The 22nd genetically encoded amino acid, pyrrolysine, plays a unique role in the key step in the growth of methanogens on mono-, di-, and tri-methylamines by activating the methyl group of these substrates for transfer to a corrinoid cofactor. Previous crystal structures of the Methanosarcina barkeri monomethylamine methyltransferase elucidated the structure of pyrrolysine and provide insight into its role in monomethylamine activation. Herein, we report the second structure of a pyrrolysine-containing protein, the M. barkeri trimethylamine methyltransferase MttB, and its structure bound to sulfite, a substrate analog of trimethylamine. We also report the structure of MttB in complex with its cognate corrinoid protein MttC, which specifically receives the methyl group from the pyrrolysine-activated trimethylamine substrate during methanogenesis. Together these structures provide key insights into the role of pyrrolysine in methyl group transfer from trimethylamine to the corrinoid cofactor in MttC.

第 22 种基因编码氨基酸吡咯赖氨酸通过激活这些底物的甲基转移到类咕啉辅因子，在产甲烷菌在单、二和三甲胺上生长的关键步骤中发挥独特作用。巴氏甲烷八叠球菌单甲胺甲基转移酶的先前晶体结构阐明了吡咯赖氨酸的结构，并提供了对其在单甲胺活化中的作用的深入了解。在此，我们报告了一种含吡咯赖氨酸的蛋白质的第二种结构，即M. barkeri三甲胺甲基转移酶 MttB，及其与亚硫酸盐结合的结构，亚硫酸盐是三甲胺的一种底物类似物。我们还报告了 MttB 与其同源类咕啉蛋白 MttC 复合物的结构，MttC 在产甲烷过程中特异性地从吡咯赖氨酸激活的三甲胺底物接收甲基。这些结构一起为吡咯赖氨酸在甲基从三甲胺转移到 MttC 中的类咕啉辅因子中的作用提供了关键见解。