Characterization of the blue protein in the shell-surface pattern of the Japanese littleneck clam (ruditapes philippinarum)

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Characterization of the blue protein in the shell pattern of Japanese littleneck clam (Ruditapes philippinarum ) was performed by purification using anion-exchange chromatography, amino acid analysis, molecular weight measurements, quantification of ferric ions, etc. Blue colored two components were found by anion exchange chromatography. The blue colored shell protein and the colorless mantle protein were found to be homologous to each other in amino acid composition; they are acidic proteins rich in aspartic acid and glutamic acid and that the isoelectric point of the blue protein was estimated as pI =3.96. The secondary structures of the blue protein were estimated as α-helix (28.6%), β-sheet (20.1%), and random coil (51.3%). From combination of molecular weight measurements and quantification of ferric ions, it was found that the blue protein is formed as trimer or tetramer from the monomer proteins of about 10 kDa by complexing with ferric ion. It was also found that there are two kinds of ferric ion existing states and that reducing agents decompose the blue colored trimer or tetramer into dimer or smaller units accompanying decolorization.

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