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Evolved Increases in Hemoglobin-Oxygen Affinity and the Bohr Effect Coincided with the Aquatic Specialization of Penguins

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Evolved Increases in Hemoglobin-Oxygen Affinity and the Bohr Effect Coincided with the Aquatic Specialization of Penguins Evolved increases in hemoglobin-oxygen affinity and the Bohr effect coincided with the aquatic specialization of penguins Anthony V. Signorea,1, Michael S. Tiftb, Federico G. Hoffmannc,d, Todd. L. Schmitte, Hideaki Moriyamaa, and Jay F. Storza aSchool of Biological Sciences, University of Nebraska, Lincoln, NE 68588; bDepartment of Biology and Marine Biology, University of North Carolina, Wilmington, NC 28403; cDepartment of Biochemistry, Molecular Biology, Entomology, and Plant Pathology, Mississippi State University, Starkville, MS 39762; dInstitute for Genomics, Biocomputing and Biotechnology, Mississippi State University, Starkville, MS 39762; and eVeterinary Services, SeaWorld of California, San Diego, CA 92109 Edited by George N. Somero, Stanford University, Pacific Grove, CA, and approved February 12, 2021 (received for review November 18, 2020) Dive capacities of air-breathing vertebrates are dictated by onboard Hb-O2 affinity have contributed to enhanced diving capacity (1). O2 stores, suggesting that physiologic specialization of diving birds It has been hypothesized that increased Hb-O2 affinity may im- such as penguins may have involved adaptive changes in convective prove pulmonary O2 extraction in diving mammals, thereby en- O2 transport. It has been hypothesized that increased hemoglobin hancing diving capacity (5), but more comparative data are (Hb)-O2 affinity improves pulmonary O2 extraction and enhances needed to assess evidence for an adaptive trend (6, 7). Experi- the capacity for breath-hold diving. To investigate evolved changes mental measurements on whole blood suggest that the emperor in Hb function associated with the aquatic specialization of pen- penguin (Aptenodytes forsteri) may have a higher blood-O2 af- guins, we integrated comparative measurements of whole-blood finity relative to nondiving waterbirds, a finding that has fostered and purified native Hb with protein engineering experiments based the view that this is a property characterizing penguins as a group on site-directed mutagenesis. We reconstructed and resurrected an- (8–10). However, blood-O2 affinity is a highly plastic trait that is cestral Hb representing the common ancestor of penguins and the influenced by changes in red cell metabolism and acid-base more ancient ancestor shared by penguins and their closest non- balance, so measurements on purified Hb under standard assay EVOLUTION diving relatives (order Procellariiformes, which includes albatrosses, conditions are needed to assess whether observed species dif- shearwaters, petrels, and storm petrels). These two ancestors ferences in blood-O2 affinity stem from genetically based bracket the phylogenetic interval in which penguin-specific changes changes in the oxygenation properties of Hb. Moreover, even if in Hb function would have evolved. The experiments revealed that species differences in Hb-O affinity are genetically based, penguins evolved a derived increase in Hb-O affinity and a greatly 2 2 comparative data from extant taxa do not reveal whether ob- augmented Bohr effect (i.e., reduced Hb-O affinity at low pH). Al- 2 served differences are attributable to a derived increase in pen- though an increased Hb-O affinity reduces the gradient for O dif- 2 2 guins, a derived reduction in their nondiving relatives, or a fusion from systemic capillaries to metabolizing cells, this can be compensated by a concomitant enhancement of the Bohr effect, combination of changes in both directions. To investigate evolved changes in Hb function associated with thereby promoting O2 unloading in acidified tissues. We suggest that the evolved increase in Hb-O affinity in combination with the aquatic specialization of penguins, we integrated experimen- 2 tal measurements of whole-blood and purified native Hb with the augmented Bohr effect maximizes both O2 extraction from the lungs and O2 unloading from the blood, allowing penguins to fully utilize their onboard O2 stores and maximize underwater Significance foraging time. In diving birds like penguins, physiologic considerations sug- hemoglobin | hypoxia | penguins | Bohr effect | adaptation gest that increased hemoglobin (Hb)-O2 affinity may improve pulmonary O2 extraction and enhance dive capacity. We inte- n air-breathing vertebrates, diving capacities are dictated by grated experimental tests on whole-blood and native Hbs of penguins with protein engineering experiments on recon- Ionboard O2 stores and the efficiency of O2 use in metabolizing tissues (1). In fully aquatic taxa, selection to prolong breath-hold structed ancestral Hbs. The experiments involving ancestral submergence and underwater foraging time may have promoted protein resurrection enabled us to test for evolved changes in Hb function in the stem lineage of penguins after divergence adaptive changes in multiple components of the O2 transport pathway, including oxygenation properties of hemoglobin (Hb). from their closest nondiving relatives. We demonstrate that Vertebrate Hb is a tetrameric protein that is responsible for penguins evolved an increased Hb-O2 affinity in conjunction with a greatly augmented Bohr effect (i.e., reduction in Hb-O2 circulatory O2 transport, loading O2 in pulmonary capillaries and affinity at low pH) that should maximize pulmonary O2 ex- unloading O2 in the systemic circulation via quaternary structural shifts between a high-affinity (predominately oxygenated) re- traction without compromising O2 delivery at systemic capillaries. laxed (R) state and a low-affinity (predominately deoxygenated) Author contributions: A.V.S. and J.F.S. designed research; A.V.S., M.S.T., F.G.H., T.L.S., and tense (T) state (2). While this mechanism of respiratory gas H.M. performed research; M.S.T. and J.F.S. contributed new reagents/analytic tools; transport is conserved in all vertebrate Hbs, amino acid variation A.V.S., M.S.T., F.G.H., H.M., and J.F.S. analyzed data; and A.V.S. and J.F.S. wrote the paper. in the constituent α- and β-type subunits may alter intrinsic O2 The authors declare no competing interest. affinity and the responsiveness to changes in temperature, red This article is a PNAS Direct Submission. cell pH, and red cell concentrations of allosteric cofactors This open access article is distributed under Creative Commons Attribution License 4.0 (nonheme ligands that modulate Hb-O2 affinity by preferentially (CC BY). binding and stabilizing the deoxy T conformation) (3, 4). 1To whom correspondence may be addressed. Email: [email protected]. While the quantity of Hb is typically increased in the blood of This article contains supporting information online at https://www.pnas.org/lookup/suppl/ diving birds and mammals compared with their terrestrial rela- doi:10.1073/pnas.2023936118/-/DCSupplemental. tives, there is no consensus on whether evolved changes in Published March 22, 2021. PNAS 2021 Vol. 118 No. 13 e2023936118 https://doi.org/10.1073/pnas.2023936118 | 1of6 Downloaded by guest on September 24, 2021 evolutionary analyses of globin sequence variation. To character- between closely related species of low- and high-altitude birds ize the mechanistic basis of evolved changes in Hb function in the (14–18, 20–22). Similar to the case of other diving vertebrates stem lineage of penguins, we performed protein engineering ex- (29), the Bohr effect of penguin Hb also greatly exceeds typical periments on reconstructed and resurrected ancestral Hb repre- avian values. senting the common ancestor of penguins and the more ancient ancestor shared by penguins and their closest nondiving relatives Ancestral Protein Resurrection. In principle, the observed differ- (order Procellariiformes, which includes albatrosses, shearwaters, ence in Hb-O2 affinity between penguins and their closest non- petrels, and storm petrels) (Fig. 1). These two ancestors bracket diving relatives could be explained by a derived increase in the phylogenetic interval in which penguin-specific changes in Hb Hb-O2 affinity in the penguin lineage (the generally assumed function would have evolved. adaptive scenario), a derived reduction in the stem lineage of Procellariiformes (the nondiving sister group), or a combination Results and Discussion of changes in both directions. To test these alternative hypoth- O2-Binding Properties of Penguin Whole-Blood and Purified Hb. Using eses, we reconstructed the Hbs of the common ancestor of blood samples from multiple individuals of six penguin species, penguins (AncSphen) and the more ancient common ancestor of we measured the partial pressure of O2 (PO2) at 50% saturation Procellariimorphae (the superorder comprising Sphenisciformes (P50) for whole-blood and purified Hb in the absence (stripped) [penguins] and Procellariiformes; AncPro) (Fig. 1 and SI Appen- and presence of allosteric cofactors (+KCl +IHP [inositol dix,Figs.S2–S4). We then recombinantly expressed and purified hexaphosphate]) (Fig. 2). Whole-blood P50 values were similar the ancestral Hb to perform in vitro functional tests. Measure- across all penguins, averaging 33.3 ± 1.1 torr (Fig. 2 and SI Ap- ments of O2 equilibrium curves revealed that the AncSphen Hb pendix,TableS1), consistent with previously published data for has a significantly higher O2 affinity than AncPro Hb (Fig. 3), emperor, Adélie, chinstrap, and gentoo penguins (8, 9, 11). indicating that penguins evolved a derived increase in Hb-O2 af- Similarly, measured O2 affinities for purified Hb exhibited very finity. In the presence of allosteric cofactors, the P50 of AncSphen little variation among species
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