A possible new mechanism of oxygen affinity modulation in mammalian hemoglobins

doi:10.1016/0301-4622(90)88014-J

Biophysical Chemistry

Volume 37, Issues 1–3, 31 August 1990, Pages 141-146

https://doi.org/10.1016/0301-4622(90)88014-J Get rights and content

Abstract

Bovine red cells do not contain appreciable amounts of 2,3-diphosphoglycerate (2,3-DPG). Bovine hemoglobin, however, has a particular sensitivity to chloride ions and as a result it can attain oxygen affinity values lower than those measured for human hemoglobin in the presence of 2,3-DPG. The interaction of bovine hemoglobin with anions is modulated by the hydrophobic characteristics of the protein. Comparison of the hydropathy plots of primate and ruminant hemoglobins indicates constant regions of opposite hydrophobicity, which have fixed amino acid differences. A model is proposed for explaining the regulation of oxygen affinity by chlorides, as an alternative to the classic modulation by 2,3-DPG.

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In cold-tolerant mammals (including reindeer, horse and cattle) the low temperature sensitivities have been suggested to correlate with the presence of the additional Cl− binding site compared to human Hb. In contrast to human Hb, where chloride ions predominantly bind at one α-chain site (involving Val1α and Ser131α) and one β-chain site (involving Val1β and Lys82β) (O'Donnell et al., 1979; Riggs, 1988), Hbs from these species have a cluster of positively charged (His and Lys) amino acid residues at positions 8, 76 and 77 of the β-chains (Fronticelli, 1990) (Table 1), that potentially increases the endothermic contribution of Cl− release upon O2 binding. In accordance with this contention, a human Hb mutant constructed to contain ‘bovine’, β-chain amino acid substitutions including Ala76β→Lys, displayed a heightened Cl− sensitivity (Fronticelli et al., 1995) — although these substitutions did not reduce O2 affinity to the level found in bovine Hb (Baudin-Creuza et al., 2002).
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A novel hemoglobin variant was observed in pure sheep (Ovis aries) breeds of the island of Chios (Greece), Egypt and Hungary. This silent variant was identified by gel electrophoresis and RP-HPLC of dissociated globin chains. Two Arg for Lys substitutions were detected, by means of MALDI TOF electrospray mass spectrometric analysis for the intact globins, at positions β66(E10) and β144(HC1) of a globin chain having the sequence of the β^B chain. Sequencing of the β-globin gene confirmed the variant gene as being an allele of the HBBB locus having the AAG → AGG and the AAA → AGA mutations at codons 66 and 144, respectively, both corresponding to the Lys → Arg substitution. The intrinsic oxygen affinity of the variant Hb (logP₅₀ = 0.79 at pH 7.0) was found to be intermediate between that of the sheep Hb B (logP₅₀ = 0.92) and that of Cypriot mouflon (O. a. ophion) Hb (logP₅₀ = 0.53), the latter having only the Lys → Arg change at β144, whereas nearly no differences were observed in the presence of the Cl⁻ physiological effector. Result supports the indication that Arg at β144 enhances the role of the ligand in decreasing oxygen affinity, this effect being partially counteracted when Arg is at β66. Data also shows that the Lys → Arg change at β66 is responsible for 1.49 fold reduction in the intrinsic oxygen affinity. This hitherto undescribed variant increases to seven the number of alleles at the sheep HBBB locus. Following the nomenclature used for human Hb variants, the new allele was termed as the Hb Chios or [β^B66(E10) Lys → Arg, 144(HC1)Lys → Arg], whereas the proposed genetic nomenclature of the locus is HBBK.
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A possible new mechanism of oxygen affinity modulation in mammalian hemoglobins

Abstract

J. Biol. Chem.

J. Mol. Biol.

J. Mol. Biol.

J. Mol. Biol.