Department of Biochemistry, Molecular Biology, Entomology, and Plant Pathology, Mississippi State University, Starkville, MS 39762, USAInstitute for Genomics, Biocomputing, and Biotechnology, Mississippi State University, Starkville, MS 39762, USA
History of structural changes in the adult-expressed HBD and HBB β-globin genes of feliform carnivores. The ancestor of laurasiatherian mammals [the supraordinal group that includes carnivores, as well as bats, pangolins, eulipotyphlans (shrews, moles and hedgehogs), perissodactyls and cetartiodactyls] possessed a tandemly linked pair of HBD and HBB genes. In the stem lineage of carnivores, an unequal cross-over event produced a duplication in the chromosome now containing a chimeric HBB/D fusion gene, flanked by the parental HBD and HBB genes on the 5′ and 3′ sides, respectively (Gaudry et al., 2014). In the common ancestor of felids (or possibly in the common ancestor of all feliform carnivores), the 5′ HBD gene was deleted after divergence from the ancestor of caniform carnivores. Consequently, adult cats co-express two structurally distinct hemoglobin (Hb) isoforms: HbA (which incorporates β-chain products of the chimeric HBB/D gene) and HbB (which incorporates β-chain products of the HBB gene). The two isoforms share identical α-chain subunits.
Isoelectric focusing gels (pH3–9) and isoHb multiplicity in big cats. (A) Red blood cell lysates from lion, leopard, jaguar, tiger and snow leopard, and (B) purified HbA and HbB components from snow leopard and lion, showing highly similar patterns. The putative α- and β-subunit composition of each of the three major bands is indicated, with βA and βB corresponding to the products of genes HBB/D and HBB, respectively. Plus sign, cathode; minus sign, anode.
Hb–O2 equilibrium curves of big cats. Hb–O2 equilibrium curves of African lion (A) and snow leopard (B) HbA and HbB are insensitive to 2,3-diphosphoglycerate (DPG) and are right-shifted compared with those of human adult Hb (open symbols, dotted lines). Data were obtained at a heme concentration of 0.3 mmol l−1, 37°C, in 0.1 mol l−1 Hepes buffer, pH 7.4, in the absence (circles) and presence of DPG (triangles), at a 2-fold molar excess of DPG over tetrameric Hb. Non-linear regression of the O2-saturation data according to the sigmoidal Hill equation is shown for each of the conditions used. 1 Torr≈133 Pa.
Allosteric regulation of Hb–O2 affinity (P50) by DPG and Cl− ions in African lion and snow leopard HbA and HbB. Sensitivity to DPG and Cl− anions is indexed by the difference in log-transformed P50 values (reported in Table 1) measured in the presence and absence (stripped) of each ionic cofactor. Human Hb–O2 affinity with a high sensitivity to both DPG and Cl− is shown as a comparison.