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EF-Hand Mimicking Calcium Binding Polymer

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journal contribution
posted on 24.02.2016, 00:00 by Hee Jung Chung, Du Young Ko, Hyo Jung Moon, Byeongmoon Jeong
There are four EF-hand polypeptides in calmodulin, a natural ubiquitous calcium binding protein that activates the enzymes involved in Ca2+-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 Ca2+. As the Ca2+ bound to CBP, small changes in the circular dichroism spectra and 13C NMR spectra were observed, indicating that Ca2+ binding to CBP induced changes in the conformation of CBP. The binding constant of CBP to Ca2+ 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 Ca2+ concentration was 5.1 × 105 M–1, which was similar to that of calmodulin. The selectivity of CBP for metal ion binding was compared among Ca2+, Cu2+, and Zn2+. The binding constant was obtained through a similar competitive binding study with murexide. The binding constants for Ca2+, Cu2+, and Zn2+ were 7.0 × 105, 4.2 × 105, and 1.7 × 105 M–1, respectively, indicating 2–4-fold higher selectivity of CBP for Ca2+ compared to Cu2+ and Zn2+. The CBP has selectivity for Ca2+, and binding affinity for Ca2+ was similar to the biological Ca2+ binding motif of calmodulin.