Conserved Histidine Adjacent to the Proximal Cluster Tunes the Anaerobic Reductive Activation of Escherichia coli Membrane‐Bound [NiFe] Hydrogenase‐1

[NiFe] hydrogenases are electrocatalysts that oxidize H(2) at a rapid rate without the need for precious metals. All membrane‐bound [NiFe] hydrogenases (MBH) possess a histidine residue that points to the electron‐transfer iron sulfur cluster closest (“proximal”) to the [NiFe] H(2)‐binding active site. Replacement of this amino acid with alanine induces O(2) sensitivity, and this has been attributed to the role of the histidine in enabling the reversible O(2)‐induced over‐oxidation of the [Fe(4)S(3)Cys(2)] proximal cluster possessed by all O(2)‐tolerant MBH. We have created an Escherichia coli Hyd‐1 His‐to‐Ala variant and report O(2)‐free electrochemical measurements at high potential that indicate the histidine‐mediated [Fe(4)S(3)Cys(2)] cluster‐opening/closing mechanism also underpins anaerobic reactivation. We validate these experiments by comparing them to the impact of an analogous His‐to‐Ala replacement in Escherichia coli Hyd‐2, a [NiFe]‐MBH that contains a [Fe(4)S(4)] center.