A diminished role for hydrogen bonds in antifreeze protein binding to ice.
The most abundant isoform (HPLC-6) of type I antifreeze protein (AFP1) in winter flounder is a 37-amino-acid-long, alanine-rich, alpha-helical peptide, containing four Thr spaced 11 amino acids apart. It is generally assumed that HPLC-6 binds ice through a hydrogen-bonding match between the Thr and neighboring Asx residues to oxygens atoms on the {2021} plane of the ice lattice. The result is a lowering of the nonequilibrium freezing point below the melting point (thermal hysteresis). HPLC-6, and two variants in which the central two Thr were replaced with either Ser or Val, were synthesized. The Ser variant was virtually inactive, while only a minor loss of activity was observed in the Val variant. CD, ultracentrifugation, and NMR studies indicated no significant structural changes or aggregation of the variants compared to HPLC-6. These results call into question the role of hydrogen bonds and suggest a much more significant role for entropic effects and van der Waals interactions in binding AFP to ice.
Chao
H
H
Protein Engineering Network of Centres of Excellence, University of Alberta, Edmonton, Canada.
Houston
M E
ME
Hodges
R S
RS
Kay
C M
CM
Sykes
B D
BD
Loewen
M C
MC
Davies
P L
PL
Sönnichsen
F D
FD
D000595:Amino Acid Sequence; D000818:Animals; D021301:Antifreeze Proteins; D002500:Centrifugation, Isopycnic; D002942:Circular Dichroism; D005432:Flounder; D005615:Freezing; D006023:Glycoproteins / Q000737:chemistry / Q000378:metabolism*; D006860:Hydrogen Bonding; D007053:Ice; D008969:Molecular Sequence Data; D019906:Nuclear Magnetic Resonance, Biomolecular; D017433:Protein Structure, Secondary; D012694:Serine / Q000737:chemistry / Q000378:metabolism; D013912:Threonine / Q000737:chemistry / Q000378:metabolism; D014633:Valine / Q000737:chemistry / Q000378:metabolism
D021301:Antifreeze Proteins; D006023:Glycoproteins; D007053:Ice; D013912:Threonine; D012694:Serine; D014633:Valine