Jixun Zhan

University of California, Los Angeles, Los Angeles, California, United States

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Publications (8)64.77 Total impact

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    ABSTRACT: The double hydroxylation of 6-pretetramid to 4-keto-anhydrotetracycline is a key tailoring reaction during the biosynthesis of the broad-spectrum antibiotic tetracyclines. It has been shown previously by heterologous reconstitution that OxyL is a dioxygenase and is the only enzyme required to catalyze the insertion of oxygen atoms at the C-12a and C-4 positions. We report here that OxyE, a flavin adenine dinucleotide (FAD)-dependent hydroxylase homologue, is an ancillary mono-oxygenase for OxyL during oxytetracycline biosynthesis in Streptomyces rimosus. By using both gene disruption and heterologous reconstitution approaches, we demonstrated that OxyE plays a nonessential, but important role in oxytetracycline biosynthesis by serving as a more efficient C-4 hydroxylase. In addition, we demonstrated that partially oxidized biosynthetic intermediates can undergo various glycosylation modifications in S. rimosus. Our results indicate that the synergistic actions of OxyE and OxyL in the double hydroxylation step prevent accumulation of shunt products during oxytetracycline biosynthesis in S. rimosus.
    ChemBioChem 06/2009; 10(9):1544-50. DOI:10.1002/cbic.200900122 · 3.06 Impact Factor
  • Jixun Zhan, Kangjian Qiao, Yi Tang
    ChemBioChem 06/2009; 10(9):1447-52. DOI:10.1002/cbic.200900082 · 3.06 Impact Factor
  • Jixun Zhan, Kenji Watanabe, Yi Tang
    ChemBioChem 07/2008; 9(11):1710-5. DOI:10.1002/cbic.200800178 · 3.06 Impact Factor
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    ABSTRACT: The cyclohexenone ring A of tetracyclines exhibits unique structural features not observed among other aromatic polyketides. These substitutions include the C2 primary amide, C4 dimethylamine, and the C12a tertiary alcohol. Here we report the identification and reconstitution of the minimum set of enzymes required for the biosynthesis of anhydrotetracycline (ATC, 5), the first intermediate in the tetracycline biosynthetic pathway that contains the fully functionalized ring A. Using a combination of in vivo and in vitro approaches, we confirmed OxyL, OxyQ, and OxyT to be the only enzymes required to convert 6-methylpretetramid 1 into 5. OxyL is a NADPH-dependent dioxygenase that introduces two oxygen atoms into 1 to yield the unstable intermediate 4-keto-ATC 2. The aminotransferase OxyQ catalyzes the reductive amination of C4-keto of 2, yielding 4-amino-ATC 3. Furthermore, the N, N-dimethyltransferase OxyT catalyzes the formation of 5 from 3 in a (S)-adenosylmethionine (SAM)-dependent manner. Finally, a "non-natural" anhydrotetracycline derivative was generated, demonstrating that our heterologous host/vector pair can be a useful platform toward the engineered biosynthesis of tetracycline analogues.
    Journal of the American Chemical Society 06/2008; 130(19):6068-9. DOI:10.1021/ja800951e · 11.44 Impact Factor
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    ABSTRACT: Resorcylic acid lactones represent a unique class of fungal polyketides and display a wide range of biological activities, such as nanomolar inhibitors of Hsp90 and MAP kinase. The biosynthesis of these compounds is proposed to involve two fungal polyketide synthases (PKS) that function collaboratively to yield a 14-membered macrolactone with a resorcylate core. We report here the reconstitution of Gibberella zeae PKS13, which is the nonreducing PKS associated with zearalenone biosynthesis. Using a small molecule mimic of the natural hexaketide starter unit, we reconstituted the entire repertoire of PKS13 activities in vitro, including starter-unit selection, iterative condensation, regioselective C2-C7 cyclization, and macrolactone formation. PKS13 synthesized both natural 14-membered and previously uncharacterized 16-membered resorcylic acid lactones, indicating relaxed control in both iterative elongation and macrocyclization. PKS13 exhibited broad starter-unit specificities toward fatty acyl-CoAs ranging in sizes between C6 and C16 and displayed the highest activity toward decanoyl-CoA. PKS13 was shown to be active in Escherichia coli and synthesized numerous alkyl pyrones and alkyl resorcylic esters without exogenously supplied precursors. We demonstrated that PKS13 can interact with E. coli fatty acid biosynthetic machinery and can be primed with fatty-acyl ACPp at low-micromolar concentrations. PKS13 synthesized new polyketides in E. coli when the culture was supplemented with synthetic precursors, showcasing its utility in precursor-directed biosynthesis. PKS13 is therefore a highly versatile polyketide macrolactone synthase that is useful in the engineered biosynthesis of polyketides, including resorcylic acid lactones that are not found in nature.
    Proceedings of the National Academy of Sciences 05/2008; 105(17):6249-54. DOI:10.1073/pnas.0800657105 · 9.81 Impact Factor
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    ABSTRACT: Regiospecific cyclizations of the nascent poly-beta-ketone backbones dictate the structures of polyketide natural products. The fungal iterative megasynthases use terminal thioesterase/claisen cyclase (TE/CLC) domains to direct the fate of the polyketide chains. In this work, we present two strategies toward redirecting the cyclization steps of fungal PKSs using the Gibberella fujikuroi PKS4. First, inactivation or removal of the TE/CLC domain resulted in the synthesis of the new polyketide SMA93 2. Complementation of the mutant PKS4 with a standalone TE/CLC domain restored the regioselective cyclization steps of PKS4 and led to the synthesis of SMA76 1, demonstrating that cyclization enzymes can interact with the megasynthase in trans. This led to the second approach in which various dissociated, bacterial tailoring enzymes were added to the megasynthase in trans. Addition of the act KR led to the synthesis of mutactin 3, while the addition of first ring and second ring cyclases yielded anthraquinone compounds DMAC 5 and SEK26 6. The cooperative activities of fungal and bacterial PKS components are especially important and enable synthesis of polyketides utilizing enzymes from two distinct families of PKSs.
    Journal of the American Chemical Society 02/2008; 130(1):38-9. DOI:10.1021/ja078091o · 11.44 Impact Factor
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    ABSTRACT: Iterative fungal polyketide synthases (PKSs) use a unique set of biochemical rules in the synthesis of complex polyketides. These rules dictate polyketide starter unit selection, chain length control, and post-PKS processing. We have demonstrated the E. coli expression and reconstitution of an iterative, unreduced fungal PKS. The Gibberella fujikuroi PKS4 was expressed at high levels, purified to homogeneity, and functionally characterized. In the presence of malonyl-CoA, PKS4 was able to synthesize the nonaketide 3,8,10,11-tetrahydroxy-l-methyl-12H -benzo[b]xanthen12-one (2) as the predominant product. PKS4 selectively used octanoyl-CoA as the starter unit and synthesized two novel benzopyrone-containing polyketides. Our work sets the stage for a comprehensive characterization of the intact PKS and its domairis and offers significant opportunity toward the enzymatic synthesis of additional compounds.
    Journal of the American Chemical Society 10/2007; 129(35):10642-3. DOI:10.1021/ja074865p · 11.44 Impact Factor
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    ABSTRACT: Aromatic polyketides are all biosynthesized from highly reactive poly-beta-ketone intermediates. Keys to introducing the vast chemical diversity seen in these natural products are the enzymatic and non-enzymatic tailoring chemistries that occur after biosynthesis of the poly-beta-keto backbone. In this work, we expand the scope of non-enzyme-catalyzed modifications and show that primary amides can act in vivo as an electrophilic center and be attacked by an intramolecular phenol to generate dibenzopyrones. Synthetic reaction sequences were developed to confirm that the displacement can take place under neutral, low temperature aqueous conditions. We demonstrate this tailoring reaction can be rationally introduced into an engineered biosynthetic pathway to produce new compounds. This mechanism is of particular note since it demonstrates the use of a "protecting group" in polyketide biosynthesis and may be the route by which pyrones are formed in other aromatic polyketide natural products.
    Journal of the American Chemical Society 09/2007; 129(30):9304-5. DOI:10.1021/ja0736919 · 11.44 Impact Factor