The molecular interactions between the Ce(IV) -substituted Keggin anion [PW11 O39 Ce(OH2 )4 ](3-) (CeK) and hen egg-white lysozyme (HEWL) were investigated by molecular dynamics simulations. The analysis of CeK was compared with the Ce(IV) -substituted Keggin dimer [(PW11 O39 )2 Ce](10-) (CeK2 ) and the Zr(IV) -substituted Lindqvist anion [W5 O18 Zr(OH2 )(OH)](3-) (ZrL) to understand how POM features such as shape, size, charge, or type of incorporated metal ion influence the POM⋅⋅⋅protein interactions. Simulations revealed two regions of the protein in which the CeK anion interacts strongly: cationic sites formed by Arg21 and by Arg45 and Arg68. The POMs chiefly interact with the side chains of the positively charged (arginines, lysines) and the polar uncharged residues (tyrosines, serines, aspargines) via electrostatic attraction and hydrogen bonding with the oxygen atoms of the POM framework. The CeK anion shows higher protein affinity than the CeK2 and ZrL anions, because it is less hydrophilic and it has the right size and shape for establishing interactions with several residues simultaneously. The larger, more negatively charged CeK2 anion has a high solvent-accessible surface, which is sub-optimal for the interaction, while the smaller ZrL anion is highly hydrophilic and cannot efficiently interact with several residues simultaneously.