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Cyc (heterocyclization) domain of nonribosomal peptide synthetases (NRPSs); belongs to the Condensation-domain family Cyc (heterocyclization) domains catalyze two separate reactions in the creation of heterocyclized peptide products in nonribosomal peptide synthesis: amide bond formation followed by intramolecular cyclodehydration between a Cys, Ser, or Thr side chain and a carbonyl carbon on the peptide backbone to form a thiazoline, oxazoline, or methyloxazoline ring. Cyc-domains are homologous to standard NRPS Condensation (C) domains. C-domains typically have a conserved HHxxxD motif at the active site; Cyc-domains have an alternative, conserved DxxxxD active site motif, mutation of the aspartate residues in this motif can abolish or diminish condensation activity. NRPS can use a large variety of acyl monomers (approximately 500 different possible monomer substrates as opposed to the 20 standard amino acids in ribosomal protein synthesis) to construct bioactive secondary metabolites of 2 to 18 units long (with various activities such as antibiotic, antifungal, antitumor and immunosuppression). There are various subtypes of C-domains such as the LCL-type which catalyzes peptide bond formation between two L-amino acids, the DCL-type which links an L-amino acid to the D-amino acid at the end of a growing peptide, starter C-domains which acylate the first amino acid with a beta-hydroxy carboxylic acid, and Cyc-domains. Typically, an NRPS module consists of an adenylation domain, a peptidyl carrier protein (PCP) domain (also known as thiolation (T) domain) and a C-domain. NRPS modules may also include specialized domains such as the terminal-module thioesterase (Te) domain that releases the product via hydrolysis or macrocyclization and any of various C-domain family members such as the epimerization (E) domain, the ester-bond forming C-domain, dual E/C (epimerization and condensation) domains, and the X-domain. |
Jiyao W et al.(2023). "The conserved domain database in 2023.",