STRUCTURAL-ANALYSIS OF MONOMERIC HEMICHROME AND DIMERIC CYANOMET HEMOGLOBINS FROM CAUDINA-ARENICOLA

The X-ray structures of two hemoglobins (Hb) from the sea cucumber Cau dina arenicola (an echinoderm) have been determined: a low spin, hemic hrome, monomeric Hb-C chain, and a cyanomet-liganded dimeric Hb-D chai n. Attempts to obtain crystal structures of the deoxy-liganded and hem ichrome forms from the same chain type have not been successful. In th is work, the Hb-C chain and Hb-D chain structures are compared, and di fferences observed in tertiary structure related to the different liga nd states for hemoglobin chains from this organism. In addition to shi fts of the distal histidine and E helix, differences are noted in the position of the heme group within the heme pocket, the hydrogen bondin g of the heme group to the protein, and the status of the D helix. The se differences are important in understanding the ligand-linked associ ation states of these hemoglobins. The quaternary structure of the Hb- D homodimer is compared with those from two other invertebrate hemoglo bins from Scapharca inaequivalvis and Urechis caupo, which also have s ubunit-subunit interactions that involve the E and E' helices. The dim er interactions of the Caudina and Urechis hemoglobins are quite dissi milar. However, the dimer interface observed in cyanomet Hb-D is strik ingly similar to that observed for the carbonmonoxy hemoglobin dimer f rom the clam, Scapharca, yet many of the key amino acid residues impli cated in the cooperative mechanism of the Scapharca hemoglobin are not conserved in the Caudina hemoglobins.