Leech H K, Raux-Deery E, Heathcote P, Warren M J
Queen Mary, University of London, Mile End Road, London E1 4NS, UK.
Biochem Soc Trans. 2002 Aug;30(4):610-3. doi: 10.1042/bst0300610.
One of the four operons required for cobalamin biosynthesis in Bacillus megaterium is also associated with sirohaem synthesis, and contains three genes, sirA, sirB and sirC. By undertaking functional complementation experiments and in vitro assays using recombinantly produced enzymes, we have been able to demonstrate that (1) SirA acts as a uroporphyrinogen III methyltransferase, transforming uroporphyrinogen III into precorrin-2, (2) SirC acts as an NAD(+) dehydrogenase, responsible for the oxidation of precorrin-2 into sirohydrochlorin, and (3) SirB acts as a ferrochelatase, responsible for the insertion of a ferrous ion into sirohydrochlorin to give sirohaem. Comparative sequence analysis reveals that the primary structure of SirB is highly similar to that of the cobalt chelatase involved in cobalamin biosynthesis in Bacillus megaterium, CbiX, with the exception that CbiX contains a C-terminal histidine-rich motif. Surprisingly, CbiX has been shown (using EPR) to contain a 4Fe-4S centre, a redox centre that is absent from SirB.
巨大芽孢杆菌中钴胺素生物合成所需的四个操纵子之一也与 siro 血红素的合成相关，它包含三个基因，即 sirA、sirB 和 sirC。通过进行功能互补实验以及使用重组产生的酶进行体外测定，我们能够证明：（1）SirA 作为尿卟啉原 III 甲基转移酶，将尿卟啉原 III 转化为前咕啉 -2；（2）SirC 作为 NAD(+) 脱氢酶，负责将前咕啉 -2 氧化为 siro 氢氯化物；（3）SirB 作为亚铁螯合酶，负责将亚铁离子插入 siro 氢氯化物中以生成 siro 血红素。比较序列分析表明，SirB 的一级结构与巨大芽孢杆菌中参与钴胺素生物合成的钴螯合酶 CbiX 的一级结构高度相似，不同之处在于 CbiX 含有一个富含组氨酸的 C 末端基序。令人惊讶的是，已证明 CbiX（使用电子顺磁共振）含有一个 4Fe-4S 中心，这是 SirB 中不存在的氧化还原中心。
