PNAS：唐功利等抗肿瘤天然产物生物合成研究获进展

对结构独特、活性显著的天然产物进行生物合成研究是从基因簇、生物合成途径及酶催化反应角度理解自然界“全合成”的生物-化学过程。中国科学院上海有机化学研究所生命有机化学国家重点实验室唐功利课题组多年来致力于复杂抗肿瘤天然产物的生物合成研究，经过几年的努力，该课题组最近在两个课题上均取得突破。

抗生素谷田霉素（Yatakemycin，YTM）可以抑制致病真菌，且对肿瘤细胞表现出极强的毒性（比抗肿瘤药物丝裂霉素的活性高约1000倍）；该家族化合物属于DNA烷基化试剂，典型的结构特征是吡咯吲哚环上的环丙烷结构。为了阐明其独特的生物合成机制，课题组利用全基因组扫描技术定位其生物合成基因簇，通过基因敲除结合生物信息学分析确定了基因簇边界。在对突变株的发酵检测中成功分离鉴定了中间体YTM-T的结构，并结合体外生化实验揭示了一类同源于粪卟啉原III-氧化酶（Coproporphyrinogen III oxidase）的甲基化酶以自由基机理催化YTM-T发生C-甲基化（J. Am. Chem. Soc.， 2012， 134， 8831-8840），这是此类蛋白催化自由基甲基化反应的首例报道。这一阶段性结果为下一步阐明YTM结构中最重要的环丙烷部分生物合成途径奠定了基础。

萘啶霉素(Naphthyridinomycin，NDM)、奎诺卡星(Quinocarcin，QNC)及Ecteinascidin 743 (ET-743)均属于四氢异喹啉生物碱家族化合物，它们都具有显著的抗肿瘤活性，其中ET-743已发展为第一例海洋天然产物来源的抗肿瘤新药。这三种化合物都具有一个独特的二碳单元结构，其生物合成来源问题一直没有得到解决。为了揭示这一谜团，唐功利课题组在克隆了NDM和QNC生物合成基因簇的基础上，通过前体喂养标记、体内相关基因敲除-回补以及体外酶催化反应等多种实验手段相结合的方式，阐明了二碳单元的独特生源合成机制：NapB/D及QncN/L在催化功能上均属于丙酮酸脱氢酶及转酮醇酶的复合体，它们负责催化二碳单元由酮糖转移至酰基承载蛋白（ACP）上，而后经过非核糖体蛋白合成（NRPS）途经进入到最终的化合物中。这一结果发表在《美国国家科学院院刊》(Proc. Natl. Acad. Sci. USA， 2012， 109， 8540-8545)上。这种将基础代谢中的酮糖直接转化为次级代谢所需要的二碳单元在非核糖体肽合成途经中是首次报道，该研究结果也有助于揭示海洋药物ET-743独特的二碳单元生物合成来源，为非核糖体聚肽类天然产物的组合生物合成带来新的前体单元。

上述研究工作得到国家自然科学基金委、科技部和中国科学院的资助。

doi:10.1073/pnas.1204232109
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Hijacking a hydroxyethyl unit from a central metabolic ketose into a nonribosomal peptide assembly line

Chao Peng1, Jin-Yue Pu1, Li-Qiang Song1, Xiao-Hong Jian1, Man-Cheng Tang, and Gong-Li Tang2

Nonribosomal peptide synthetases (NRPSs) usually catalyze the biosynthesis of peptide natural products by sequential selection, activation, and condensation of amino acid precursors. It was reported that some fatty acids, α-ketoacids, and α-hydroxyacids originating from amino acid metabolism as well as polyketide-derived units can also be used by NRPS assembly lines as an alternative to amino acids. Ecteinascidin 743 (ET-743), naphthyridinomycin (NDM), and quinocarcin (QNC) are three important antitumor natural products belonging to the tetrahydroisoquinoline family. Although ET-743 has been approved as an anticancer drug, the origin of an identical two-carbon (C2) fragment among these three antibiotics has not been elucidated despite much effort in the biosynthetic research in the past 30 y. Here we report that two unexpected two-component transketolases (TKases), NapB/NapD in the NDM biosynthetic pathway and QncN/QncL in QNC biosynthesis, catalyze the transfer of a glycolaldehyde unit from ketose to the lipoyl group to yield the glycolicacyl lipoic acid intermediate and then transfer the C2 unit to an acyl carrier protein (ACP) to form glycolicacyl-S-ACP as an extender unit for NRPS. Our results demonstrate a unique NRPS extender unit directly derived from ketose phosphates through (α,β-dihydroxyethyl)-thiamin diphosphate and a lipoyl group-tethered ester intermediate catalyzed by the TKase-ACP platform in the context of NDM and QNC biosynthesis, all of which also highlights the biosynthesis of ET-743. This hybrid system and precursor are distinct from the previously described universal modes involving the NRPS machinery. They exemplify an alternate strategy in hybrid NRPS biochemistry and enrich the diversity of precursors for NRPS combinatorial biosynthesis.