![]() Still, for most IBSs the number of hydrogen bonds formed between the protein and liquid as well as between the protein and solid water is similar 40. The enthalpy gain strongly depends on the positions of the hydrogen bond donors and acceptors. Later on, several authors investigated the enthalpy gain upon binding of different AFPs to various crystallographic ice planes 28, 39. The proposed mechanism was rejected soon, as experimental studies showed that the OH groups of the IBS only have a negligible role for ice-binding 4, 5, 37, 38. Initial studies focused on the enthalpic part of the binding process and suggested that AFPs bind directly to the growing ice crystal via hydrogen bonds as sketched in Fig. Nevertheless, not only the correct shape, but also the adequate interactions of the protein with ice and water play a significant role. Experimental observations confirmed this behavior, as AFPs were found to preferably bind to one specific ice site matching these IBS criteria 34. This was also confirmed by subsequent reports and is still believed to be one of the key properties of IBSs 32, 33. They highlighted that the distances between regularly spaced residues at the IBS, or “active” site of an AFP, are comparable with the distances found between oxygen atoms in ice crystals 29, 30, 31. The first computational studies focused on the shape of the IBS 29, 30, 31. Unfortunately, the adsorption mechanism is still not understood in detail 28.Ĭomputer investigations based on experimentally obtained, three-dimensional structures have been performed for an atomistic view on this problem. In essence, the attached proteins force the ice surface to grow in a highly curved surface, resulting in a freezing point depression. The second step of this process, the inhibition, can be explained quite well with the principle of the Gibbs-Thomson (Kelvin) effect and is very well described in the literature 12, 28. Consequently, the binding of AFPs to ice-nuclei is likely one of the most challenging recognition problems posed by nature 24.ĪFPs prevent freezing by adsorbing to the growing ice crystal and subsequently inhibiting further ice growth 25, 26, 27. ![]() This is remarkable and unique, as the protein’s natural solvent as well as ligand is water itself: in the liquid state acting as solvent, in the frozen state as ligand. These have resulted in the currently accepted theory, stating that AFPs act by binding to the growing ice crystal 19. The techniques include mutation studies 20, ice-etching studies 20, as well as structure determination methods like X-ray crystallography 21, 22 and NMR 23. In order to understand how AFPs are able to prevent freezing, a variety of experimental methods have been applied to this protein family. The IBS is usually a rather apolar surface 19 formed typically by threonine residues and, to a smaller extent, by other apolar amino acids (AAs) like valine (VAL), glycine (GLY), and alanine (ALA). Their structural variety indicates different evolutionary origins, whereby the specific face of the protein binding to the ice crystal, the ice-binding site (IBS), is similar for most of these proteins. The cryo-protection ability of those proteins also makes them interesting for industrial use like cryopreservation 14, 15, ice cream production 16, frozen food storage 17, and deicing 18. Therefore, these proteins are called thermal hysteresis proteins. AFPs lower the freezing temperature and slightly increase the melting temperature of water 13. fish 1, 2, 3, 4, 5, insects 6, 7, 8, plants 9, 10, and bacteria 11 to survive at temperatures below 0 ☌ 12. Especially, high enthalpic interactions between the protein surface and water can hinder the ice-binding activity.Īntifreeze Proteins (AFPs) are a structurally diverse class of proteins helping a variety of organisms, e.g. Based on these observations, we propose a new, thermodynamically more refined mechanism of the ice recognition process showing that the appropriate balance between entropy and enthalpy facilitates ice-binding of proteins. In contrast to most of the recently published studies, our analyses show that enthalpic interactions are as important as an ice-like pre-ordering. The observed enthalpic and entropic differences between the ice-binding sites and the inactive surface reveal key properties essential for proteins in order to bind ice: While entropic contributions are similar for all sites, the enthalpic gain for all ice-binding sites is lower than for the rest of the protein surface. We investigated three AFPs using Molecular Dynamics simulations in combination with Grid Inhomogeneous Solvation Theory, exploring their hydration thermodynamics. The detailed binding mechanism is, however, still not fully understood. Antifreeze Proteins (AFPs) inhibit the growth of an ice crystal by binding to it.
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