Structural Effect of Gγ and Aγ Globin Chains in Fetal Hemoglobin Tetramer

Introduction. Hemoglobin (Hb) is a tetrameric oxigen and carbon dioxide transportation protein; it is composed of four polypeptide chains, two of alpha type, which can be ζ, α₂ or α₁; and two beta chains, which include ε, ^Gγ, ^Aγ, δ or β (Antonini and Brunori, 1970).The synthesis of these globin chains depends on the stage of human development; in embryonic stages the hemoglobins Gower I (ζ₂ε₂), Gower II (α₂ε₂) and Portland (ζ₂γ₂) are synthesized; subsequently, from the third week of gestation begins the synthesis of fetal hemoglobin (HbF) formed by two alpha chains and two gamma chains (α₂γ₂); finally during the last trimester the second hemoglobin switching takes place consisting on decreased synthesis of gamma globin chains and the beginning of beta and delta globin chain synthesis that will be forming adult hemoglobin (α₂β₂) and hemoglobin A₂ (α₂δ₂), which will remain constant throughout extrauterine life (Wilber, 2011).

One of the molecular mechanisms that has allowed the evolutionary success of humans has been gene duplication, although it results from a random process it favors DNA variation that increases the adaptive capacity of a population (Magadum, 2013). The high homology between duplicated genes is clear, however, it allows differences to arise because of genetic conversion, globin genes HBG2, HBG1, for instance ,which can be observed in their functional products; ^Gγ and ^Aγ globin chains (Chen, 2007), both formed by 146 residues, with their distinct difference a position 136 where there is a change of glycine in ^Gγ by alanine in ^Aγ and maintain a ratio of 3:1 (^Gγ: ^Aγ) during fetal stages; while in extra uterine life this ratio is reversed to 2:3 (^Gγ: ^Aγ), the heterogeneity of γ globins available for HbF synthesis allows the formation of three variants of the same molecule α₂^Gγ₂; ₂α^A/Gγ and ₂α^Aγ₂; these γ globin chains differences potential implication on HbF interaction with oxygen has not been elucidated as well as whether the presence of a specific type occurs to a greater or lesser extent in different pathologies (Alter 1979).

Objective. To evaluate the structural effect of the type of γ globin chain in three models of Fetal Hemoglobin.

Methodology. Modeling of the three HbF possible structures: α₂^Gγ₂; ₂α^A/Gγ and ₂α^Aγ₂ was performed on UCSF Chimera using the crystallography structures of a HbA (1a3n) and Hb Bart (1i3d) hemoglobins as atomic position templates of α and ^Aγ globin chains. HbF models as well as HbA and Bart hemoglobin (as reference) where then uploaded to CHARMM-GUI platform to establish system conditions for molecular dynamic simulation such as force field (c36m), water box size (90 Å) and temperature (310 K), molecular dynamics simulation was run for 100 ns on GROMCS (by triplicate), results were analyzed by Root Mean Square Deviation (RMDS) and Root Mean Square Fluctuation (RMSF) of γ globin chains.

Results. Three different HbF tetrameric structures where created based on γ globin chains available α₂^Gγ₂; ₂α^A/Gγ and ₂α^Aγ₂, RMSD shows fluctuation in atomic positions through time among the three HbF models (figure 1: a, b, c), it is shown that the HbF behavior most similar to HbA (figure 1d) is the ₂α^A/Gγ (figure 1b) which could indicate that this particular type of HbF would be the more abundant during healthy extrauterine life since HbA responds to a normoxic environment that can become hypoxic at medular level when some disease are present, leukemia for instance.

By comparing Bart hemoglobin to HbF models is clear that ^Aγ reduces residues movement, limiting tetramer flexibility thus restraining allosteric changes needed for gases exchange, which can be seen when analyzing HbF γ globin chains at 100-146 residues region where there is no fluctuation in the ^Aγ₂ chains opposite to what is shown by ^Gγ₂ (Figure 2a, b and c). To evaluate these RMSD results, cluster analysis was performed for each replica in the three HbF structures, resulting in values of α₂^Gγ₂: 0.13, ₂α^A/Gγ and ₂α^Aγ₂: 0.12 with differences below 0.20 among replicates showing the validity of the results.

Conclusion and perspectives. There are structural changes in HbF depending on the presence of ^Gγ or ^Aγ globin chains. Further studies are required to identify the type of tetramer present in healthy individuals as in patients with different blood malignancies where HbF has been reported as a therapeutic target or risk factor to understand its effect and association with opposite outcomes.