S-Adenosyl-S-carboxymethyl-L-homocysteine: a novel cofactor found in the putative tRNA-modifying enzyme CmoA.
S-Adenosyl-S-carboxymethyl-L-homocysteine: a novel cofactor found in the putative tRNA-modifying enzyme CmoA.
复制标题
DOI：
10.1107/s0907444913004939
复制
发表时间：
2013-06
期刊：
影响因子：
--
通讯作者：
Waterman DG
中科院分区：
文献类型：
--
作者：
Byrne RT;Whelan F;Aller P;Bird LE;Dowle A;Lobley CM;Reddivari Y;Nettleship JE;Owens RJ;Antson AA;Waterman DG
推测的甲基转移酶CmoA参与转运RNA的核苷修饰。X射线晶体学和质谱分析表明，它含有一种新的SAM衍生物，S-腺苷-S-羧甲基-L-同型半胱氨酸，其中供体甲基被羧甲基取代。细菌转移RNA的位置34处的尿苷通常被修饰为尿苷-5-氧乙酸（cmo 5 U）以增加解码能力。蛋白质CmoA参与cmo 5 U的形成，并且基于其与其它含SAM的酶的序列同源性而被注释为S-腺苷-L-甲硫氨酸依赖性（SAM依赖性）甲基转移酶。然而，大肠杆菌CmoA在1.73 nm分辨率下的晶体结构和质谱都表明它含有一种新的辅因子，S-腺苷-S-羧甲基-L-高半胱氨酸（SCM-SAH），其中供体甲基被羧甲基取代。 羧基部分与Arg 199形成盐桥相互作用，Arg 199在一大群CmoA相关蛋白中是保守的，但在其他含SAM的酶中不保守。这提出了一种可能性，即以前被注释为SAM依赖性的许多酶实际上是SCM-SAH依赖性的。实际上，对具有已知X射线结构的一种这样的酶（PDB条目1 im 8）的电子密度的检查表明，活性位点含有SCM-SAH而不是SAM。
The putative methyltransferase CmoA is involved in the nucleoside modification of transfer RNA. X-ray crystallography and mass spectrometry are used to show that it contains a novel SAM derivative, S-adenosyl-S-carboxymethyl-l-homocysteine, in which the donor methyl group is replaced by a carboxymethyl group. Uridine at position 34 of bacterial transfer RNAs is commonly modified to uridine-5-oxyacetic acid (cmo5U) to increase the decoding capacity. The protein CmoA is involved in the formation of cmo5U and was annotated as an S-adenosyl-l-methionine-dependent (SAM-dependent) methyltransferase on the basis of its sequence homology to other SAM-containing enzymes. However, both the crystal structure of Escherichia coli CmoA at 1.73 Å resolution and mass spectrometry demonstrate that it contains a novel cofactor, S-adenosyl-S-carboxymethyl-l-homocysteine (SCM-SAH), in which the donor methyl group is substituted by a carboxymethyl group. The carboxyl moiety forms a salt-bridge interaction with Arg199 that is conserved in a large group of CmoA-related proteins but is not conserved in other SAM-containing enzymes. This raises the possibility that a number of enzymes that have previously been annotated as SAM-dependent are in fact SCM-SAH-dependent. Indeed, inspection of electron density for one such enzyme with known X-ray structure, PDB entry 1im8, suggests that the active site contains SCM-SAH and not SAM.