The crystal structure of a tetrameric hemoglobin in a partial hemichrome state

• Published in: Proceedings of the National Academy of Sciences
• Main Authors: Riccio, Antonio, Vitagliano, Luigi, di Prisco, Guido, Zagari, Adriana, Mazzarella, Lelio
• Format: Text
• Language: English
• Published: The National Academy of Sciences 2002
• Subjects: Biological Sciences; Antarctic; The Antarctic; Antarc*
• Online Access: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC125021; http://www.ncbi.nlm.nih.gov/pubmed/12093902; https://doi.org/10.1073/pnas.132182099

Tetrameric hemoglobins are the most widely used systems in studying protein cooperativity. Allosteric effects in hemoglobins arise from the switch between a relaxed (R) state and a tense (T) state occurring upon oxygen release. Here we report the 2.0-Å crystal structure of the main hemoglobin component of the Antarctic fish Trematomus newnesi, in a partial hemichrome form. The two α-subunit iron atoms are bound to a CO molecule, whereas in the β subunits the distal histidine residue is the sixth ligand of the heme iron. This structure, a tetrameric hemoglobin in the hemichrome state, demonstrates that the iron coordination by the distal histidine, usually associated with denaturing states, may be tolerated in a native-like hemoglobin structure. In addition, several features of the tertiary and quaternary organization of this structure are intermediate between the R and T states and agree well with the R → T transition state properties obtained by spectroscopic and kinetic techniques. The analysis of this structure provides a detailed pathway of heme–heme communication and it indicates that the plasticity of the β heme pocket plays a role in the R → T transition of tetrameric hemoglobins.