*5.2.3 Macrocyclization*

PatA protease from patellamide gene cluster catalyzes the N-terminal protease cleavage from the precursor peptide removing the leader sequence. This reaction catalyzed by the N-terminal protease A and C-terminal protease G, under subtilisin protein family encoded by cyanobactin gene cluster. A kind of protease called PatG isolated from Prochlorom was found to macrocyclize a wide range of synthetic substrates with non-proteinogenic and D-amino acids. Macrocyclase consists of PatG and PagG structural domains representing a catalytic triad. Macrocyclase crystal structure represents a domain of PatG, showing subtilisin folds containing two helices presented by the macrocyclization insert without any change in sequence length. This domain is insensitive to the identity of the residues within the core peptides, as PatG acts on RSIII residues and catalyzes the C-N macrocyclization. During this process, PatA protease removes the amino terminal linked to the core, producing a free amino terminal and PatG protease removes a catalytic terminal flanking the core. Cleaving site is protected by the PatG protease preventing access to water and continues hydrolysis until the transduction reactiobn is completed. PatG emphasizes macrocyclic peptide formation, by removing the C-terminal protease.
