Two of nature’s recurring binding motifs in metalloproteins are the CxxxCxxC motif in radical SAM enzymes and the 2-His-1-carboxylate motif found both in zincins and α-ketoglutarate and non-haem iron enzymes. Here we show the confluence of these two domains in a single post-translational modifying enzyme containing an N-terminal radical S-adenosylmethionine domain fused to a C-terminal 2-His-1-carboxylate (HExxH) domain. The radical SAM domain catalyses three-residue cyclophane formation and is the signature modification of triceptides, a class of ribosomally synthesized and post-translationally modified peptides. The HExxH domain is a defining feature of zinc metalloproteases. Yet the HExxH motif-containing domain studied here catalyses β-hydroxylation and is an α-ketoglutarate non-haem iron enzyme. We determined the crystal structure for this HExxH protein at 2.8 Å, unveiling a distinct structural fold, thus expanding the family of α-ketoglutarate non-haem iron enzymes with a class that we propose to name αKG-HExxH. αKG-HExxH proteins represent a unique family of ribosomally synthesized and post-translationally modified peptide modifying enzymes that can furnish opportunities for genome mining, synthetic biology and enzymology.

自然界在金属蛋白中反复出现的两个结合基序是自由基 SAM 酶中的 CxxxCxxC 基序和锌蛋白和 α-酮戊二酸和非血红素铁酶中的 2-His-1-羧酸盐基序。在这里，我们展示了这两个结构域在单个翻译后修饰酶中的汇合，该酶包含一个 N 末端自由基 S-腺苷甲硫氨酸结构域与 C 端 2-His-1-羧酸盐 （HExxH） 结构域融合。自由基 SAM 结构域催化三残基环烷形成，是 triceptides 的标志性修饰，triceptides 是一类核糖体合成和翻译后修饰的肽。HExxH 结构域是锌金属蛋白酶的一个定义特征。然而，这里研究的包含 HExxH 基序的结构域催化 β-羟基化，是一种 α-酮戊二酸非血红素铁酶。我们确定了这种 HExxH 蛋白在 2.8 Å 处的晶体结构，揭示了一个独特的结构折叠，从而扩展了 α-酮戊二酸非血红素铁酶家族，我们建议将其命名为 αKG-HExxH。αKG-HExxH 蛋白代表了一个独特的核糖体合成和翻译后修饰肽修饰酶家族，可为基因组挖掘、合成生物学和酶学提供机会。