Sterol 14α-demethylase encoded by CYP51 is a mixed-function oxidase involved in sterol synthesis in eukaryotic organisms. Completion of the
Mycobacterium tuberculosis
genome project revealed that a protein having homology to mammalian 14α-demethylases might be present in this bacterium. Using genomic DNA from mycobacterial strain H
37
Rv, we have established unambiguously that the CYP51-like gene encodes a bacterial sterol 14α-demethylase. Expression of the
M. tuberculosis
CYP51 gene in
Escherichia coli
yields a P450, which, when purified to homogeneity, has the predicted molecular mass,
ca.
50 kDa on SDS/PAGE, and binds both sterol substrates and azole inhibitors of P450 14α-demethylases. It catalyzes 14α-demethylation of lanosterol, 24,25-dihydrolanosterol, and obtusifoliol to produce the 8,14-dienes stereoselectively as shown by GC/MS and
1
H NMR analysis. Both flavodoxin and ferredoxin redox systems are able to support this enzymatic activity. Structural requirements of a 14α-methyl group and Δ
8(9)
-bond were established by comparing binding of pairs of sterol substrate that differed in a single molecular feature, e.g., cycloartenol paired with lanosterol. These substrate requirements are similar to those established for plant and animal P450 14α-demethylases. From the combination of results, the interrelationships of substrate functional groups within the active site show that oxidative portions of the sterol biosynthetic pathway are present in prokaryotes.
CYP51编码的类固醇14α-去甲基酶是参与真核生物类固醇合成的混合功能氧化酶。完成结核分枝杆菌基因组项目揭示,可能存在一种与哺乳动物14α-去甲基酶同源的蛋白质。使用结核分枝杆菌H37Rv菌株的基因组DNA,我们已经明确确定CYP51类似基因编码细菌类固醇14α-去甲基酶。在大肠杆菌中表达M. tuberculosis CYP51基因会产生P450,纯化后在SDS/PAGE上显示预测的分子量约为50 kDa,并结合类固醇底物和P450 14α-去甲基酶的唑类抑制剂。它能催化蜡甾醇、24,25-二氢蜡甾醇和钝叶桐酚的14α-去甲基化反应,产生8,14-二烯,经GC/MS和1H NMR分析表明它有立体选择性。类黄酮蛋白和铁蛋白氧化还原系统都能支持这种酶活性。通过比较具有单一分子特征的一对类固醇底物的结合,如环油菜醇与蜡甾醇,确定了14α-甲基和Δ8(9)-键的结构要求。这些底物要求类似于已经确定的植物和动物P450 14α-去甲基酶的要求。从结果的组合来看,底物功能基团在活性位点内的相互关系表明类固醇生物合成途径的氧化部分存在于原核生物中。