Molecular Characterization of Hemopexin in the Siberian Sturgeon (Acipenser baerii): Evolutionary Insights and Differential Expression Under Immune and Thermal Stresses

Hemopexin (HPX) is a heme-binding glycoprotein involved in detoxification, oxidative stress regulation, and immune defense. In teleosts, gene duplication gave rise to <i>Wap65-1</i> and <i>Wap65-2</i>, but the structure and function of ancestral <i>HPX</i> in basal actinopterygians remain poorly understood. Here, we characterized <i>HPX</i> from the Siberian sturgeon (<i>Acipenser baerii</i>), a slow-evolving chondrostean species that diverged prior to the teleost-specific duplication. Structural modeling and superimposed 3D alignment revealed high similarity between sturgeon HPX, human HPX, and Wap65-2, but not Wap65-1. Phylogenetic analysis placed sturgeon HPX in a distinct basal clade within the Actinopterygian lineage, positioned below the divergence of Wap65 paralogs. Tissue expression was liver-dominant but also evident in skin and kidney, and <i>HPX</i> transcripts increased during larval development. Under <i>Aeromonas hydrophila</i> challenge, <i>HPX</i> was strongly upregulated in immune-relevant tissues. Thermal challenge also induced modest, tissue-specific changes, particularly in peripheral tissues. These results indicate that ancestral <i>HPX</i> possessed multivalent stress responsiveness-primarily immune-related-with supplementary thermal sensitivity. The observed functional flexibility is consistent with the hypothesis that ancestral HPX functions were partitioned following <i>Wap65</i> gene duplication in teleosts, with <i>Wap65-1</i> evolving toward a specialized thermal response role.