Meurer G, Gerlitz M, Wendt-Pienkowski E, Vining L C, Rohr J, Hutchinson C R
School of Pharmacy, University of Wisconsin, 425 N. Charter St, Madison, WI 53706, USA.
Chem Biol. 1997 Jun;4(6):433-43. doi: 10.1016/s1074-5521(97)90195-2.
Iterative type II polyketide synthases (PKSs) produce polyketide chains of variable but defined length from a specific starter unit and a number of extender units. They also specify the initial regiospecific folding and cyclization pattern of nascent polyketides either through the action of a cyclase (CYC) subunit or through the combined action of site-specific ketoreductase (KR) and CYC subunits. Additional CYCs and other modifications may be necessary to produce linear aromatic polyketides. The principles of the assembly of the linear aromatic polyketides, several of which are medically important, are well understood, but it is not clear whether the assembly of the angular aromatic (angucyclic) polyketides follows the same rules.
We performed an in vivo evaluation of the subunits of the PKS responsible for the production of the angucyclic polyketide jadomycin (jad), in comparison with their counterparts from the daunorubicin (dps) and tetracenomycin (tcm) PKSs which produce linear aromatic polyketides. No matter which minimal PKS was used to produce the initial polyketide chain, the JadD and DpsF CYCs produced the same two polyketides, in the same ratio; neither product was angularly fused. The set of jadABCED PKS plus putative jadl CYC genes behaved similarly. Furthermore, no angular polyketides were isolated when the entire set of jad PKS enzymes and Jadl or the jad minimal PKS, Jadl and the TcmN CYC were present. The DpsE KR was able to reduce decaketides but not octaketides; in contrast, the KRs from the jad PKS (JadE) or the actinorhodin PKS (ActIII) could reduce octaketide chains, giving three distinct products.
It appears that the biosynthesis of angucyclic polyketides cannot be simply accomplished by expressing the known PKS subunits from artificial gene cassettes under the control of a non-native promoter. The characteristic structure of the angucycline ring system may arise from a kinked precursor during later cyclization reactions involving additional, but so far unknown, components of the extended decaketide PKS. Our results also suggest that some KRs have a minimal chain length requirement and that CYC enzymes may act aberrantly as first-ring aromatases that are unable to perform all of the sequential cyclization steps. Both of these characteristics may limit the widespread application of CYC or KR enzymes in the synthesis of novel polyketides.
迭代型II聚酮合酶（PKSs）从特定的起始单元和多个延伸单元产生长度可变但确定的聚酮链。它们还通过环化酶（CYC）亚基的作用或通过位点特异性酮还原酶（KR）和CYC亚基的联合作用来指定新生聚酮的初始区域特异性折叠和环化模式。可能需要额外的CYC和其他修饰来产生线性芳香聚酮。线性芳香聚酮的组装原理已得到充分理解，其中一些在医学上很重要，但尚不清楚角状芳香（环角）聚酮的组装是否遵循相同的规则。
我们对负责产生环角聚酮柔红霉素（jad）的PKS亚基进行了体内评估，并与产生线性芳香聚酮的柔红霉素（dps）和四环素霉素（tcm）PKS的对应亚基进行了比较。无论使用哪种最小PKS来产生初始聚酮链，JadD和DpsF CYC都会以相同的比例产生相同的两种聚酮；两种产物都没有角状稠合。jadABCED PKS加上推定的jadl CYC基因的组合表现相似。此外，当存在整套jad PKS酶和Jadl或jad最小PKS、Jadl和TcmN CYC时，未分离到角状聚酮。DpsE KR能够还原十酮，但不能还原八酮；相比之下，来自jad PKS（JadE）或放线紫红素PKS（ActIII）的KR能够还原八酮链，产生三种不同的产物。
看来环角聚酮的生物合成不能简单地通过在非天然启动子的控制下从人工基因盒中表达已知的PKS亚基来完成。环角素环系统的特征结构可能源于在涉及延伸的十酮PKS的其他但迄今未知的成分的后期环化反应中形成的扭结前体。我们的结果还表明，一些KR有最小链长要求，并且CYC酶可能异常地作为无法执行所有连续环化步骤的第一环芳香化酶起作用。这两个特征都可能限制CYC或KR酶在新型聚酮合成中的广泛应用。
