Research

Biophysical Journal 2007; 93:1442-1451

Antifreeze Proteins at the Ice/Water Interface: Three Calculated Discriminating Properties for Orientation of Type I Proteins

ABSTRACT :

The number of hydrogen bond count, for example; and the sidechain Antifreeze proteins (AFPs protect many plants and organisms from freezing in low temperatures. Of the different AFPs, the most studied AFP Type I from winter flounder is used in the current computational studies to gain molecular insight into its adsorption at the ice/water interface. Employing molecular dynamics simulations, we calculate the free energy difference between the hydrophilic and hydrophobic faces of the protein interacting with ice. Furthermore, we identify three properties of Type I "antifreeze" proteins that discriminate among these two orientations of the protein at the ice/water interface. The three properties are: the "surface area" of the protein; a measure of the interaction of the protein with neighboring water molecules as determined by orientation angles of the threonine residues. All three discriminants are consistent with our free energy results which clearly show that the hydrophilic protein face orientations towards the ice/water interface, as hypothesized from experimental and ice/vacuum simulations, is incorrect, while supporting the hydrophobic protein face orienting towards the interface. The adsorption free energy is calculated to be 2-3 kJ/mol. Read full abstract

234th ACS National Meeting, Boston, MA, August 19-23, 2007

Division of Computers in Chemistry
Ions at water interfaces
ADJ Haymet, Scripps Institution of Oceanography, UC San Diego, 9500 Gilman Drive, La Jolla, CA 92093-0210, Fax: 858-453-0167, haymet@ucsd.edu, and Taras Bryk, Institute for Condensed Matter Physics, National Academy of Sciences of Ukraine
The behavior of ions and molecules at the ice/gas and ice/water interfaces is a fundamental process encountered in a range of physical and biological systems and has relevance to unsolved problems in atmospheric chemistry, for example charge separation at the ice/water interfaces known as Workman-Reynolds effect. Recent studies on the behavior of Na+ and Cl- ions at the ice/water interface question the inherent (or induced) electrostatic potential at ice/water interfaces. Others have reported very thorough calculations on the electrostatic potential for water/vapor interfaces. Our results on free energy profiles for Na+ and Cl- ions across the basal ice/water interface indicate that the ice/water interface can function as a capacitor. Similar profiles have been calculated for Na+ and Cl- at the basal ice surface / vacuum interface. For each solute ion at the ice surface, the minimum in the free energy profiles is located in the top surface liquid-like layers.

234th ACS National Meeting, Boston, MA, August 19-23, 2007

Division of Computers in Chemistry
Molecular dynamics simulations of Antifreeze proteins at a lipid/water interface
E. J. Smith, Math, Science and Healthcare, Kingwood College, 20000 Kingwood Dr, Kingwood, TX 77339, erica.j.smith@nhmccd.edu, Pranav Dalal, Schrodinger, Inc, Jeffry D. Madura, Department of Chemistry and Biochemistry, Center for Computational Sciences, Duquesne University, and ADJ Haymet, Scripps Institution of Oceanography, UC San Diego

Abstract

Antifreeze proteins (AFP's), found in certain fish, insects and plants, allow the host organism to survive in lower than the equilibrium freezing point of their blood and/or internal fluids via suppression of ice crystal growth and the protection of cell membranes from cold-induced damage. Using molecular dynamics simulation techniques, a solvated dimyristolyphosphatidylcholine (DMPC) lipid bilayer containing an AFP type I at the lipid/water interface was constructed, simulated and characterized at several temperatures (310, 295, 288 and 260 K). Lipid structural characteristics were monitored and all results agreed well with other lipid bilayer simulation and experimental data. The simulations performed at 260 K showed (i) a distinct broadening of the lipid headgroup distribution in the pure system that was not observed in the system containing the AFP and (ii) that the phosphate-nitrogen (P-N) tilt angle decreased for the pure solvated system but remained the same for the system containing the AFP.