Structural Basis for the Inhibition of Gas Hydrates by α -Helical Antifreeze Proteins

Abstract

Kinetic hydrate inhibitors (KHIs) are used commercially to inhibit gas hydrate formation and growth in pipelines. However, improvement of these polymers has been constrained by the lack of verified molecular models. Since antifreeze proteins (AFPs) act as KHIs, we have used their solved x-ray crystallographic structures in molecular modeling to explore gas hydrate inhibition. The internal clathrate water network of the fish AFP Maxi, which extends to the protein's outer surface, is remarkably similar to the {100} planes of structure type II (sII) gas hydrate. The crystal structure of this water web has facilitated the construction of in silico models for Maxi and type I AFP binding to sII hydrates. Here, we have substantiated our models with experimental evidence of Maxi binding to the tetrahydrofuran sII model hydrate. Both in silico and experimental evidence support the absorbance-inhibition mechanism proposed for KHI binding to gas hydrates. Based on the Maxi crystal structure we suggest that the inhibitor adsorbs to the gas hydrate lattice through the same anchored clathrate water mechanism used to bind ice. These results will facilitate the rational design of a next generation of effective green KHIs for the petroleum industry to ensure safe and efficient hydrocarbon flow.

Ice-Binding Proteins in Plants

“…This model suggests that the IBP structure itself is responsible for the organization of surrounding water molecules into an ice-like lattice, by forming water cages around the methyl groups of outward facing residues on the ice-binding surface. This model is consistent with the ability of IBPs to also adsorb to gas hydrates ( Sun et al, 2015 ; Walker et al, 2015 ). By forming structures that resemble the quasi-liquid layer existing between water and ice, IBPs are then able to merge with ice crystal surfaces.…”

Adsorption‐Inhibition of Clathrate Hydrates by Self‐Assembled Nanostructures

“…[69,70,73] Still, there is no evidence that such an ordered layer of waters is created around the SFO lamellar structure, or that this structure indeed binds to ice or hydrates. However, the fact that SFO, like other AFPs, [44,45,59] binds to both ice and THF hydrates suggests that SFO self-assembled structures bind to the crystal surface via the clathrate water mechanism. [69] In contrast, PVP binds to hydrates but not to ice, probably via filling cavities (i. e. open cages) with its pyrrolidone ring [17,29] rather than by the clathrate water mechanism.…”

“…[39] Luckily for the fish that proliferate in subzero temperatures, an elegant solution to this problem evolved in the form of AFPs, [40] which bind to the surface of ice crystals and inhibit their growth. [34][35][36]41,42] The discovery that various AFPs inhibit clathrate hydrates [23,[43][44][45][46] as well, suggests that these proteins bind to the hydrate surface and inhibit further growth, similar to the mechanism by which they inhibit ice growth. [35,41] Moreover, the structure of many AFPs has been determined by X-ray crystallography, which simplifies studies that include molecular dynamics simulations.…”

“…[54] We hypothesize that KHIs inhibit the growth of clathrate hydrates via a similar mechanism as AFPs, which includes binding to the surface of the crystal [45,46] and inhibiting its growth via the Kelvin effect. [12,42] In this study, using a coldstage, microfluidics, fluorescence microscopy and cryo-TEM, we examined the effect of SFO, phenosafranine (PSF), and Polyvinylpyrrolidone (PVP), [4,17,20,30,31] (Figure 1), on the growth of tetrahydrofuran (THF) hydrates, which form SII hydrates [22,43,45,[59][60][61] that are obtained at ambient pressure. This model system is technically simpler and requires less costly instruments than those used for high pressures for small hydrocarbons, [25,32,62,63] which form SI hydrates.…”

Peptide backbone circularization enhances antifreeze protein thermostability

“…As a result, surrounding water remains liquid at lower temperatures and further ice growth is suppressed. This has led to use of AFPs in the frozen food industry for preserving product texture and improving cold storage and for assessing AFPs as kinetic hydrate inhibitors to prevent the build‐up of gas hydrate in oil pipelines …”