Mitchell D T, Kitto G B, Hackert M L
Department of Chemistry and Biochemistry, University of Texas at Austin 78712, USA.
J Mol Biol. 1995 Aug 18;251(3):421-31. doi: 10.1006/jmbi.1995.0445.
The X-ray structures of two hemoglobins (Hb) from the sea cucumber Caudina arenicola (an echinoderm) have been determined: a low spin, hemichrome, monomeric Hb-C chain, and a cyanomet-liganded dimeric Hb-D chain. Attempts to obtain crystal structures of the deoxy-liganded and hemichrome forms from the same chain type have not been successful. In this work, the Hb-C chain and Hb-D chain structures are compared, and differences observed in tertiary structure related to the different ligand states for hemoglobin chains from this organism. In addition to shifts of the distal histidine and E helix, differences are noted in the position of the heme group within the heme pocket, the hydrogen bonding of the heme group to the protein, and the status of the D helix. These differences are important in understanding the ligand-linked association states of these hemoglobins. The quaternary structure of the Hb-D homodimer is compared with those from two other invertebrate hemoglobins from Scapharca inaequivalvis and Urechis caupo, which also have subunit-subunit interactions that involve the E and E' helices. The dimer interactions of the Caudina and Urechis hemoglobins are quite dissimilar. However, the dimer interface observed in cyanomet Hb-D is strikingly similar to that observed for the carbonmonoxy hemoglobin dimer from the clam, Scapharca, yet many of the key amino acid residues implicated in the cooperative mechanism of the Scapharca hemoglobin are not conserved in the Caudina hemoglobins.
已确定了来自海黄瓜（一种棘皮动物）的两种血红蛋白（Hb）的X射线结构：一种是低自旋、高铁血红素单体Hb-C链，另一种是氰化高铁配体的二聚体Hb-D链。尝试从相同链类型获得脱氧配体和高铁血红素形式的晶体结构未成功。在这项工作中，对Hb-C链和Hb-D链结构进行了比较，并观察到与该生物体血红蛋白链不同配体状态相关的三级结构差异。除了远端组氨酸和E螺旋的位移外，还注意到血红素口袋内血红素基团的位置、血红素基团与蛋白质的氢键以及D螺旋的状态存在差异。这些差异对于理解这些血红蛋白的配体连接缔合状态很重要。将Hb-D同型二聚体的四级结构与来自不等边毛蚶和加州星虫的另外两种无脊椎动物血红蛋白的四级结构进行了比较，它们也具有涉及E和E'螺旋的亚基-亚基相互作用。海黄瓜和加州星虫血红蛋白的二聚体相互作用非常不同。然而，在氰化高铁Hb-D中观察到的二聚体界面与在毛蚶的一氧化碳血红蛋白二聚体中观察到的界面惊人地相似，然而，许多与毛蚶血红蛋白协同机制有关的关键氨基酸残基在海黄瓜血红蛋白中并不保守。
