An Order-Disorder Transition Plays a Role in Switching Off the Root Effect in Fish Hemoglobins*

• Published in: Journal of Biological Chemistry
• Main Authors: Vergara, Alessandro, Vitagliano, Luigi, Merlino, Antonello, Sica, Filomena, Marino, Katia, Verde, Cinzia, di Prisco, Guido, Mazzarella, Lelio
• Format: Text
• Language: English
• Published: American Society for Biochemistry and Molecular Biology 2010
• Subjects: Protein Structure and Folding; Antarctic; The Antarctic; Antarc*
• Online Access: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2952259; http://www.ncbi.nlm.nih.gov/pubmed/20610398; https://doi.org/10.1074/jbc.M110.143537

The Root effect is a widespread property among fish hemoglobins whose structural basis remains largely obscure. Here we report a crystallographic and spectroscopic characterization of the non-Root-effect hemoglobin isolated from the Antarctic fish Trematomus newnesi in the deoxygenated form. The crystal structure unveils that the T state of this hemoglobin is stabilized by a strong H-bond between the side chains of Asp95α and Asp101β at the α1β2 and α2β1 interfaces. This unexpected finding undermines the accepted paradigm that correlates the presence of this unusual H-bond with the occurrence of the Root effect. Surprisingly, the T state is characterized by an atypical flexibility of two α chains within the tetramer. Indeed, regions such as the CDα corner and the EFα pocket, which are normally well ordered in the T state of tetrameric hemoglobins, display high B-factors and non-continuous electron densities. This flexibility also leads to unusual distances between the heme iron and the proximal and distal His residues. These observations are in line with Raman micro-spectroscopy studies carried out both in solution and in the crystal state. The findings here presented suggest that in fish hemoglobins the Root effect may be switched off through a significant destabilization of the T state regardless of the presence of the inter-aspartic H-bond. Similar mechanisms may also operate for other non-Root effect hemoglobins. The implications of the flexibility of the CDα corner for the mechanism of the T-R transition in tetrameric hemoglobins are also discussed.