The Radical SAM (RS) superfamily of enzymes catalyzes a wide array of enzymatic reactions. The majority of these enzymes employ an electron from a reduced [4Fe–4S]⁺¹ cluster to facilitate the reductive cleavage of S-adenosyl-l-methionine, thereby producing a highly reactive 5′-deoxyadenosyl radical (5′-dA⋅) and l-methionine. Typically, RS enzymes use this 5′-dA⋅ to extract a hydrogen atom from the target substrate, starting the cascade of an expansive and impressive variety of chemical transformations. While a great deal of understanding has been gleaned for 5′-dA⋅ formation, because of the chemical diversity within this superfamily, the subsequent chemical transformations have only been fully elucidated in a few examples. In addition, with the advent of new sequencing technology, the size of this family now surpasses 700,000 members, with the number of uncharacterized enzymes and domains also rapidly expanding. In this review, we outline the history of RS enzyme characterization in what we term “epochs” based on advances in technology designed for stably producing these enzymes in an active state. We propose that the state of the field has entered the fourth epoch, which we argue should commence with a protein structure initiative focused solely on RS enzymes to properly tackle this unique superfamily and uncover more novel chemical transformations that likely exist.

Radical SAM (RS)酶超家族可催化多种酶促反应。这些酶中的大多数利用来自还原的 [4Fe–4S] ^{+1簇的电子来促进 S-腺苷}-l-甲硫氨酸的还原裂解，从而产生高反应性的 5′-脱氧腺苷自由基 (5′-dA⋅)和L-蛋氨酸。通常，RS 酶使用这个 5′-dA⋅ 从目标底物中提取氢原子，启动一系列广泛而令人印象深刻的化学转化。虽然人们对 5′-dA⋅ 的形成有了很多了解，但由于这个超家族内的化学多样性，随后的化学转化仅在几个例子中得到充分阐明。此外，随着新测序技术的出现，该家族的成员数量现已超过70万，未表征的酶和结构域的数量也在迅速扩大。在这篇综述中，我们概述了 RS 酶表征的历史，即我们所说的“时代”，其基础是旨在稳定生产这些处于活性状态的酶的技术进步。我们认为该领域的现状已经进入第四个时代，我们认为应该从仅关注 RS 酶的蛋白质结构倡议开始，以正确解决这个独特的超家族并发现可能存在的更多新颖的化学转化。