Deer mouse hemoglobin exhibits a lowered oxygen affinity owing to mobility of the E helix

Commentary: Hierarchical reductionism approach to understanding adaptive variation in animal performance

Aerobic capacity is a complex performance trait with important consequences for fitness, and is determined by the integrated function of the O₂ transport pathway. The components of the O₂ pathway interact and function as an integrated physiological system, which could strongly influence the contribution of each component to variation in aerobic capacity. In this commentary, we highlight the value of hierarchical reductionism - combining studies of how component parts work in isolation with studies of how components interact within integrated systems - for understanding the evolution of aerobic capacity. This is achieved by focussing on the role of haemoglobin in adaptive increases in aerobic capacity in high-altitude deer mice (Peromyscus maniculatus). High-altitude deer mice have evolved increased aerobic capacity in hypoxia, in association with evolved changes in several subordinate traits across the O₂ pathway. This includes an evolved increase in Hb-O₂ affinity - which helps safeguard arterial O₂ saturation in hypoxia - and reductionist approaches have been successful at identifying the genetic, structural, and biochemical underpinnings of variation in this trait. However, theoretical modelling and empirical measurements suggest that increased Hb-O₂ affinity may not augment aerobic capacity on its own. The adaptive benefit of increased Hb-O₂ affinity in high-altitude deer mice appears to have been contingent upon antecedent changes in other traits in the O₂ pathway, particularly an increased capacity for O₂ diffusion and utilization in active tissues. These findings highlight the importance of understanding the interactions between the components of integrated systems for fully appreciating the evolution of complex performance phenotypes.

Crystal structure of hemoglobin from mouse (Mus musculus) compared with those from other small animals and humans

Mice (Mus musculus) are nocturnal small animals belonging to the rodent family that live in burrows, an environment in which significantly high CO2 levels prevail. It is expected that mouse hemoglobin (Hb) plays an important role in their adaptation to living in such a high-CO2 environment, while many other species cannot. In the present study, mouse Hb was purified and crystallized at a physiological pH of 7 in the orthorhombic space group P212121; the crystals diffracted to 2.8 Å resolution. The primary amino-acid sequence and crystal structure of mouse Hb were compared with those of mammalian Hbs in order to investigate the structure-function relationship of mouse Hb. Differences were observed from guinea pig Hb in terms of amino-acid sequence and from cat Hb in overall structure (in terms of r.m.s.d.). The difference in r.m.s.d. from cat Hb may be due to the existence of the molecule in a conformation other than the R-state. Analysis of tertiary- and quaternary-structural features, the α1β2 interface region and the heme environment without any ligands in all four heme groups showed that mouse methemoglobin is in an intermediate state between the R-state and the T-state that is much closer to the R-state conformation.

Alteration of the α1β2/α2β1 subunit interface contributes to the increased hemoglobin-oxygen affinity of high-altitude deer mice

BACKGROUND

Deer mice (Peromyscus maniculatus) that are native to high altitudes in the Rocky Mountains have evolved hemoglobins with an increased oxygen-binding affinity relative to those of lowland conspecifics. To elucidate the molecular mechanisms responsible for the evolved increase in hemoglobin-oxygen affinity, the crystal structure of the highland hemoglobin variant was solved and compared with the previously reported structure for the lowland variant.

RESULTS

Highland hemoglobin yielded at least two crystal types, in which the longest axes were 507 and 230 Å. Using the smaller unit cell crystal, the structure was solved at 2.2 Å resolution. The asymmetric unit contained two tetrameric hemoglobin molecules.

CONCLUSIONS

The analyses revealed that αPro50 in the highland hemoglobin variant promoted a stable interaction between αHis45 and heme that was not seen in the αHis50 lowland variant. The αPro50 mutation also altered the nature of atomic contacts at the α1β2/α2β1 intersubunit interfaces. These results demonstrate how affinity-altering changes in intersubunit interactions can be produced by mutations at structurally remote sites.

Peromyscus as a model of human disease

Animals of the genus Peromyscus have been a particularly informative model for many areas of study, including behavior, evolution, anatomy, physiology and genetics. While their use in modeling human disease and pathology has been relatively restricted, certain qualities of Peromyscine mice may make them a good candidate for such studies. Pathophysiological conditions where Peromyscus may be of particular value involve aging, reactive oxygen species-associated pathologies, metabolism and detoxification, diabetes, and certain cancers. In this review article we will summarize pathological conditions where Peromyscus have been used effectively, we will discuss factors limiting the use of Peromyscus in studying pathology and we will indicate areas at which the use of this model may be of special value.

Bohr effect and temperature sensitivity of hemoglobins from highland and lowland deer mice

An important means of physiological adaptation to environmental hypoxia is an increased oxygen (O2) affinity of the hemoglobin (Hb) that can help secure high O2 saturation of arterial blood. However, the trade-off associated with a high Hb-O2 affinity is that it can compromise O2 unloading in the systemic capillaries. High-altitude deer mice (Peromyscus maniculatus) have evolved an increased Hb-O2 affinity relative to lowland conspecifics, but it is not known whether they have also evolved compensatory mechanisms to facilitate O2 unloading to respiring tissues. Here we investigate the effects of pH (Bohr effect) and temperature on the O2-affinity of high- and low-altitude deer mouse Hb variants, as these properties can potentially facilitate O2 unloading to metabolizing tissues. Our experiments revealed that Bohr factors for the high- and low-altitude Hb variants are very similar in spite of the differences in O2-affinity. The Bohr factors of deer mouse Hbs are also comparable to those of other mammalian Hbs. In contrast, the high- and low-altitude variants of deer mouse Hb exhibited similarly low temperature sensitivities that were independent of red blood cell anionic cofactors, suggesting an appreciable endothermic allosteric transition upon oxygenation. In conclusion, high-altitude deer mice have evolved an adaptive increase in Hb-O2 affinity, but this is not associated with compensatory changes in sensitivity to changes in pH or temperature. Instead, it appears that the elevated Hb-O2 affinity in high-altitude deer mice is compensated by an associated increase in the tissue diffusion capacity of O2 (via increased muscle capillarization), which promotes O2 unloading.

Intraspecific polymorphism, interspecific divergence, and the origins of function-altering mutations in deer mouse hemoglobin

Major challenges for illuminating the genetic basis of phenotypic evolution are to identify causative mutations, to quantify their functional effects, to trace their origins as new or preexisting variants, and to assess the manner in which segregating variation is transduced into species differences. Here, we report an experimental analysis of genetic variation in hemoglobin (Hb) function within and among species of Peromyscus mice that are native to different elevations. A multilocus survey of sequence variation in the duplicated HBA and HBB genes in Peromyscus maniculatus revealed that function-altering amino acid variants are widely shared among geographically disparate populations from different elevations, and numerous amino acid polymorphisms are also shared with closely related species. Variation in Hb-O₂ affinity within and among populations of P. maniculatus is attributable to numerous amino acid mutations that have individually small effects. One especially surprising feature of the Hb polymorphism in P. maniculatus is that an appreciable fraction of functional standing variation in the two transcriptionally active HBA paralogs is attributable to recurrent gene conversion from a tandemly linked HBA pseudogene. Moreover, transpecific polymorphism in the duplicated HBA genes is not solely attributable to incomplete lineage sorting or introgressive hybridization; instead, it is mainly attributable to recurrent interparalog gene conversion that has occurred independently in different species. Partly as a result of concerted evolution between tandemly duplicated globin genes, the same amino acid changes that contribute to variation in Hb function within P. maniculatus also contribute to divergence in Hb function among different species of Peromyscus. In the case of function-altering Hb mutations in Peromyscus, there is no qualitative or quantitative distinction between segregating variants within species and fixed differences between species.

Epistasis among adaptive mutations in deer mouse hemoglobin

Epistatic interactions between mutant sites in the same protein can exert a strong influence on pathways of molecular evolution. We performed protein engineering experiments that revealed pervasive epistasis among segregating amino acid variants that contribute to adaptive functional variation in deer mouse hemoglobin (Hb). Amino acid mutations increased or decreased Hb-O2 affinity depending on the allelic state of other sites. Structural analysis revealed that epistasis for Hb-O2 affinity and allosteric regulatory control is attributable to indirect interactions between structurally remote sites. The prevalence of sign epistasis for fitness-related biochemical phenotypes has important implications for the evolutionary dynamics of protein polymorphism in natural populations.

Deer mouse hemoglobin exhibits a lowered oxygen affinity owing to mobility of the E helix. Corrigendum

A corrigendum to the article by Inoguchi et al. [(2013). Acta Cryst. F69, 393–398].

The affiliation of two authors in the article by Inoguchi et al. [(2013). Acta Cryst. F69, 393–398] is corrected.