Cyanobacteria are abundant producers of natural products well recognized for their bioactivity and utility in drug discovery and biotechnology applications. In the last decade, characterization of several modular gene clusters that code for the biosynthesis of these compounds has revealed a number of unusual enzymatic reactions. In this article, we review several mechanistic transformations identified in marine cyanobacterial biosynthetic pathways, with an emphasis on modular polyketide synthase(PKS)/non-ribosomal peptide synthetase (NRPS) gene clusters. In selected instances, we also make comparisons between cyanobacterial gene clusters derived from marine and freshwater strains. We then provide an overview of recent developments in cyanobacterial natural products biosynthesis made available through genome sequencing and new advances in bioinformatics and genetics.

A biocatalytic platform that employs the final two monomodular type I polyketide synthases (PKS) of the pikromycin pathway in vitro followed by direct appendage of D-desosamine and final C-H oxidation(s) in vivo was developed and applied toward the synthesis of a suite of 12-and 14-membered ring macrolide natural products. This methodology delivered both compound classes in thirteen steps (longest linear sequence) from commercially available (R)-Roche ester in >10% overall yields.

Synergistic gathering: Biochemical studies reveal a highly efficient strategy in biosynthesis to promote consecutive multienzyme reactions. In the curacin A pathway, catalytic efficiency of a multi‐step modification process and one‐step translocation is significantly enhanced by tandem organization and dimerization of the ACP3 tridomain with an unexpected synergistic effect (see scheme).

Polyketide synthases (PKSs) and non-ribosomal peptide synthetases (NRPSs) are large multidomain proteins present in microorganisms that produce bioactive compounds. Curacin A is such a bioactive compound with potent anti-proliferative activity. During its biosynthesis the growing substrate is bound covalently to an acyl carrier protein (ACP) that is able to access catalytic sites of neighboring domains for chain elongation and modification. While ACP domains usually occur as monomers, the curacin A cluster codes for a triplet ACP (ACPI-ACPII-ACPIII) within the CurA PKS module. We have determined the structure of the isolated holo-ACPI and show that the ACPs are independent of each other within this tridomain system. In addition, we have determined the structure of the 3-hydroxyl-3-methylglutaryl-loaded holo-ACPI, which is the substrate for the unique halogenase (Hal) domain embedded within the CurA module. We have identified the interaction surface of both proteins using mutagenesis and MALDI-based identification of product formation. Amino acids affecting product formation are located on helices II and III of ACPI and form a contiguous surface. Since the CurA Hal accepts substrate only when presented by one of the ACPs within the ACPI-ACPII-ACPIII tridomain, our data provide insight into the specificity of the chlorination reaction.

Sulfated molecules with diverse functions are common in biology, but sulfonation as a method to activate a metabolite for chemical catalysis is rare. Catalytic activity was characterized and crystal structures were determined for two such “activating” sulfotransferases (STs) that sulfonate β-hydroxyacyl thioester substrates. The CurM polyketide synthase (PKS) ST domain from the curacin A biosynthetic pathway of Moorea producens and the olefin synthase (OLS) ST from a hydrocarbon-producing system of Synechococcus PCC 7002 both occur as a unique acyl carrier protein (ACP), ST and thioesterase (TE) tridomain within a larger polypeptide. During pathway termination, these cyanobacterial systems introduce a terminal double bond into the β-hydroxyacyl-ACP-linked substrate by the combined action of the ST and TE. Under in vitro conditions, CurM PKS ST and OLS ST acted on β-hydroxy fatty acyl-ACP substrates; however, OLS ST was not reactive toward analogs of the natural PKS ST substrate bearing a C5-methoxy substituent. The crystal structures of CurM ST and OLS ST revealed that they are members of a distinct protein family relative to other prokaryotic and eukaryotic sulfotransferases. A common binding site for the sulfonate donor 3'-phosphoadenosine-5'-phosphosulfate was visualized in complexes with the product 3'-phosphoadenosine-5'-phosphate. Critical functions for several conserved amino acids in the active site were confirmed by site-directed mutagenesis, including a proposed glutamate catalytic base. A dynamic active-site flap unique to the “activating” ST family affects substrate selectivity and product formation, based on the activities of chimeras of the PKS and OLS STs with exchanged active-site flaps.