Metal-enzyme frameworks: role of metal ions in promoting enzyme self-assembly on α-zirconium(IV) phosphate nanoplates.
Previously, an ion-coupled protein binding (ICPB) model was proposed to explain the thermodynamics of protein binding to negatively charged α-Zr(IV) phosphate (α-ZrP). This model is tested here using glucose oxidase (GO) and met-hemoglobin (Hb) and several cations (Zr(IV), Cr(III), Au(III), Al(III), Ca(II), Mg(II), Zn(II), Ni(II), Na(I), and H(I)). The binding constant of GO with α-ZrP was increased ∼380-fold by the addition of either 1 mM Zr(IV) or 1 mM Ca(II), and affinities followed the trend Zr(IV) ≃ Ca(II) > Cr(III) > Mg(II) ≫ H(I) > Na(I). Binding studies could not be conducted with Au(III), Al(III), Zn(II), Cu(II), and Ni(II), as these precipitated both proteins. Zr(IV) increased Hb binding constant to α-ZrP by 43-fold, and affinity enhancements followed the trend Zr(IV) > H(I) > Mg(II) > Na(I) > Ca(II) > Cr(III). Zeta potential studies clearly showed metal ion binding to α-ZrP and affinities followed the trend, Zr(IV) ≫ Cr(III) > Zn(II) > Ni(II) > Mg(II) > Ca(II) > Au(III) > Na(I) > H(I). Electron microscopy showed highly ordered structures of protein/metal/α-ZrP intercalates on micrometer length scales, and protein intercalation was also confirmed by powder X-ray diffraction. Specific activities of GO/Zr(IV)/α-ZrP and Hb/Zr(IV)/α-ZrP ternary complexes were 2.0 × 10(-3) and 6.5 × 10(-4) M(-1) s(-1), respectively. While activities of all GO/cation/α-ZrP samples were comparable, those of Hb/cation/α-ZrP followed the trend Mg(II) > Na(I) > H(I) > Cr(III) > Ca(II) ≃ Zr(IV). Metal ions enhanced protein binding by orders of magnitude, as predicted by the ICPB model, and binding enhancements depended on charge as well as the phosphophilicity/oxophilicity of the cation.
Department of Chemistry, University of Connecticut, U-3060, Storrs, Connecticut 06269, USA.
Ajith
A
Pattammattel
Inoka K
IK
Deshapriya
Ruma
R
Chowdhury
Challa V
CV
Kumar
D000818:Animals; D001234:Aspergillus niger / Q000201:enzymology; D002417:Cattle; D005949:Glucose Oxidase / Q000737:chemistry* / Q000378:metabolism; D008670:Metals / Q000494:pharmacology*; D008706:Methemoglobin / Q000737:chemistry* / Q000378:metabolism; D049329:Nanostructures / Q000737:chemistry*; D011485:Protein Binding / Q000187:drug effects; D013499:Surface Properties; D015040:Zirconium / Q000737:chemistry*
D008670:Metals; C027006:zirconium phosphate; D008706:Methemoglobin; D015040:Zirconium; D005949:Glucose Oxidase