Primary Structure of Freezing Point-depressing Glycoproteins

The freezing point-depressing glycoproteins, which are present in the blood of the Antarctic fish, Trematomus borchgrevinki, were sequenced. The first 6 residues of the native glycoprotein and the β-eliminated protein were identified by phenyl isothiocyanate degradation. The native glycoproteins were hydrolyzed into several small glycopeptides by the proteolytic enzymes subtilisin or elastase. The resulting hepta, hexa, penta, tetra, tri, and diglycopeptides were separated on Dowex 50 and were sequenced by phenyl isothiocyanate degradation. The sequences of these glycopeptides showed that the molecule contained the same repeating unit found in the first 6 residues of the unhydrolyzed protein. The repeating structural unit is a tripeptide of alanine-alanine-threonine in which the last residue is glycosidically linked to the disaccharide galactosyl-N-acetylgalactosamine.