucleotide and peptide substrate co-complex structures of LynD reveal the substrate leader peptide binds to and extends the β-sheet of a conserved domain of LynD, whilst catalysis is accomplished in another conserved domain. The spatial segregation of catalysis from recognition combines seemingly contradictory properties of regioselectivity and promiscuity; it appears to be a conserved strategy in other peptide modifying enzymes. A variant of LynD that efficiently processes substrates without a leader peptide has been engineered.

Peptide macrocycles are found in many biologically active natural products. Their versatility, resistance to proteolysis and ability to traverse membranes has made them desirable molecules. Although technologies exist to synthesize such compounds, the full extent of diversity found among natural macrocycles has yet to be achieved synthetically. Cyanobactins are ribosomal peptide macrocycles encompassing an extraordinarily diverse range of ring sizes, amino acids and chemical modifications. We report the structure, biochemical characterization and initial engineering of the PatG macrocyclase domain of Prochloron sp. from the patellamide pathway that catalyzes the macrocyclization of linear peptides. The enzyme contains insertions in the subtilisin fold to allow it to recognize a three-residue signature, bind substrate in a preorganized and unusual conformation, shield an acyl-enzyme intermediate from water and catalyze peptide bond formation. The ability to macrocyclize a broad range of nonactivated substrates has wide biotechnology applications.

Counting backwards: The cyanobactin class of heterocyclases, exemplified by TruD, possess an almost unique combination of processivity, specificity, chemical versatility, and promiscuity. TruD is shown by biochemical assay to be an adenylase, and processes cysteines in a defined order. The entire substrate leader can be removed and TruD will process a single specific cysteine residue; however the role of leader is to permit processivity through a balance of recognition. ATP/AMP=adenosine tri/monophosphate; PPi=pyrophosphate.