In general, proteins in the biological system interact with nanoparticles (NPs) via adsorption on the particle surface. Understanding the adsorption at the molecular level is crucial to explore NP-protein interactions. The increasing concerns about the risk to human health on NP exposure have been explored through the discovery of a handful protein biomarkers and biochemical analysis. However, detailed information on structural perturbation and associated functional changes of proteins on interaction with NPs is limited. Erythrocytes (red blood cells) are devoid of defense mechanism of protecting NP penetration through endocytosis. Therefore, it is important to investigate the interaction of erythrocyte proteins with NPs. Hemoglobin, the most abundant protein of human erythrocyte, is a tetrameric molecule consisting of alpha- and beta-globin chains in duplicate. In the present study, we have used hemoglobin as a model system to investigate NPprotein interaction with ferric pyrophosphate NPs [NP-Fe-4(P2O7)(3)]. We report the formation of a bioconjugate of hemoglobin upon adsorption to NP-Fe-4(P2O7)(3) surface. Analysis of the bioconjugate indicated that Fe3+ ion of NP-Fe-4(P2O7)(3) contributed in the bioconjugate formation. Using hydrogen/deuterium exchange based mass spectrometry, it was observed that the amino termini of alpha- and beta-globin chains of hemoglobin were involved in the adsorption on NP surface whereas the carboxy termini of both chains became more flexible in its conformation compared to the respective regions of the normal hemoglobin. Circular dichroism spectra of desorbed hemoglobin indicated an adsorption induced localized structural change in the protein molecule. The formation of bioconjugate led to functional alteration of hemoglobin, as probed by oxygen binding assay. Thus, we hypothesize that the large amount of energy released upon adsorption of hemoglobin to NP surface might be the fundamental cause of structural perturbation of human hemoglobin and subsequent formation of the bioconjugate with an altered function.