Cane David E, He Xiaofei, Kobayashi Seiji, Omura Satoshi, Ikeda Haruo
Department of Chemistry, Brown University, Box H, Providence, Rhode Island 02912-9108, USA.
J Antibiot (Tokyo). 2006 Aug;59(8):471-9. doi: 10.1038/ja.2006.66.
Geosmin (1) is responsible for the characteristic odor of moist soil. The Gram-positive soil bacterium Streptomyces avermitilis produces geosmin (1) as well as its precursor germacradienol (3). The S. avermitilis gene SAV2163 (geoA) is extremely similar to the S. coelicolor A3(2) SCO6073 gene that encodes a germacradienol/geosmin synthase. S. avermitilis mutants with a deleted geoA were unable to produce either germacradienol (3) or geosmin (1). Biosynthesis of both compounds was restored by introducing an intact geoA gene into the mutants. Incubation of recombinant GeoA, encoded by the SAV2163 gene of S. avermitilis, with farnesyl diphosphate (2) in the presence of Mg2+ gave a mixture of (4S,7R)-germacra-1(10)E,5E-diene-11-ol (3) (66%), (7S)-germacrene D (4) (24%), geosmin (1) (8%), and a hydrocarbon, tentatively assigned the structure of octalin 5 (2%). Incubation of this germacradienol/geosmin synthase with [1,1-(2)H2] FPP (2a) gave geosmin-d1 (1a), as predicted. When recombinant GeoA from either S. avermitilis or S. coelicolor A3(2) was incubated with nerolidyl diphosphate (8), only the acyclic elimination products beta3-farnesene (10), (Z)-alpha-farnesene (11), and (E)-alpha-farnesene (12) were formed, thereby ruling out nerolidyl diphosphate as an intermediate in the conversion of farnesyl diphosphate to geosmin, germacradienol, and germacrene D.
土臭素（1）是潮湿土壤特有气味的来源。革兰氏阳性土壤细菌阿维链霉菌（Streptomyces avermitilis）能产生土臭素（1）及其前体牻牛儿基香叶烯醇（3）。阿维链霉菌基因SAV2163（geoA）与天蓝色链霉菌A3（2）中编码牻牛儿基香叶烯醇/土臭素合酶的SCO6073基因极为相似。geoA缺失的阿维链霉菌突变体无法产生牻牛儿基香叶烯醇（3）或土臭素（1）。将完整的geoA基因导入突变体后，这两种化合物的生物合成得以恢复。在Mg2+存在的情况下，用阿维链霉菌SAV2163基因编码的重组GeoA与法呢基二磷酸（2）一起孵育，得到了（4S,7R）-牻牛儿-1（10）E,5E-二烯-11-醇（3）（66%）、（7S）-牻牛儿烯D（4）（24%）、土臭素（1）（8%）和一种暂定为八氢化萘5结构的烃（2%）的混合物。正如所预测的那样，用[1,1-（2）H2]FPP（2a）与这种牻牛儿基香叶烯醇/土臭素合酶一起孵育得到了土臭素-d1（1a）。当来自阿维链霉菌或天蓝色链霉菌A3（2）的重组GeoA与橙花叔基二磷酸（8）一起孵育时，只形成了无环消除产物β3-法呢烯（10）、（Z）-α-法呢烯（11）和（E）-α-法呢烯（12），从而排除了橙花叔基二磷酸作为法呢基二磷酸转化为土臭素、牻牛儿基香叶烯醇和牻牛儿烯D过程中的中间体。
