A 43Ca NMR study of the binding of calcium to parvalbumins

Abstract

Association between ions and biological molecules (proteins, nucleic acids, sugars) is important in the structure and function of many biological systems. Many of the ions so involved (alkali, alkaline earth, halide ions) lack characteristic spectroscopic properties. However, NMR detectable isotopes can sometimes be used. The spin quantum numbers of these nuclei are usually greater than l/2 so that the nuclear magnetic relaxation is governed mainly by quadrupolar effects which show important variations when an ionmacromolecule complex is formed, due to the local dynamics and the electrical environment of the bound ion. The quadrupolar relaxation of alkali and halide nuclei has been extensively studied and several chemical and biological applications have been developed [ 1,2] . For biological systems, interest is mainly on Mg” and Ca” but because of the low inherent sensitivity of the NMR detectable isotopes there have been very few studies using these nuclei [31. H owever, *sMg and 43Ca NMR investigations of isotopicallyenriched samples on Fourier transform spectrometers should be most significant for many biological problems. We report here on the feasibility of using the resonance of calcium-43 in the study of parvalbumins. These calcium-binding proteins, present in the muscles of almost all vertebrates, have a low molecular weight (about 11 500), a high water solubility, an acidic isoelectric point, a high phenylalanine content and a strong affinity for calcium ions [4,5] . The tertiary structure of a parvalbumin component from carp muscle (characterized by its isoelectric point, ~14.25;