Limulus polyphemus Hemocyanin. A Nuclear Magnetic Resonance Study of Its Subunits
Hemocyanin from Limulus polyphemus is a 3.3 million dalton oligomeric protein made up of about eight different kinds of polypeptide chains. The subunit mixture obtained after dissociation of the oligomer can be fractionated into five distinct chromatographic zones. While homogeneous in molecular weight, the subunits in these zones appear to have unique functional properties. This uniqueness is also manifest in their NMR spectra. A simulated NMR spectrum, based on a random coil configuration, has been compared with the spectrum of the unfractionated subunit mixture. Notable differences between the observed and simulated spectra are apparent in the region of the spectrum, corresponding to aromatic amino acid side chains. These differences may provide information about the conformation of the native subunits and the environment of the copper atoms in the active site. Chloride ions are known to alter the oxygen affinities of some of the Limulus hemocyanin subunits. This functional differentiation is supported by the marked effects of chloride ions on the NMR spectra of these particular subunits.
KeywordsAmino Acid Composition Nuclear Magnetic Resonance Spectroscopy Simulated Spectrum Horseshoe Crab Nuclear Magnetic Resonance Study
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