Nanoparticles (NPs), when exposed to biofluids, become coated with proteins. As the protein is adsorbed on the surface, the extent of adsorption and the consequent effect on protein conformation and activity depend on the chemical nature, shape, and size of the nanoparticle. We have carried out a detailed study on the interaction of a-lactalbumin (a protein which forms the regulatory subunit of lactose synthase) with zinc oxide nanoparticles. The NPs were prepared by the sol gel route and characterized by transmission electron microscopy, X-ray diffraction, UV-visible, and photoluminescence spectroscopy. ZnO particles were found to have a size of 4-7 nm with hexagonal structure. The interaction of protein with NP was examined using a combination of spectroscopic and computational methods. The binding was studied by ITC (isothermal calorimetry), and the result revealed that the complexation is mostly entropy driven and involves hydrophobic interaction. There is alteration in secondary structures in protein on binding ZnO nanoparticle, as revealed by circular dichroism (CD) and Fourier transform infrared spectroscopy (FITR). Finally, a comparison of structure, function, and stability of the alpha-lactalbumin-NP complex has been made by binding ZnO to other model proteins to get a better insight into the process of protein nanoparticle interaction. The present study thus provides useful insights into issues such as protein nanoparticle recognition.