Aflatoxin Biosynthesis and Iterative Type I Polyketide Synthases

描述（由申请人提供）：黄曲霉毒素是真菌毒素中的一种范例，在环境毒理学中占据中心地位。肝癌是世界上第三大最常见的癌症死亡原因。肝炎感染和饮食中的黄曲霉毒素是导致这种疾病发生的主要危险因素。产黄曲霉A. parasticus，A. flavus和A. nomius通常感染谷物和食品，其中明确的流行病学数据与人类疾病相关。长期摄入是亚洲、非洲和中美洲过早死亡的一个主要原因。在毒素的代谢活化形式与DNA的相互作用之间建立了直接联系，特别是在p53基因的热点中，导致其重要的编码细胞周期调节蛋白的突变。了解它的生物合成将导致控制这种环境致癌物。黄曲霉毒素是通过异常漫长和复杂的生物合成产生的。关键的分子重排，一些显着的机制，是由细胞色素P450催化。这些裂解和重排反应的机制将通过化学模拟物进行研究和建模，以了解其潜在的化学性质。第二个，也是主要的，目标是利用在当前资助期内取得的令人兴奋的进展，了解黄曲霉毒素生物合成和许多其他真菌天然产物的非还原迭代聚酮酶的功能。在PKS的三种主要类型中，对迭代I型系统的编程知之最少;即起始单元是如何补充和合成是如何开始的，链长是如何确定的，规范的和固有反应性的聚2-酮中间体是如何稳定的，在链伸长过程中氧化还原状态是如何控制的，最后，如何控制分子内环化为特定的环形式优先于其他环形式。叠加在此之上的是迭代催化的基本问题，这是一种罕见但令人印象深刻的有效过程，在这种过程中，这些多结构域酶中的活性位点被一次又一次地使用，适应一系列不断增长的底物，同时忠实地执行合成程序。我们已经发现了两个以前未被识别的域在这些域中，我们现在相信是一般的这类酶，一个起始单位：酰基载体蛋白转酰酶（SAT）域，这可能是经典的观察起始单位的效果，和产品模板（PT）域，在初步结果中明确具有聚酮稳定和产品模板的作用。生物化学，X射线晶体学和质谱实验概述了这些域的功能，单独和一起工作，已经出现了一个新的图片迭代催化。 黄曲霉毒素在环境毒理学中占有中心地位。肝炎感染和来自受感染谷物和食品的饮食黄曲霉毒素是肝癌的主要风险因素，肝癌是世界上第三大常见癌症死亡原因。了解它的生物合成将导致它的控制。

DESCRIPTION (provided by applicant): The aflatoxins are a paradigm among mycotoxins and occupy a central place in environmental toxicology. Hepatocarcinomas are the third most common cause of cancer death in the world. Hepatitis infections and dietary aflatoxin are major risk factors contributing to the incidence of this disease. The aflatoxigenic Aspergillus species, A. parasticus, A. flavus and A. nomius, commonly infect grains and food stuffs where clear epidemiological data correlate to human disease. Chronic ingestion is a major cause of premature death in Asia, Africa and Central America. A direct link has been forged between the interaction of the metabolically activated form of the toxin and DNA, particularly in a hot spot in the p53 gene leading to mutation of its important encoded cell cycle regulating protein. Understanding its biosynthesis will lead to control of this environmental carcinogen. The aflatoxins are created by an unusually long and complex biosynthesis. Key molecular rearrangements, some remarkable mechanistically, are catalyzed by cytochromes P450. The mechanisms of these cleavage and rearrangement reactions will be studied and modeled by chemical mimetics to understand their underlying chemistry. The second, and principal, goal is to capitalize upon exciting progress made in the current grant period to understand the function of non-reducing iterative polyketide synthases central to aflatoxin biosynthesis and many other fungal natural products. Of the three principal types of PKSs, least is known about the programming of iterative Type I systems; that is, how are starter units recruited and synthesis begun, how is chain length determined, how is the canonical and intrinsically reactive poly 2-keto intermediate stabilized, how is redox state controlled during chain elongation, and, finally, how are intramolecular cyclizations controlled to specific ring forms in preference to others. Superimposed on this is the fundamental question of iterative catalysis, a rare but impressively efficient process in which active sites in these polydomainal enzymes are used over and over again, accommodating a sequence of growing substrates yet faithfully executing a synthetic program. We have discovered two previously unrecognized domains in these domains we now believe are general to this class of enzymes, a starter unit:acyl-carrier protein transacylase (SAT) domain, which might account for the classical observation of a starter unit effect, and a product template (PT) domain, which in preliminary results clearly has a role in polyketide stabilization and product templating. Biochemical, X-ray crystallographic and mass spectrometric experiments are outlined to understand the functions of these domains individually and working together in what is already emerging as a new picture of iterative catalysis. PUBLIC HEALTH RELEVANCE The aflatoxins occupy a central place in environmental toxicology. Hepatitis infections and dietary aflatoxin from infected grains and foodstuffs are major risk factors for hepatocarcinoma, the third most common cause of cancer death in the world. Understanding its biosynthesis will lead to its control.