Effects of a type I antifreeze protein (AFP) on the melting of frozen AFP and AFP+solute aqueous solutions studied by NMR microimaging experiment

Analysis of the Sequence Characteristics of Antifreeze Protein

Antifreeze protein (AFP) is a proteinaceous compound with improved antifreeze ability and binding ability to ice to prevent its growth. As a surface-active material, a small number of AFPs have a tremendous influence on the growth of ice. Therefore, identifying novel AFPs is important to understand protein–ice interactions and create novel ice-binding domains. To date, predicting AFPs is difficult due to their low sequence similarity for the ice-binding domain and the lack of common features among different AFPs. Here, a computational engine was developed to predict the features of AFPs and reveal the most important 39 features for AFP identification, such as antifreeze-like/N-acetylneuraminic acid synthase C-terminal, insect AFP motif, C-type lectin-like, and EGF-like domain. With this newly presented computational method, a group of previously confirmed functional AFP motifs was screened out. This study has identified some potential new AFP motifs and contributes to understanding biological antifreeze mechanisms.

Cryoprotective Activity and Action Mechanism of Antifreeze Peptides Obtained from Tilapia Scales on Streptococcus thermophilus during Cold Stress

Cold stress adversely affects cell viability and acidification, and new cryoprotective methods continue to be needed in cold-chain food industry. Given this, we investigated the cryoprotective effects and action mechanism of antifreeze peptides obtained from tilapia scales (TSAPP) on Streptococcus thermophilus during cold stress. Our results showed that the molecular weight of TSAPP ranged from 180 to 2000 Da and its thermal hysteresis activity was 0.29 °C. Growth of S. thermophilus was improved after treatment with TSAPP (1 mg/mL) under cold stress. This growth was notable when compared with the effects of other cryoprotectants. Furthermore, TSAPP improved the metabolic activity of S. thermophilus during cold stress. TSAPP likely offered its cellular protection by maintaining cell membrane fluidity through hydrogen bonding of the phospholipid bilayer. These results indicate that TSAPP has potential as a novel biological peptide material with cryoprotective activity for future use in probiotic or other processed food applications.

Mechanisms of antifreeze proteins investigated via the site-directed spin labeling technique

The site-directed spin labeling (SDSL) technique was used to examine the antifreeze mechanisms of type-I antifreeze proteins (AFPs). The effects on the growth of seed ice crystals by the spin-label groups attached to different side chains of the AFPs were observed, and the states of water molecules surrounding the spin-label groups were probed via analyses of variable-temperature (VT) dependent electron paramagnetic resonance (EPR) spectra. The first set of experiments revealed the antifreeze activities of the spin-labeled AFPs at the microscopic level, while the second set of experiments displayed those at the molecular level. The experimental results confirmed the putative ice-binding surface (IBS) of type-I AFPs. The VT EPR spectra indicate that type-I AFPs can inhibit the nucleation of seed ice crystals down to ~ - 20 °C in their aqueous solutions. Thus, the present authors believe that AFPs protect organisms from freezing damage in two ways: (1) inhibiting the nucleation of seed ice crystals, and (2) hindering the growth of seed ice crystals once they have formed. The first mechanism should play a more significant role in protecting against freezing damage among organisms living in cold environments. The VT EPR spectra also revealed that liquid-like water molecules existed around the spin-labeled non-ice-binding side chains of the AFPs frozen within the ice matrices, and ice surrounding the spin-label groups melted at subzero temperatures during the heating process. This manuscript concludes with the proposed model of antifreeze mechanisms of AFPs based on the experimental results.

Induced ice melting by the snow flea antifreeze protein from molecular dynamics simulations

Antifreeze proteins (AFP) allow different life forms, insects as well as fish and plants, to survive in subzero environments. AFPs prevent freezing of the physiological fluids. We have studied, through molecular dynamics simulations, the behavior of the small isoform of the AFP found in the snow flea (sfAFP), both in water and at the ice/water interface, of four different ice planes. In water at room temperature, the structure of the sfAFP is found to be slightly unstable. The loop between two polyproline II helices has large fluctuations as well as the C-terminus. Torsional angle analyses show a decrease of the polyproline II helix area in the Ramachandran plots. The protein structure instability, in any case, should not affect its antifreeze activity. At the ice/water interface the sfAFP triggers local melting of the ice surface. Bipyramidal, secondary prism, and prism ice planes melt in the presence of AFP at temperatures below the melting point of ice. Only the basal plane is found to be stable at the same temperatures, indicating an adsorption of the sfAFP on this ice plane as confirmed by experimental evidence.