Abstract:The molecular interactions between the Ce(IV)-substituted Keggin anion [PW11O39Ce(OH2)4] 3-(CeK) and hen egg white lysozyme (HEWL), was investigated by molecular dynamics (MD) simulations. We compared the analysis of CeK with the Ce(IV)-substituted Keggin dimer [(PW11O39)2Ce] 10-(CeK2) and the Zr(IV)-substituted Lindqvist anion [W5O18Zr(OH2)(OH)] 3-(ZrL) in order to understand how POM features such as the shape, the size, the charge or the type of incorporated metal ion influence the POM···protein interactions. Simulations revealed two regions of the protein, in which the CeK anion interacts strongly: the cationic sites formed by Arg21 on one hand and by Arg45 and Arg68 on the other. The two sites can be related with the observed selectivity in the hydrolytic cleavage of HEWL. The POMs chiefly interact with the side chains of the positively charged (arginines and lysines) and the polar uncharged (tyrosines, serines and aspargines) residues via electrostatic attraction and hydrogen bonding with the oxygens 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 stablishing interactions with several residues simultaneously. The larger and more negatively charged CeK2 anion has a high solvent-accessible surface, which is suboptimal for the interaction, while the smaller ZrL anion is highly hydrophilic and it cannot interact simultaneously with several residues so efficiently.

The interaction between the plenary Keggin H3PW12O40, lacunary Keggin K7PW11O39 and the Eu(III)-substituted Keggin K4EuPW11O39 (Eu-Keggin) type polyoxometalates (POMs), and the proteins human and bovine serum albumin (HSA and BSA) was studied using steady state and time-resolved Eu(III) luminescence and tryptophan (Trp) fluorescence spectroscopy. The excitation spectrum of the Eu-Keggin POM is dominated by a ligand-to-metal charge transfer band at 291 nm. In the absence of proteins, the number of water molecules coordinated in the first coordination sphere of the Eu(III) center of Eu-Keggin was determined to be 4, indicating that Eu(III) occurs as a 1 : 1 isomer in solution. In the presence of HSA or BSA, the number of coordinated water molecules decreased to 0 and 1, respectively, suggesting interaction between the Eu-Keggin POM and the protein surface. As a result of this interaction, a five-fold increase of the hypersensitive (5)D0 → (7)F2 transition in the luminescence intensity was observed for the Eu-Keggin-HSA complex. The association constants were calculated to be 1.5 × 10(2) M(-1) and 2.0 × 10(3) M(-1) for the Eu-Keggin-HSA and Eu-Keggin-BSA complexes, respectively. Tryptophan fluorescence quenching studies were performed and the quenching constants were calculated using a Stern-Volmer analysis. The obtained values of the quenching constants were 6.1 × 10(4) M(-1) and 2.0 × 10(6) M(-1) for the Eu-Keggin-HSA and Eu-Keggin-BSA complexes, respectively. The surface map of both proteins shows that the cavity containing the tryptophan has a positive surface potential, providing a specific binding site at the surface of albumin proteins for the negatively charged POM.

A multitechnique approach has been applied in order to identify the thermodynamic and kinetic parameters related to the regioselective hydrolysis of human serum albumin (HSA) or Cys392ÀGlu393 peptide bonds. This is in agreement with the interaction studies as the Arg114ÀLeu115 peptide bond is located in the positive cleft of HSA and the three remaining peptide bonds are each located near an upstream acidic residue, which can be expected to coordinate to the metal ion. A detailed kinetic study has evidenced the formation of additional fragments upon prolonged reaction times. Edman degradation of the additional reaction products has shown that these fragments result from further hydrolysis at the initially observed cleavage positions, indicating a fixed selectivity for K 15 H[Zr(a 2 -P 2 W 17 O 61 ) 2 ].

This study represents the first example of protein hydrolysis at pH = 7.4 and 60 °C by a metal-substituted polyoxometalate (POM) in the presence of a zwitterionic surfactant. Edman degradation results show that in the presence of 0.5% w/v 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate (CHAPS) detergent, a Zr(IV)-substituted Wells–Dawson-type POM, K15H[Zr(α2-P2W17O61)2]·25H2O (Zr1-WD2), selectively hydrolyzes human serum albumin exclusively at peptide bonds involving Asp or Glu residues, which contain carboxyl groups in their side chains. The selectivity and extent of protein cleavage are tuned by the CHAPS surfactant by an unfolding mechanism that provides POM access to the hydrolyzed peptide bonds.

Abstract:The interaction between human serum albumin (HSA) and a series of Zr(IV)-substituted polyoxometalates (POMs) (Lindqvist type POM ((