Department of Chemistry, University of British Columbia, Vancouver, British Columbia, Canada.
J Biol Chem. 2011 Jun 3;286(22):19392-8. doi: 10.1074/jbc.M111.229781. Epub 2011 Apr 13.
Menaquinone (vitamin K(2)) serves as an electron carrier in the electron transport chain required for respiration in many pathogenic bacteria. Most bacteria utilize a common menaquinone biosynthetic pathway as exemplified by Escherichia coli. Recently, a novel biosynthetic pathway, the futalosine pathway, was discovered in Streptomyces. Bioinformatic analysis strongly suggests that this pathway is also operative in the human pathogens Campylobacter jejuni and Helicobacter pylori. Here, we provide compelling evidence that a modified futalosine pathway is operative in C. jejuni and that it utilizes 6-amino-6-deoxyfutalosine instead of futalosine. A key step in the Streptomyces pathway involves a nucleosidase called futalosine hydrolase. The closest homolog in C. jejuni has been annotated as a 5'-methylthioadenosine nucleosidase (MTAN). We have shown that this C. jejuni enzyme has MTAN activity but negligible futalosine hydrolase activity. However, the C. jejuni MTAN is able to hydrolyze 6-amino-6-deoxyfutalosine at a rate comparable with that of its known substrates. This suggests that the adenine-containing version of futalosine is the true biosynthetic intermediate in this organism. To demonstrate this in vivo, we constructed a C. jejuni mutant strain deleted for mqnA2, which is predicted to encode for the enzyme required to synthesize 6-amino-6-deoxyfutalosine. Growth of this mutant was readily rescued by the addition of 6-amino-6-deoxyfutalosine, but not futalosine. This provides the first direct evidence that a modified futalosine pathway is operative in C. jejuni. It also highlights the tremendous versatility of the C. jejuni MTAN, which plays key roles in S-adenosylmethionine recycling, the biosynthesis of autoinducer molecules, and the biosynthesis of menaquinone.
甲萘醌（维生素 K(2)）作为电子载体在许多病原细菌的呼吸电子传递链中发挥作用。大多数细菌利用一种常见的甲萘醌生物合成途径，以大肠杆菌为例。最近，在链霉菌中发现了一种新的生物合成途径，即 futalosine 途径。生物信息学分析强烈表明，该途径也存在于人类病原体空肠弯曲菌和幽门螺杆菌中。在这里，我们提供了令人信服的证据表明，一种改良的 futalosine 途径在空肠弯曲菌中起作用，并且它利用 6-氨基-6-脱氧 futalosine 而不是 futalosine。该途径的一个关键步骤涉及一种称为 futalosine 水解酶的核酶。在空肠弯曲菌中最接近的同源物被注释为 5'-甲基硫代腺苷核酶（MTAN）。我们已经表明，这种空肠弯曲菌酶具有 MTAN 活性，但 futalosine 水解酶活性可忽略不计。然而，空肠弯曲菌 MTAN 能够以与已知底物相当的速率水解 6-氨基-6-脱氧 futalosine。这表明腺嘌呤含量的 futalosine 是该生物体内真正的生物合成中间体。为了在体内证明这一点，我们构建了一个空肠弯曲菌突变菌株，该菌株缺失 mqnA2，该基因预测编码合成 6-氨基-6-脱氧 futalosine 的酶。该突变体的生长很容易通过添加 6-氨基-6-脱氧 futalosine 而不是 futalosine 得到挽救。这首次直接证明了改良的 futalosine 途径在空肠弯曲菌中起作用。它还突出了空肠弯曲菌 MTAN 的巨大多功能性，该酶在 S-腺苷甲硫氨酸循环、自动诱导分子生物合成和甲萘醌生物合成中发挥关键作用。
