A natural variant of type I antifreeze protein with four ice-binding repeats is a particularly potent antifreeze.
A 4.3-kDa variant of Type I antifreeze protein (AFP9) was purified from winter flounder serum by size exclusion chromatography and reversed-phase HPLC. By the criteria of mass, amino acid composition, and N-terminal sequences of tryptic peptides, this variant is the posttranslationally modified product of the previously characterized AFP gene 21a. It has 52 amino acids and contains four 11-amino acid repeats, one more than the major serum AFP components. The larger protein is completely alpha-helical at 0 degree C, with a melting temperature of 18 degrees C. It is considerably more active as an antifreeze than the three-repeat winter flounder AFP and the four-repeat yellowtail flounder AFP, both on a molar and a mg/mL basis. Several structural features of the four-repeat winter flounder AFP, including its larger size, additional ice-binding residues, and differences in ice-binding motifs might contribute to its greater activity. Its abundance in flounder serum, together with its potency as an antifreeze, suggest that AFP9 makes a significant contribution to the overall freezing point depression of the host.
Chao
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Protein Engineering Network of Centres of Excellence, University of Alberta, Edmonton, Canada.
Hodges
R S
RS
Kay
C M
CM
Gauthier
S Y
SY
Davies
P L
PL
D000595:Amino Acid Sequence; D000596:Amino Acids / Q000032:analysis / Q000737:chemistry; D000818:Animals; D021301:Antifreeze Proteins; D002850:Chromatography, Gel; D002851:Chromatography, High Pressure Liquid; D002942:Circular Dichroism; D005432:Flounder / Q000097:blood; D005615:Freezing; D006023:Glycoproteins / Q000032:analysis / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D007053:Ice; D013058:Mass Spectrometry; D008969:Molecular Sequence Data; D010446:Peptide Fragments / Q000032:analysis; D017433:Protein Structure, Secondary; D017421:Sequence Analysis; D013056:Spectrophotometry, Ultraviolet; D014357:Trypsin / Q000378:metabolism
D000596:Amino Acids; D021301:Antifreeze Proteins; D006023:Glycoproteins; D007053:Ice; D010446:Peptide Fragments; D014357:Trypsin