Marquet A, Bui B T, Florentin D
Laboratoire de Chimie Organique Biologique, Université Pierre et Marie Curie, 75252 Paris, France.
Vitam Horm. 2001;61:51-101. doi: 10.1016/s0083-6729(01)61002-1.
The genetics and mechanistic enzymology of biotin biosynthesis have been the subject of much investigation in the last decade, owing to the interest for biotin production by fermentation, on the one hand, and for the design of inhibitors with potential herbicidal properties, on the other hand. Four enzymes are involved in the synthesis of biotin from its two precursors, alanine and pimeloyl-CoA. They are now well-characterized and the X-ray structures of the first three have been published. 8-Amino-7-oxopelargonic acid synthase is a pyridoxal 5'-phosphate (PLP) enzyme, very similar to other acyl-CoA alpha-oxoamine synthases, and its detailed mechanism has been determined. The origin of its specific substrate, pimeloyl-CoA, however, is not completely established. It could be produced by a modified fatty acid pathway involving a malonyl thioester as the starter. 7,8-Diaminopelargonic acid (DAPA) aminotransferase, although sharing sequence and folding homologies with other transaminases, is unique as it uses S-adenosylmethionine (AdoMet) as the NH2 donor. The mechanism of dethiobiotin synthethase is also now well understood. It catalyzes the formation of the ureido ring via a DAPA carbamate activated with ATP. On the other hand, the mechanism of the last enzyme, biotin synthase, which has long raised a very puzzling problem, is only starting to be unraveled and appears indeed to be very complex. Biotin synthase belongs to the family of AdoMet-dependent enzymes that reductively cleave AdoMet into a deoxyadenosyl radical, and it is responsible for the homolytic cleavage of C-H bonds. A first radical formed on dethiobiotin is trapped by the sulfur donor, which was found to be the iron-sulfur (Fe-S) center contained in the enzyme, and cyclization follows in a second step. Two important features come from these results: (1) a new role for an Fe-S center has been revealed, and (2) biotin synthase is not only a catalyst but also a substrate for the reaction. Lipoate synthase, which catalyzes the formation of two C-S bonds from octanoic acid, has a very high sequence similarity with biotin synthase. Although no in vitro enzymology has been carried out with lipoate synthase, the sequence homology as well as the results of in vivo studies support the conclusion that both enzymes are strongly mechanistically related.
在过去十年中，生物素生物合成的遗传学和机制酶学一直是众多研究的主题，一方面是由于对发酵生产生物素感兴趣，另一方面是由于设计具有潜在除草特性的抑制剂。从其两种前体丙氨酸和庚二酰辅酶A合成生物素涉及四种酶。它们现在已得到充分表征，并且前三种酶的X射线结构已经发表。8-氨基-7-氧代壬酸合酶是一种磷酸吡哆醛（PLP）酶，与其他酰基辅酶Aα-氧代胺合酶非常相似，其详细机制已经确定。然而，其特定底物庚二酰辅酶A的来源尚未完全明确。它可能由一种涉及丙二酰硫酯作为起始物的修饰脂肪酸途径产生。7,8-二氨基壬酸（DAPA）转氨酶虽然与其他转氨酶具有序列和折叠同源性，但却是独特的，因为它使用S-腺苷甲硫氨酸（AdoMet）作为NH2供体。脱硫生物素合酶的机制现在也已得到很好的理解。它通过由ATP激活的DAPA氨基甲酸盐催化脲基环的形成。另一方面，最后一种酶生物素合酶的机制长期以来一直是一个非常令人困惑的问题，现在才刚刚开始被揭示，并且看起来确实非常复杂。生物素合酶属于依赖于AdoMet的酶家族，该家族将AdoMet还原裂解为脱氧腺苷自由基，并且它负责C-H键的均裂。在脱硫生物素上形成的第一个自由基被硫供体捕获，发现硫供体是该酶中含有的铁硫（Fe-S）中心，第二步是环化反应。这些结果有两个重要特征：（1）揭示了Fe-S中心的新作用，（2）生物素合酶不仅是反应的催化剂，也是反应的底物。硫辛酸合酶催化从辛酸形成两个C-S键，与生物素合酶具有非常高的序列相似性。虽然尚未对硫辛酸合酶进行体外酶学研究，但序列同源性以及体内研究结果支持这两种酶在机制上密切相关的结论。
