A novel two-over-two alpha-helical sandwich fold is characteristic of the truncated hemoglobin family

Small hemoproteins displaying amino acid sequences 20-40 residues shorter t han (non-)vertebrate hemoglobins (Hbs) have recently been identified in sev eral pathogenic and non-pathogenic unicellular organisms, and named 'trunca ted hemoglobins' (trHbs). They have been proposed to be involved not only i n oxygen transport but also in other biological functions, such as protecti on against reactive nitrogen species, photosynthesis or to act as terminal oxidases. Crystal structures of trHbs from the ciliated protozoan Parameciu m caudatum and the green unicellular alga Chlamydomonas eugametos show that the tertiary structure of both proteins is based on a 'two-over-two' a-hel ical sandwich, reflecting an unprecedented editing of the classical 'three- over-three' alpha-helical globin fold, Based on specific Gly-Gly motifs the tertiary structure accommodates the deletion of the N-terminal A-helix and replacement of the crucial heme-binding F-helix with an extended polypepti de loop. Additionally, concerted structural modifications allow burying of the heme group and define the distal site, which hosts a TyrB10, GlnE7 resi due pair. A set of structural and amino acid sequence consensus rules for s tabilizing the fold and the bound heme in the trHbs homology subfamily is d educed.