How corrinoids are synthesized without oxygen: nature's first pathway to vitamin B12.
How corrinoids are synthesized without oxygen: nature's first pathway to vitamin B12.
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类咕啉如何在无氧的情况下合成：自然界合成维生素 B12 的第一个途径。
DOI：
10.1016/s1074-5521(97)90221-0
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发表时间：
1997
影响因子：
--
通讯作者：
A. Scott
中科院分区：
文献类型：
--
作者：
P. J. Santander;C. Roessner;N. Stolowich;M. Holderman;A. Scott
背景：在维生素B12的生物合成过程中，好氧细菌——反硝化假单胞菌（pseudomonas反硝化假单胞菌）利用CobG和CobJ两种酶将预蛋白-3转化为环收缩中间体预蛋白-4。CobG是一种单加氧酶，它在C-20上添加一个羟基，而CobJ是双功能酶，在大环的C-17上插入一个甲基，并催化环收缩。产生维生素b12的厌氧细菌和古细菌成员无法获得分子氧，因此问题出现了，这些微生物如何完成关键的环收缩过程。结果：鼠伤寒沙门氏菌的克隆和过表达导致了一个单一的酶，CbiH，在厌氧生物合成维生素B12的过程中负责环收缩。当CbiH与预科林斯-3孵育时，该过程发生，但只有在钴存在的情况下。CbiH作为一种C-17甲基转移酶，介导环收缩和内酯化，产生中间体钴-预corrin-4，分离为钴因子iv。13c标记研究证明钴-预corrin-4被纳入钴酸中，从而证实钴-预corrin-4是维生素b12生物合成的中间体。结论：自然界中存在两种不同的卟啉类化合物环收缩成corcorinids的机制——一种是古老的厌氧途径，需要钴络合才能进行非氧化重排，另一种是最近的有氧途径，其中分子氧作为辅助因子。目前的结果为维生素B12的有氧和厌氧生物合成之间的主要差异提供了一个基本原理。因此，厌氧菌在C-27乙酸位点进行氧交换，挤出乙醛，并在早期插入钴，而好氧菌在C-27位点没有氧交换，在环收缩发生后，在生物合成途径中很晚才出现挤出乙酸和插入钴。这些通往维生素b12的平行途径现在已经通过它们不同的环收缩机制被清楚地区分出来。
Background: During the biosynthesis of vitamin B12, the aerobic bacteriumPseudomonas denitrificansuses two enzymes, CobG and CobJ, to convert precorrin-3 to the ring-contracted intermediate, precorrin-4. CobG is a monooxygenase that adds a hydroxyl group, derived from molecular oxygen, to C-20, whereas CobJ is bifunctional, inserting a methyl group at C-17 of the macrocycle and catalyzing ring contraction. Molecular oxygen is not available to vitamin B12-producing anaerobic bacteria and members of the ancient Archaea, so the question arises of how these microbes accomplish the key ring-contraction process.Results: Cloning and overexpression ofSalmonella typhimuriumgenes has led to the discovery that a single enzyme, CbiH, is responsible for ring contraction during anaerobic biosynthesis of vitamin B12. The process occurs when CbiH is incubated with precorrin-3, but only in the presence of cobalt. CbiH functions as a C-17 methyltransferase and mediates ring contraction and lactonization to yield the intermediate, cobalt-precorrin-4, isolated as cobalt-factor IV.13C labeling studies have proved that cobalt-precorrin-4 is incorporated into cobyrinic acid, thereby confirming that cobalt-precorrin-4 is an intermediate in vitamin B12biosynthesis.Conclusions: Two distinct mechanisms exist in nature for the ring contraction of porphyrinoids to corrinoids — an ancient anaerobic pathway that requires cobalt complexation prior to nonoxidative rearrangement, and a more recent aerobic route in which molecular oxygen serves as the cofactor. The present results offer a rationale for the main differences between aerobic and anaerobic biosynthesis of vitamin B12. Thus, in anaerobes there is exchange of oxygen at the C-27 acetate site, extrusion of acetaldehyde and early insertion of cobalt, whereas the aerobes show no exchange of oxygen at C-27, extrude acetic acid and insert cobalt very late in the biosynthetic pathway, after ring contraction has occurred. These parallel routes to vitamin B12have now been clearly distinguished by their differing mechanisms for ring contraction.