The catalytic cycle of [FeFe] hydrogenase: A tale of two sites

[Display omitted] •[FeFe] hydrogenases use earth abundant metals for efficient hydrogen activation.•Understanding their catalytic mechanism may help synthesise new catalysts.•Their active site H-cluster contains two sites: a [4Fe-4S] site and a [2Fe] site.•Proton-coupled electron transfer and hydride formation are important steps.•We discuss the dominant models for the mechanism and future research directions. 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.