Binding of an Oligopeptide to a Specific Plane of Ice

Abstract

The ␣-helical antifreeze protein (AFP) from winter flounder inhibits ice growth by binding to a specific set of pyramidal surface planes that are not otherwise macroscopically expressed. The 37-residue AFP contains three 11-amino acid repeats that make a stereo-specific fit to the ice lattice along the ͗01-12͘ direction of the {20 -21} and equivalent binding planes. When the AFP was shortened to delete two of the three 11-amino acid ice-binding repeats, the resulting 15-residue peptide and its variants were less helical and showed no antifreeze activity. However, when the helicity of the peptide was reinforced by an internal lactam bridge between Glu-7 and Lys-11, the minimized AFP was able to stably express the pyramidal plane {20 -21} on the surface of growing ice crystals. This dynamic shaping of the ice surface by a single ice-binding repeat provides evidence that AFP adsorption to the ice lattice is not an "all-or-nothing" interaction. Instead, a partial interaction can help develop the binding site on ice to which the remainder of the AFP (or other AFP molecules) can orient and bind.Type I AFP 1 from winter flounder, represented by the abundant serum isoform HPLC-6 (1) is a remarkably long freestanding ␣-helix (2). Its helicity can be attributed to several structural features, which include an abundance of alanine, an extensive capping network at both termini (3), and an internal salt bridge (1, 4). From ice etching studies, it was shown that this AFP binds to the {20 -21} hexagonal bipyramidal planes of ice along the ͗01-12͘ direction (5). It has been suggested that the critical connection between structure and function in this protein is that putative ice-binding residues (Thr, Asx, and Leu) are aligned and regularly spaced along one face of the helix (5, 6). Each residue (e.g. Thr-2, Thr-13, Thr-24, and Thr-35) is spaced 11 amino acids apart (16.5 Å), which closely matches the 16.7-Å distance between repeating features of the ridge and valley topology along the ͗01-12͘ direction of the {20 -12} binding plane. Adsorption of type I AFP to these lattice binding sites through a precise distance and geometry match involving H-bonding and van der Waals interactions (5, 7) leads to inhibition of ice growth by the Kelvin effect (8, 9). In the process, seed ice crystals are constrained to form hexagonal bipyramids with a c:a axial ratio of 3.3:1, which matches the ratio predicted from the adsorption planes revealed by ice etching studies (5, 6).Several attempts have been made to model the helix in contact with the ice surface (3, 7, 10, 11). As a result, a common concern is that the number and strength of the potential interactions between ice and AFP are barely sufficient for tight binding (3, 7). One solution proposed is that all four ice-binding threonines share the same rotamer configuration and bind to ice in a "zipper-like fashion" (11). A similar hypothesis, stemming directly from the x-ray structure, is that tight binding relies on the simultaneous docking of similarly aligned and constrained ice-binding...

Programmable biofilm-based materials from engineered curli nanofibres

“…The library members, detailed in Table 1, range in length from 7-59 amino acids and encode a wide variety of functions such as binding to various inorganic substrates (GBP, CNBP, QBP, MBD and AFP8), nucleation of mineral and metallic nanostructures (A3, CLP12, CT43 and Mms6), and a highly specific catalytic interaction with a protein (SpyTag) 15,[21][22][23][24][25][26][27][28][29][30][31] . Each peptide domain was cloned as C-terminal fusions to CsgA with an intervening six-amino-acid flexible linker (Table 1) and these plasmids were expressed in LSR10 cells to produce 12 different BIND biofilms.…”

Antifreeze Protein-induced Morphological Modification Mechanisms Linked to Ice Binding Surface

“…This exceptional occurrence of variable face orientations in AFP-modified ice has not been dealt with. The unusual behavior of the AFP of one insect (Tenebrio molitor) (13) in producing pyramidal ice crystallites, as opposed to hexagonal disk-shaped ones, has puzzled the literature because, as will be shown in this work, previous studies failed to distinguish the predetermined face indices of the primary pyramid from the variable face indices of the secondary pyramids (17). The superior activity of the insect AFPs (13) in depressing the freezing point of water has also remained unexplained; it can, however, be attributed to the difference between the surface poisoning mechanism on one hand and the mechanism of bridge-induced surface reconstruction on the other.…”

“…Less elongated pyramids with indices in the range (102)-(201), although not excluding the occurrence of (101), have also been observed (26,27) as well as combinations of such pyramids with the basal face (001) and various prisms (hk0) (17). The addition of foreign compounds can change the AFP conformation, thereby causing a transition in the ice morphology between pyramidal and prismatic (29).…”

“…Experimental accounts report that the binding of fish AFP to hexagonal pyramids is indexed frequently as (201) (14,16,17,19,21,23); but more elongated pyramids with higher indices such as (301) or (401) (24,25) and even extremely elongated pyramids with indices (501) or higher (15) have been reported. Less elongated pyramids with indices in the range (102)-(201), although not excluding the occurrence of (101), have also been observed (26,27) as well as combinations of such pyramids with the basal face (001) and various prisms (hk0) (17).…”

The Role of N and C Termini in the Antifreeze Activity of Winter Flounder (Pleuronectes americanus) Antifreeze Proteins

“…A study of a minimized peptide consisting of the N and C termini of wflAFP-6 (38) and mutagenesis studies performed on the first and last Thr residues of wflAFP-6 (21) have suggested a role for the N terminus in activity. The Thr 3 Ser mutation in [L(T2S)]Skin clearly demonstrated that Thr-2 is involved in antifreeze activity.…”