Tetrameric hemoglobins represent the most commonly used model for the description of the basic concepts of protein allostery. The classical stereochemical model assumes a concerted transition of the protein, upon oxygen release, from the relaxed (R) to the tense (T) state. Despite the large amount of data accumulated on the end-points of the transition, scarce structural information is available on the intermediate species along the pathway. Here we report a spectroscopic characterization of the autoxidation process of the Trematomus newnesi major Hb component and the atomic resolution structure (1.25 A) of an intermediate form along the pathway characterized by a different binding and oxidation state of the alpha and beta chains. In contrast to the alpha-heme iron, which binds a CO molecule, the beta iron displays a pentacoordinated oxidized state, which is rare in tetrameric hemoglobins. Interestingly, the information provided by the present analysis is not limited to the characterization of the peculiar oxidation process of Antarctic fish hemoglobins. Indeed, this structure represents the most detailed snapshot of hemoglobin allosteric transition hitherto achieved. Upon ligand release at the beta heme, a cascade of structural events is observed. Notably, several structural features of the tertiary structure of the alpha and beta chains closely resemble those typically observed in the deoxygenated state. The overall quaternary structure also becomes intermediate between the R and the T state. The analysis of the alterations induced by the ligand release provides a clear picture of the temporal sequence of the events associated with the transition. The implications of the present findings have also been discussed in the wider context of tetrameric Hbs.
The Root effect is a widespread property in fish hemoglobins (Hbs) that produces a drastic reduction of cooperativity and oxygen-binding ability at acidic pH. Here, we report the high-resolution structure of the deoxy form of Hb isolated from the Antarctic fish Trematomus bernacchii (HbTb) crystallized at pH 6.2 and 8.4. The structure at acidic pH has been previously determined at a moderate resolution (Ito et al., J Mol Biol 1995;250:648-658). Our results provide a clear picture of the events occurring upon the pH increase from 6.2 to 8.4, observed within a practically unchanged crystal environment. In particular, at pH 8.4, the interaspartic hydrogen bond at the alpha(1)beta(2) interface is partially broken, suggesting a pK(a) close to 8.4 for Asp95alpha. In addition, a detailed survey of the histidine modifications, caused by the change in pH, also indicates that at least three hot regions of the molecule are modified (Ebeta helix, Cbeta-tail, CDalpha corner) and can be considered to be involved at various levels in the release of the Root protons. Most importantly, at the CDalpha corner, the break of the salt bridge Asp48alpha-His55alpha allows us to describe a detailed mechanism that transmits the modification from the CDalpha corner far to the alpha heme. More generally, the results shed light on the role played by the histidine residues in modulating the strength of the Root effect and also support the emerging idea that the structural determinants, at least for a part of the Root effect, are specific of each Hb endowed with this property.
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