Structural Characterization of the Complex between Hen Egg-White Lysozyme and ZrIV-Substituted Keggin Polyoxometalate as Artificial Protease

Successful co-crystallization of a noncovalent complex between hen egg-white lysozyme (HEWL) and the monomeric Zr-IV-substituted Keggin polyoxometalate (POM) (Zr1K1), (Et2NH2)(3)[Zr(PW11O39)] (1), has been achieved, and its single-crystal X-ray structure has been determined. The dimeric Zr-IV-substituted Keggin-type polyoxometalate (Zr1K2), (Et2NH2)(10)[Zr(PW11O39)(2)] (2), has been previously shown to exhibit remarkable selectivity towards HEWL hydrolysis. The reported X-ray structure shows that the hydrolytically active Zr-IV-substituted Keggin POM exists as a monomeric species. Prior to hydrolysis, this monomer interacts with HEWL in the vicinity of the previously identified cleavage sites found at Trp28-Val29 and Asn44-Arg45, through water-mediated H-bonding and electrostatic interactions. Three binding sites are observed at the interface of the negatively charged Keggin POM and the positively charged regions of HEWL at: 1)Gly16, Tyr20, and Arg21; 2)Asn44, Arg45, and Asn46; and 3)Arg128.