An enzymatic globin from a marine worm: Protein structure

Some marine worms, such as Thelepus crispus and Notomastus lobatus , secrete brominated aromatic molecules and other halogenated metabolites as repellants 1 . Other species, such as Amphitrite ornata , do not produce repellants but are adapted to the chemical warfare of N. lobatus and cohabit with them in estuarine mudflats. The halocompounds are tolerated by A. ornata as they are degraded by dehaloperoxidase (DHP) 2 . We have determined the amino-acid sequence and crystal structure of DHP and find that its fold is typical of the globin family, indicating that the enzyme evolved from an oxygen carrier protein. Residues at the dimer interface do not correspond to those in tetrameric and dimeric haemoglobins and the spatial arrangement of the dimers is different. The complete amino-acid sequence is most similar to that of myoglobin from the sea hare, with 20.6% identity among 126 overlapping amino acids.