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Dennis Huwald, Sabrina Duda, Raphael Gasper, Vincent Olieric, Eckhard Hofmann, Anja Christine Hemschemeier

• Hemoglobins (Hbs) utilize heme b as a cofactor and are found in all kingdoms of life. The current knowledge reveals an enormous variability of Hb primary sequences, resulting in topological, biochemical and physiological individuality. As Hbs appear to modulate their reactivities through specific combinations of structural features, predicting the characteristics of a given Hb is still hardly possible. The unicellular green alga \(\textit {Chlamydomonas reinhardtii}\) contains 12 genes encoding diverse Hbs of the truncated lineage, several of which possess extended N- or C-termini of unknown function. Studies on some of the \(\it {Chlamydomonas}\) Hbs revealed yet unpredictable structural and biochemical variations, which, along with a different expression of their genes, suggest diverse physiological roles. \(\it {Chlamydomonas}\) thus represents a promising system to analyze the diversification of Hb structure, biochemistry and physiology. Here, we report the crystal structure, resolved to 1.75 Å, of the heme-binding domain of cyanomet THB11 (Cre16.g662750), one of the pentacoordinate algal Hbs, which offer a free Fe-coordination site in the reduced state. The overall fold of THB11 is conserved, but individual features such as a kink in helix E, a tilted heme plane and a clustering of methionine residues at a putative tunnel exit appear to be unique. Both N- and C-termini promote the formation of oligomer mixtures, and the absence of the C terminus results in reduced nitrite reduction rates. This work widens the structural and biochemical knowledge on the 2/2Hb family and suggests that the N- and C-terminal extensions of the \(\it {Chlamydomonas}\) 2/2Hbs modulate their reactivity by intermolecular interactions.