背景:曲霉毒素(OTs)是世界范围内受到调节的霉菌毒素,污染了各种食品环境和农业环境。几种曲霉属和青霉属从六基因生物合成基因簇(BGC)合成OTs以产生高毒性的最终产物OTA。尽管对OTA降解酶进行了许多研究,非常需要具有强稳定性的高效酶,和OTA降解机制知之甚少。
目的:本研究旨在探索OT降解酶,并探讨其在绿僵菌中的降解机制,其中包含一个OT生物合成基因簇。
方法:利用系统发育关系结合RNA表达分析探讨OT-BGC在真菌中的分布。进行了生物活性指导的分离和蛋白质质谱测定,以追踪绿僵菌属中的降解酶。,酶在大肠杆菌中异源表达,并通过体外试验进行验证。进行结构预测和点突变以揭示MbAmh1的催化机理。
结果:除了曲霉属和青霉属物种,三个遥远的系统发育分类单元的绿僵菌包含表达的OT样BGC,但缺乏otaD基因。出乎意料的是,在某些绿僵菌物种中未发现OTBGC产品。相反,绿杆菌将OTA和OTB代谢为其无毒降解产物。Brunneum的这种活性归因于细胞内水解酶MbAmh1,其通过生物活性指导的蛋白质组学分析与体外反应结合进行跟踪。重组MbAmh1(5μg/mL)在3分钟内完全降解1μg/mLOTA,对OTA表现出很强的降级能力。此外,MbAmh1显示出在30至70°C范围内的相当大的温度适应性和在4.0至7.0范围内的酸性pH稳定性。活性位点的鉴定支持了金属铁在该酶促反应中的关键作用。
结论:这些发现揭示了真菌中OT合成的不同模式,并为工业应用提供了潜在的OTA降解酶。
BACKGROUND: Ochratoxins (OTs) are worldwide regulated mycotoxins contaminating a variety of food-environment and agro-environment. Several Aspergillus and Pencillium species synthesize OTs from a six-gene biosynthetic gene cluster (BGC) to produce the highly toxic final product OTA. Although many studies on OTA-degrading enzymes were performed, high efficiency enzymes with strong stability are extremely needed, and the OTA degrading mechanism is poorly understood.
OBJECTIVE: The study aimed to explore the OT-degradation enzyme and investigate its degradation mechanisms in Metarhizium, which contain an OT biosynthetic gene cluster.
METHODS: Phylogenomic relationship combined with RNA expression analysis were used to explore the distribution of OT BGC in fungi. Bioactivity-guided isolation and protein mass spectrometry were conducted to trace the degrading enzymes in Metarhizium spp., and the enzymes were heterologously expressed in E. coli and verified by in vitro assays. Structure prediction and point mutation were performed to reveal the catalytic mechanism of MbAmh1.
RESULTS: Beyond Aspergillus and Pencillium species, three species of the distant phylogenetic taxon Metarhizium contain an expressed OT-like BGC but lack an otaD gene. Unexpectedly, no OT BGC products were found in some Metarhizium species. Instead, Metarhizium metabolized both OTA and OTB to their non-toxic degradation products. This activity of M. brunneum was attributed to an intracellular hydrolase MbAmh1, which was tracked by bioactivity-guided proteomic analysis combined with in vitro reaction. Recombinant MbAmh1 (5 μg/mL) completely degraded 1 μg/mL OTA within 3 min, demonstrating a strong degrading ability towards OTA. Additionally, MbAmh1 showed considerable temperature adaptability ranging from 30 to 70 °C and acidic pH stability ranging from 4.0 to 7.0. Identification of active sites supported the crucial role of metal iron for this enzymatic reaction.
CONCLUSIONS: These findings reveal different patterns of OT synthesis in fungi and provide a potential OTA degrading enzyme for industrial applications.