Hydrogen Exchange Study of Winter Flounder Type I Antifreeze Protein

Abstract

Antifreeze proteins (AFPs) are found in various organisms, such as fish, insects, plants, bacteria, and fungi, to promote survival at subzero temperatures by decreasing the freezing point of bodily fluids. AFPs, which are isolated from Arctic and Antarctic fish, comprise several structurally diverse classes of proteins and four classes of structurally independent proteins are identified: type I AFPs, alanine-rich αhelical proteins of 3.3 to 4.5 kDa; type II AFPs, cysteinerich globular proteins containing five disulfide bonds; type III AFPs, 6-kDa globular proteins; and Type IV AFPs, glutamateand glutamine-rich α-helical proteins. The type I AFPs are alanine-rich amphiphilic α-helical proteins found in winter flounder, yellowtail flounder, Alaskan plaice, shorthorn sculpin, and Arctic sculpin. The winter flounder AFP isoform, HPLC6, which is the most extensively studied AFP, contains three 11-amino acid repeats of the sequence Thr-X2-Asx-X7, where X is generally alanine. X-ray crystallographic studies revealed that the type I AFP is completely α-helical in conformation, with the exception of the last residue (R37), which adopts a 310helix. The hydroxyl and methyl groups of four threonine residues, particularly of the central two residues, T13 and T24, play key roles in the ice-binding properties of the type I AFP. For example, T13S/T24S mutations of the type I AFP caused a 90-100% loss in activity relative to wild-type AFP. The mutants, A17L and A21L, where the substitution lies adjacent to the Thr-rich face, caused a significant decrement of the thermal hysteresis activity, whereas the mutants A19L and A20L exhibited wild-type activity. It was reported that termini of the HPLC6 isoform possess greater helix-stabilizing ability compared to the HPLC2 isoform, leading to a 50% difference in the thermal hysteresis activity between these two AFPs. These results suggested that helical stabilization of the N and C termini is the critical component for antifreeze activity of the type I AFP. It was recently reported that flexibility of the C-terminal region causes a loss of thermal hysteresis activity because its dynamic nature strongly prevents binding to the ice surface. Here, to further understand the correlation between the dynamic properties and function of type I AFPs, we performed NMR hydrogen exchange experiments on the winter flounder type I AFP (HPLC6 isoform) (Fig. 1, referred to as wt-AFP). The hydrogen exchange rate constants for the two mutants, A17Land A20L-AFP (Fig. 1), which exhibit no and wild-type activities in thermal hysteresis respectively, are also determined. Comparison of the wtAFP and mutant AFPs can provide valuable insights into the thermal hysteresis mechanism of type I AFPs.