AMINO-TERMINAL OXYGEN-BINDING FUNCTIONAL UNIT OF THE RAPANA-THOMASIANA GROSSE (GASTROPOD) HEMOCYANIN - CARBOHYDRATE CONTENT, MONOSACCHARIDECOMPOSITION AND AMINO-ACID-SEQUENCE STUDIES

The amino-terminal oxygen-binding unit Rta of the Rapana thomasiana he mocyanin is a glycoprotein with a carbohydrate content of 4.8% (w/w). Sugar analysis revealed as monosaccharide constituents xylose, fucose, 3-O-methylgalactose, mannose, galactose, N-acetylgalactosamine and N- acetylglucosamine residues. On subtracting the carbohydrate contributi on from the molecular mass of 49,698 Da, determined by laser desorptio n mass spectrometry for Rta, an M-r value of 47,318 Da was determined for the polypeptide part of the functional unit. The Rapana hemocyanin oxygen-binding unit Rta contains 400 residues in a single polypeptide chain. The nearly complete amino acid sequence (about 90%) is determi ned. This is the first report on a sequence of a marine gastropod oxyg en-binding unit and also on a molluscan hemocyanin amino-terminal unit . Comparison of the Rta sequence with those of other molluscan hemocya nin units. localized in the C-terminus or in the middle of the respect ive multidomain polypeptide chains, revealed 42-46% homology (52-55%, including isofunctional residues). Probably, all molluscan oxygen-bind ing units evolved from a common ancestral gene. (C) 1997 Elsevier Scie nce Inc.