Antifreeze proteins from the ocean pout, Macrozoarces americanus: circular dichroism spectral studies on the native and denatured states

CD spectral data on the antifreeze proteins from the ocean pout (Macrozoarces americanus) have been obtained under native and denaturing conditions. At low temperatures (near 0°C), the far-ultraviolet CD spectra of the two known immunologically distinct groups of ocean pout proteins are found to be qualitatively similar to those of the antifreeze glycoproteins, with a distinct positive band near 225 nm. However, unlike the latter, the ocean pout proteins exhibit CD bands in the 250–300 nm region, indicating the presence of asymmetrically situated aromatic residues. On heating from 0 to 60°C, a sigmoidal conformational transition is seen to occur when monitored by means of the 225 nm CD band. Addition of 6 M guanidine hydrochloride produces a CD spectrum typical of denatured proteins. On treatment with SDS, the spectrum of the native protein changes over to that of a protein having α-helical segments. The SDS-treated protein was found to be devoid of antifreeze activity. Taken together, the CD data point to the presence of well-defined secondary and tertiary structures in the ocean pout proteins which are quite different from those obtained in other antifreeze proteins.