EF-Hand Mimicking Calcium Binding Polymer

Abstract

There are four EF-hand polypeptides in calmodulin, a natural ubiquitous calcium binding protein that activates the enzymes involved in Ca(2+)-mediated signal transduction. An EF-hand polypeptide has six carboxylate functional groups in the middle loop region between two rigid polypeptides. In this study, a calcium binding polymer (CBP) with a structure of poly(L-alanine)-poly(L-alanine-co-L-glutamic acid)-poly(ethylene glycol)-poly(L-alanine-co-L-glutamic acid)-poly(L-alanine) (PA-PAE-PEG-PAE-PA; A11.1-A3.4E3.2-EG40.1-A3.4E3.2-A11.1) was synthesized by mimicking the EF-hand polypeptide. The 6-7 carboxylate functional groups from PAE are expected to form a binding site for Ca(2+). As the Ca(2+) bound to CBP, small changes in the circular dichroism spectra and (13)C NMR spectra were observed, indicating that Ca(2+) binding to CBP induced changes in the conformation of CBP. The binding constant of CBP to Ca(2+) was investigated by using the competitive binding of 2,2',2″,2‴-{ethane-1,2-diylbis[oxy(4-bromo-2,1-phenylene)nitrilo]} tetraacetic acid (5,5-Br2-BAPTA). The binding constant obtained with a CaLigator program by least-squares fitting of the absorbance profile as a function of Ca(2+) concentration was 5.1 × 10(5) M(-1), which was similar to that of calmodulin. The selectivity of CBP for metal ion binding was compared among Ca(2+), Cu(2+), and Zn(2+). The binding constant was obtained through a similar competitive binding study with murexide. The binding constants for Ca(2+), Cu(2+), and Zn(2+) were 7.0 × 10(5), 4.2 × 10(5), and 1.7 × 10(5) M(-1), respectively, indicating 2-4-fold higher selectivity of CBP for Ca(2+) compared to Cu(2+) and Zn(2+). The CBP has selectivity for Ca(2+), and binding affinity for Ca(2+) was similar to the biological Ca(2+) binding motif of calmodulin.