polyketide synthases (PKSs) polymerize simple fatty acids into a large variety of different products, called polyketides, by successive decarboxylating Claisen condensations. PKSs can be divided into 2 groups, modular type I PKSs consisting of one or more large multifunctional proteins and iterative type II PKSs, complexes of several monofunctional subunits.

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.

The adenylation domain of nonribosomal peptide synthetases (NRPS), including Streptoalloteichus tallysomycin biosynthesis genes. The adenylation (A) domain of NRPS recognizes a specific amino acid or hydroxy acid and activates it as an (amino) acyl adenylate by hydrolysis of ATP. The activated acyl moiety then forms a thioester to the enzyme-bound cofactor phosphopantetheine of a peptidyl carrier protein domain. NRPSs are large multifunctional enzymes which synthesize many therapeutically useful peptides in bacteria and fungi via a template-directed, nucleic acid independent nonribosomal mechanism. These natural products include antibiotics, immunosuppressants, plant and animal toxins, and enzyme inhibitors. NRPS has a distinct modular structure in which each module is responsible for the recognition, activation, and in some cases, modification of a single amino acid residue of the final peptide product. The modules can be subdivided into domains that catalyze specific biochemical reactions. This family includes the TLM biosynthetic gene cluster from Streptoalloteichus that consists of nine NRPS genes; the N-terminal module of TlmVI (NRPS-5) and the starter module of BlmVI (NRPS-5) are comprised of the acyl CoA ligase (AL) and acyl carrier protein (ACP)-like domains, which are thought to be involved in the biosynthesis of the beta-aminoalaninamide moiety.

Fatty acyl-AMP ligase (FAAL). FAAL belongs to the class I adenylate forming enzyme family and is homologous to fatty acyl-coenzyme A (CoA) ligases (FACLs). However, FAALs produce only the acyl adenylate and are unable to perform the thioester-forming reaction, while FACLs perform a two-step catalytic reaction; AMP ligation followed by CoA ligation using ATP and CoA as cofactors. FAALs have insertion motifs between the N-terminal and C-terminal subdomains that distinguish them from the FACLs. This insertion motif precludes the binding of CoA, thus preventing CoA ligation. It has been suggested that the acyl adenylates serve as substrates for multifunctional polyketide synthases to permit synthesis of complex lipids such as phthiocerol dimycocerosate, sulfolipids, mycolic acids, and mycobactin.

Acyl transferase domain in polyketide synthase (PKS) enzymes [Secondary metabolites biosynthesis, transport and catabolism].

ChainA, Dihydroaeruginoic acid synthetase [Pseudomonas aeruginosa PAO1],7EMY_B Chain B, Dihydroaeruginoic acid synthetase [Pseudomonas aeruginosa PAO1],7EN1_A Chain A, Dihydroaeruginoic acid synthetase [Pseudomonas aeruginosa PAO1],7EN1_B Chain B, Dihydroaeruginoic acid synthetase [Pseudomonas aeruginosa PAO1],7EN2_A Chain A, Dihydroaeruginoic acid synthetase [Pseudomonas aeruginosa PAO1],7EN2_B Chain B, Dihydroaeruginoic acid synthetase [Pseudomonas aeruginosa PAO1]

ChainA, BmdB, Bacillamide NRPS [Thermoactinomyces vulgaris]

ChainE, BmdB, bacillamide NRPS [Thermoactinomyces vulgaris]

ChainA, Nonribosomal peptide synthetase [Streptomyces sp. Sp080513GE-23],6LTB_A Chain A, Nonribosomal peptide synthetase [Streptomyces sp. Sp080513GE-23],6LTC_A Chain A, Nonribosomal peptide synthetase [Streptomyces sp. Sp080513GE-23],6LTD_A Chain A, Nonribosomal peptide synthetase [Streptomyces sp. Sp080513GE-23],6LTD_B Chain B, Nonribosomal peptide synthetase [Streptomyces sp. Sp080513GE-23]

ChainA, CurL [Moorena producens 3L],4MZ0_B Chain B, CurL [Moorena producens 3L]

High-molecular-weight protein 2 OS=Yersinia enterocolitica serotype O:8 / biotype 1B (strain NCTC 13174 / 8081) OX=393305 GN=irp2 PE=3 SV=1

Pyochelin synthase PchE OS=Pseudomonas aeruginosa (strain UCBPP-PA14) OX=208963 GN=pchE PE=1 SV=1

Pyochelin synthase PchE OS=Pseudomonas aeruginosa (strain ATCC 15692 / DSM 22644 / CIP 104116 / JCM 14847 / LMG 12228 / 1C / PRS 101 / PAO1) OX=208964 GN=pchE PE=1 SV=1

Pyochelin synthase PchF OS=Pseudomonas aeruginosa (strain ATCC 15692 / DSM 22644 / CIP 104116 / JCM 14847 / LMG 12228 / 1C / PRS 101 / PAO1) OX=208964 GN=pchF PE=2 SV=1

Pyochelin synthase PchF OS=Pseudomonas aeruginosa (strain UCBPP-PA14) OX=208963 GN=pchF PE=1 SV=1