Truncated Hemoglobins: A New Family of Hemoglobins Widely Distributed in Bacteria, Unicellular Eukaryotes, and Plants* 210

(1) constitute a family ofsmall oxygen-binding heme proteins distributed in eubacteria,cyanobacteria, protozoa, and plants (Table I, SupplementalMaterial) forming a distinct group within the hemoglobin (Hb)superfamily (2). They are nearly ubiquitous in the plant king-dom, occur in many aggressively pathogenic bacteria, and areheld to be of very ancient origin. None have been detected inthe genomes of archaea or metazoa. Characteristically, trHbsoccur at nano- to micromolar intracellular concentration, hint-ing at a possible role as catalytic proteins.Many trHbs display amino acid sequences 20–40 residuesshorter than non-vertebrate hemoglobins to which they arescarcely related by sequence similarity. Crystal structures (1,3) show that trHb tertiary structure is based on a 2-on-2 -hel-ical sandwich, which represents an unprecedented editing ofthe highly conserved globin fold. Moreover, an almost continu-ous hydrophobic tunnel, traversing the protein matrix from themolecular surface to the heme distal site, may provide a pathfor ligand diffusion to the heme.

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