Soru

Passage:
Every spring and autumn, the bar-headed goose (*Anser indicus*) undertakes a trans-Himalayan migration, traversing high-altitude passes exceeding 7,000 meters above sea level where the partial pressure of atmospheric oxygen ($P_{\text{O}_2}$) is less than half that at sea level. Sustaining the immense metabolic power output required for flapping flight in such severely hypoxic conditions requires a suite of specialized physiological adaptations operating across every tier of the oxygen transport cascade.

At the level of pulmonary gas exchange, *Anser indicus* exhibits significantly deeper and more rapid ventilation than lowland waterfowl species. In most mammals and lowland birds, severe hyperventilation precipitates extreme hypocapnia (a critical drop in arterial carbon dioxide tension), which typically triggers cerebral vasoconstriction and subsequent cerebral ischemia. Bar-headed geese, however, possess specialized cerebral vascular autoregulatory mechanisms: their cerebral arterioles remain refractory to hypocapnia-induced constriction, thereby maintaining stable cerebral blood perfusion and preventing hypoxia-mediated syncope during strenuous flight.

Complementing this respiratory control is a structurally modified hemoglobin molecule with an exceptionally high intrinsic oxygen affinity. Comparative crystallographic analysis reveals that this elevated affinity arises primarily from a single amino acid substitution on the $\alpha$-globin subunit—specifically, the replacement of a bulky proline residue at position 119 with an alanine residue ($\alpha 119\text{Pro}\rightarrow\text{Ala}$). This substitution eliminates an interface van der Waals contact between the $\alpha_1$ and $\beta_2$ subunits, destabilizing the low-affinity deoxygenated (Tense) quaternary conformation and thereby shifting the allosteric equilibrium toward the high-affinity oxygenated (Relaxed) state.

Finally, oxygen utilization in peripheral flight tissues is enhanced through distinct histological and biochemical modifications in the pectoralis muscle. Capillary density per muscle fiber in *Anser indicus* is approximately thirty percent greater than in related lowland geese, effectively reducing diffusion distances between circulating erythrocytes and the sarcolemma. Furthermore, subsarcolemmal mitochondria within these fibers are localized in close proximity to adjacent capillaries rather than clustered deep within the myofibrillar core, minimizing intracellular oxygen diffusion pathways. Crucially, metabolic enzyme profiling indicates that during high-altitude transit, the flight musculature relies almost exclusively on fatty acid beta-oxidation rather than glycogenolysis, maximizing energy yield per unit mass of substrate.

Statement: According to the passage, the structural basis for the elevated oxygen affinity of bar-headed goose hemoglobin is a single amino acid substitution located on the $\alpha$-globin subunit that destabilizes the molecule's deoxygenated conformation.