The Histidine Brace: Nature's Copper Alternative to Haem?

The copper histidine brace is a structural unit in metalloproteins. 1 It consists of a copper ion chelated by the NH 2 and π-N atom of an N-terminal histidine, and the τ-N atom of a further histidine, in an overall T-shaped coordination geometry. 2 Like haem-containing proteins, histidine-brace-containing proteins have peroxygenase and/or oxygenase activity, where the substrates are notable for resistance to oxidation, e.g. lytic polysaccharide monooxygenases (LPMOs). Moreover, the histidine brace is an invariant unit around which different protein structures exert different activities. Given the similarities in the diversity of function of proteins that contain either the copper histidine brace or haem, the question arises as to whether the functions of histidine brace-containing proteins duplicate those containing haem groups.