Ever since the discovery of hydrogenases over 90 years ago (Stephenson and Stickland, 1931), their structure and mechanism of action have been intensively investigated. Of the three classes of hydrogenases, the [FeFe] hydrogenases show the highest activity and reversibility, and, thus, have garnered special interest. In this review, we tell the story of how structural, spectroscopic, functional and theoretical studies have all contributed substantially to our modern understanding of the catalytic cycle. We present a brief historical overview of the main events preceding the X-ray crystal structure determination, and then discuss how this defining moment over 20 years ago (Peters et al., 1998; Nicolet et al., 1999) revolutionised our understanding. We then detail the studies leading up to one model for the catalytic cycle in the simple enzyme from Chlamydomonas reinhardtii, containing only the active site H-cluster, as well as how the situation differs in enzymes containing additional iron-sulphur clusters. We then discuss the studies that led to a second model in the literature. Finally, we highlight the open questions and discuss how these could be answered.

自从 90 多年前发现氢化酶以来（Stephenson 和 Stickland，1931），人们对其结构和作用机制进行了深入研究。在三类氢化酶中，[FeFe] 氢化酶显示出最高的活性和可逆性，因此引起了人们的特别关注。在这篇综述中，我们讲述了结构、光谱、功能和理论研究如何对我们对催化循环的现代理解做出重大贡献的故事。我们对 X 射线晶体结构确定之前的主要事件进行了简要的历史概述，然后讨论了 20 多年前的这一决定性时刻（Peters 等人，1998 年；Nicolet 等人，1999 年）如何彻底改变了我们的理解。然后，我们详细介绍了导致简单酶催化循环模型的研究Chlamydomonas reinhardtii，仅包含活性位点 H 簇，以及含有额外铁硫簇的酶的情况有何不同。然后我们讨论了导致文献中第二个模型的研究。最后，我们强调未解决的问题并讨论如何回答这些问题。