The molecular basis of Celmer's rules: The stereochemistry of the condensation step in chain extension on the erythromycin polyketide synthase
The molecular basis of Celmer's rules: The stereochemistry of the condensation step in chain extension on the erythromycin polyketide synthase
复制标题
DOI：
10.1021/bi971566b
复制
发表时间：
1997-11-11
期刊：
影响因子：
2.9
通讯作者：
Leadlay, PF
中科院分区：
文献类型：
--
作者：
Weissman, KJ;Timoney, M;Leadlay, PF
模块化聚酮化合物脱氢酶（PKS），例如负责合成大环内酯抗生素红霉素的糖苷配基核心的6-脱氧腺苷酸B合酶（DEBS），在其聚酮化合物产物中产生令人印象深刻的不对称中心多样性。然而，正如Celmer所指出的，大环内酯类在所有可比的立体中心处具有相同的绝对构型，因此，理解链延伸的立体化学如何被控制对于确定这些酶的共同作用机制至关重要。我们旨在通过DEBS 1-TE（DEBS的双模衍生物，来自多孢酵母菌）的体外研究来阐明Celmer规则的分子基础，其使用（2S）-甲基丙二酰-辅酶A在其三酮内酯产物中产生D-和L-甲基中心。我们在这里表明，在模块2中的（2S）-甲基丙二酰辅酶A的缩合进行脱羧转化，而不裂解邻近甲基的C-H键;相反，在模块1中的扩链过程涉及连接到甲基丙二酰辅酶A前体的C-2的氢的损失。在模块2中产生D-甲基中心而没有从（2S）-甲基丙二酰-CoA的不对称中心损失氢，这确立了如在脂肪酸生物合成中那样发生构型反转的缩合。从（2S）-甲基丙二酰-CoA损失关键氢以产生模块1中产生的L-甲基中心意味着在该模块中发生另外的强制性差向异构化步骤。差向异构化的性质和时间仍有待确定。
Modular polyketide synthases (PKSs), for example, the 6-deoxyerythronolide B synthase (DEBS) responsible for synthesis of the aglycone core of the macrolide antibiotic erythromycin, generate an impressive diversity of asymmetric centers in their polyketide products. However, as noted by Celmer, macrolides have the same absolute configuration at all comparable stereocenters, Understanding how the stereochemistry of chain extension is controlled is therefore crucial to determining the common mechanism of action of these enzymes, We aimed to elucidate the molecular basis of Celmer's rules through in vitro studies with DEBS 1-TE, a bimodular derivative of DEBS from Saccharopolyspora erythraea, which uses (2S)-methylmalonyl-coenzyme A to produce both D-and L-methyl centers in its triketide lactone product. We show here that condensation of (2S)-methylmalonyl-CoA in module 2 proceeds with decarboxylative inversion without cleavage of the C-H bond adjacent to the methyl group; in contrast, in module 1 the chain extension process involves loss of the hydrogen attached to C-2 of the methylmalonyl-CoA precursor. The production of the D-methyl center in module 2 without loss of hydrogen from the asymmetric center of the (2S)-methylmalonyl-CoA establishes that condensation takes place with inversion of configuration as in fatty acid biosynthesis, The loss of the key hydrogen from the (2S)-methylmalonyl-CoA to produce the L-methyl center generated in module 1 implies that an additional obligatory epimerization step takes place in that module. The nature and timing of the epimerization remain to be established.