If an enzyme name is shown in bold, there is experimental evidence for this enzymatic activity.
|Superclasses:||Biosynthesis → Secondary Metabolites Biosynthesis → Antibiotic Biosynthesis|
Some taxa known to possess this pathway include : Streptomyces coelicolor A3(2)
Expected Taxonomic Range:
The dimeric, benzoisochromanequinone antibiotic, actinorhodin, is an aromatic polyketide. Polyketides have been found to posses extensive biological activities and therefore, the actinorhodin pathway has undergone intensive genetic and biochemical studies for several years. The actinorhodin cluster is composed of 22 genes (SCO5071- SCO5092), which, in addition to the genes that encode catalytic enzymes, include genes that encode regulatory (SCO5082, SCO5085) and actinorhodin transport (SCO5083) proteins.
About This Pathway
Actinorhodin biosynthesis starts with 8 malonyl-CoAs and the minimal PKS, composed of a ketoacylsynthase, KSα, a chain-length factor, chain length factor, and an acyl carrier protein, actinorhodin polyketide synthase [acp]-like protein [McDaniel93]. The KSα and CLF form a heterodimer [FernandezMoreno92] and create a tunnel that stabilizes the growing polyketide backbone and controls chain length [Gualdi92][KeatingeClay04].
The first malonyl group is loaded onto the ACPPKS [Khosla92][Arthur06] by either the ACPPKS [Matharu98][Arthur05][Arthur06][Hitchman98] or a malonyltransferase from primary metabolism in a phenomenon termed 'cross talk' [Revill95][Khosla99] [Tang04][Carreras98]. Once loaded, it is decarboxylated to the starter acetyl group by the KSα [KeatingeClay04] and subsequently bound to KSα via a thioester bond. Once KSα is primed, a second malonyl group is transferred to ACPPKS and undergoes a KSα-catalyzed condensation with the acetyl starter group. 6 more malonyl groups are inidividually transferred to ACPPKS and condensed to the growing polyketide chain in the same manner [Sherman89a][Bibb89][Meurer95][Arthur06]. The resultant chain is attached to the ACPPKS by a thioester bond.
As the chain grows into the KSα-CLF tunnel, it buckles, which is believed to initiate the first ring cyclization. Further molecular interactions complete formation of the first ring [KeatingeClay04].
Following first ring cyclization and with the carbon chain still attached to the ACPPKS, the C-9 keto-group is reduced to a hydroxyl group by ActIII [Hallam88][Bartel90] [Yu94][Teartasin04][Hadfield04][Sherman89a][McDaniel93a][McDaniel93] [Fu94]. Subsequently, the first ring is aromatized by ActVII [McDaniel94] and then a second ring is formed by ActIV via an intramolecular aldol condensation [McDaniel94]. The compound is then released from ACPPKS as it was determined that a non-enzyme bound acid is the likely substrate for the next enzyme, ActVI-ORF1 [BookerMilburn05].
ActVI-ORF1 then reduces the keto-group at C-3 resulting in a chiral secondary alcohol [FernandezMoreno94] [Ichinose99][Taguchi00][Taguchi04b]. A pyran ring is then formed with the assistance of ActVI-ORF3, followed by a spontaneous dehydration [FernandezMoreno94][Ichinose99] [Taguchi00]. The resulting compound, (S)-DNPA, was found to be the best trigger for actinorhodin export via a 'feed-forward' mechanism. (S)-DNPA, and to a lesser extent, actinorhodin itself, eliminates the interaction between the repressor, ActII-ORF1, and the operon, actII-ORF2-actII-ORF3 (SCO5083-SCO5084), allowing transcription of the operon. actII-ORF3 may encode an enzyme involved in the synthesis of a second actinorhodin pigment, γ-actinorhodin, while actII-ORF2 encodes the protein that exports both actinorhodins before their intracellular levels become toxic [Bystrykh96][Tahlan07].
In the next biosynthesis step, the double bond between C-14 and C-15 is reduced by ActVI-ORF2, while ActVI-ORF4 assists in formation of the chemically favored isomer [FernandezMoreno94] [Taguchi00]. ActVA-ORF6 then oxidizes the substrate to form a quinone [Sciara03][Kendrew00][Kendrew97], which is most likely dimerized at this point. Though this step has yet to be determined experimentally, basic chemistry favors dimerization before hydroxylation since the C-11 hydroxyl of the quinone substrate could easily direct coupling to the C-10 ortho position [Kendrew95].
Finally, the actinorhodin precursor is oxidized by a two-component NADH:flavin-dependent monooxygenase system, composed of ActVII and ActVA-ORF5. ActVB is the NADH:flavin reductase which reduces FMN in the presence of cofactor, NADH. ActVA-ORF5 then binds the reduced FMN along with molecular oxygen, forming an electrophillic flavin hydroperoxide intermediate. This intermediate then oxidizes the actinorhodin precursor, subsequently releasing an oxidized FMN which diffuses from ActVA-ORF5 to ActVB for another cycle [Valton06][Filisetti05][Valton04][Filisetti03][Kendrew95][Cole87].
Citations: [Shen93, Aceti98, Rudd79, He00, Wietzorrek97, Fu94a, Parro91, Leavitt94, Kang97, Sherman92a, Taguchi07, Kim95a, Hillen82, Arias99, Bisang99, Crump96, FernandezMoreno91, Gramajo93, Ichinose98, Fu94b, Hopwood04, Caballero91, Omura86, McDaniel94a, Caballero91a, Tang03, Crump97, Malpartida, Malpartida86, Hopwood, Zhang90, Hesketh03, Kanehisa05, Kanehisa05a, Ichinose01, Hopwood07, Summers95]
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Arthur06: Arthur CJ, Szafranska AE, Long J, Mills J, Cox RJ, Findlow SC, Simpson TJ, Crump MP, Crosby J (2006). "The malonyl transferase activity of type II polyketide synthase acyl carrier proteins." Chem Biol 13(6);587-96. PMID: 16793516
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Caballero91a: Caballero JL, Malpartida F, Hopwood DA (1991). "Transcriptional organization and regulation of an antibiotic export complex in the producing Streptomyces culture." Mol Gen Genet 228(3);372-80. PMID: 1716725
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Crump96: Crump MP, Crosby J, Dempsey CE, Murray M, Hopwood DA, Simpson TJ (1996). "Conserved secondary structure in the actinorhodin polyketide synthase acyl carrier protein from Streptomyces coelicolor A3(2) and the fatty acid synthase acyl carrier protein from Escherichia coli." FEBS Lett 391(3);302-6. PMID: 8764994
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Khosla92: Khosla C, Ebert-Khosla S, Hopwood DA (1992). "Targeted gene replacements in a Streptomyces polyketide synthase gene cluster: role for the acyl carrier protein." Mol Microbiol 6(21);3237-49. PMID: 1453961
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McDaniel93a: McDaniel, R., Ebert-Kosla, S., Hopwood, D.A., Khosla, C. (1993). "Engineerred Biosynthesis of Novel Polyketides: Manipulation and Analysis of an Aromatic Polyketide Synthase with Unproven Catalytic Specificities." J. Am. Chem. Soc., 115, 11671-11675.
McDaniel94: McDaniel, R., Ebert-Khosla, S., Hopwood, D.A., Khosla, C. (1994). "Engineered Biosynthesis of Novel Polyketides: actVII and actIV Genes Encode Aromatase and Cyclase Enzymes, Respectively." J. Am. Chem. Soc. 116(24): 10855-10859.
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