ABSTRACT
Tetrahydrofolate is a ubiquitous C
1
carrier in many biosynthetic pathways in bacteria, importantly, in the biosynthesis of formylmethionyl tRNA
fMet
, which is essential for the initiation of translation. The final step in the biosynthesis of tetrahydrofolate is carried out by the enzyme dihydrofolate reductase (DHFR). A search of the complete genome sequence of
Helicobacter pylori
failed to reveal any sequence that encodes DHFR. Previous studies demonstrated that the
H. pylori
dihydropteroate synthase gene
folP
can complement an
Escherichia coli
strain in which
folA
and
folM
, encoding two distinct DHFRs, are deleted. It was also shown that
H. pylori
FolP possesses an additional N-terminal domain that binds flavin mononucleotide (FMN). Homologous domains are found in FolP proteins of other microorganisms that do not possess DHFR. In this study, we demonstrated that
H. pylori
FolP is also a dihydropteroate reductase that derives its reducing power from soluble flavins, reduced FMN and reduced flavin adenine dinucleotide. We also determined the stoichiometry of the enzyme-bound flavin and showed that half of the bound flavin is exchangeable with the soluble flavins. Finally, site-directed mutagenesis of the most conserved amino acid residues in the N-terminal domain indicated the importance of these residues for the activity of the enzyme as a dihydropteroate reductase.
摘要
四氢叶酸是一种无处不在的 C
1
载体,尤其是在甲酰基蛋氨酰 tRNA 的生物合成过程中。
fMet
的生物合成过程中,甲酰蛋氨酰 tRNA fMet 是启动翻译所必需的。四氢叶酸生物合成的最后一步由二氢叶酸还原酶(DHFR)完成。对幽门螺旋杆菌的完整基因组序列进行的检索显示
幽门螺杆菌
未能发现任何编码 DHFR 的序列。以前的研究表明
幽门螺杆菌
二氢蝶酸合成酶基因
folP
可以与
大肠杆菌
菌株的
folA
和
folM
的大肠杆菌菌株中,删除了编码两种不同 DHFR 的 folA 和 folM。研究还表明
幽门螺杆菌
FolP 具有一个额外的 N 端结构域,可结合黄素单核苷酸(FMN)。在其他不具有 DHFR 的微生物的 FolP 蛋白中也发现了同源结构域。在这项研究中,我们证明了
幽门螺杆菌
FolP 也是一种二氢蝶酸还原酶,其还原力来自可溶性黄素、还原型 FMN 和还原型黄素腺嘌呤二核苷酸。我们还测定了酶结合黄素的化学计量学,结果表明结合黄素的一半可与可溶性黄素交换。最后,对 N 端结构域中最保守的氨基酸残基进行了定点突变,结果表明这些残基对该酶作为二氢蝶酸还原酶的活性非常重要。