Complete amino acid sequence of the Glycera dibranchiata monomer hemoglobin component IV: structural implications.

The globin derived from the monomer Component IV hemoglobin of the marine amnelid, Glycera dibranchiata, has been completely sequenced, and the resulting information has been used to create a structural model of the protein. The most important result is that the consensus sequence of Component IV differs by 3 amino acids from a cDNA-predicted amino acid sequence thought earlier to encode the Component IV hemoglobin. This work reveals that the histidine (E7), typical of most heme-containing globins, is replaced by leucine in Component IV. Also significant is that this sequence is not identical to any of the previously reported Glycera dibranchiata monomer hemoglobin sequences, including the sequence from a previously reported crystal structure, but has high identity to all. A three-dimensional structural model for monomer Component IV hemoglobin was constructed using the published 1.5 A crystal structure of a monomer hemoglobin from Glycera dibranchiata as a template. The model shows several interesting features: (1) a Phe31 (B10) that is positioned in the active site; (2) a His39 occurs in an interhelical region occupied by Pro in 98.2% of reported globin sequences; and (3) a Met41 is found at a position that emerges from this work as a previously unrecognized heme contact.