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Biofoams
land vertebrates is the sweat of horses. Even the fact that horses, like humans, keep cool by copious sweating sets them apart from most of the animal kingdom. While our sweat contains a lot of salt and little protein, the situation is the other way round for horses.
X-ray crystallography reveals
that ranasmurfin has a new type fold with novel crosslinks between amino acid side chains
One of the abundant proteins in horse sweat is latherin, a protein with strong surfactant properties. The most likely explanation for its presence is that it helps the sweat to wet the pelt hairs, which are water-repellent. The sweat water can thereby move to the surface of the hairs more quickly, making the cooling more efficient. For a herbivore whose main protection from predators is endurance in fast running, this kind of adaptation is of vital importance. The amino acid sequence of the protein is highly conserved between the four equid species: horse, zebra, onager, and ass. However, the latherin protein also occurs in the saliva of horses, and researchers suspect that the horse’s mouth is the place where it serves its more ancestral function, namely wetting dry and fibrous food and helping with its mastication.
How does this protein form stable biofoams? Back in 2009, the groups of Cooper and Kennedy first determined the complete amino acid sequence and the basic physical parameters of the protein when it acts as a surfactant.4 Using neutron reflection experiments, they found that it is spread out very thinly at the water–air interface, producing a layer of only 10Å (10-10
biophysical investigations of the protein in solution suggested a globular shape. Determining the structure of the
protein in aqueous solution, however, proved challenging. Using NMR, the groups of Brian Smith, also at Glasgow, and Malcolm Kennedy found that the leucine residues, which account for almost a quarter of the amino acid sequence, are extremely difficult to tell apart in the spectra. Before reporting the structure, the teams published a separate paper describing the details of how they eventually managed to assign the signals to the amino acid residues.5 When the structure was solved, the
overall shape of the soluble protein turned out to be a hollow cylinder defined by β-sheets on one side and by α-helices on the other.6
The helices are aligned
parallel to the cylinder axis, and the researchers suspect that they could part company to allow the cylinder to unfurl and the protein molecule to flatten out on the surface. The fold isn’t novel in this case,
but close relatives are hard to find. The m) thickness, while
closest and most interesting match is probably with the PLUNC (Palate, LUng and Nasal epithelium Carcinoma) proteins, some of which may serve as surfactants in saliva, but their functions remain a mystery. For these proteins there is no experimental structure yet, but the Glaswegian groups successfully applied modelling on the basis of the latherin structure to predict a plausible structure for a human PLUNC protein BPIFA1, which is produced in the trachea.
Surfactant
proteins have many potential applications in foods and medicine
Foaming food Although the research into these surfactant proteins was purely curiosity- driven, applications could be found in several areas, for instance, in food processing. Unilever, for example, has filed patents for the use of a group of surfactant proteins from fungi, the hydrophobins, to stabilise the size of air bubbles in foamed desserts, ice cream and milk shakes. Hydrophobins are small – around 100 amino acids – cystein-rich proteins, which appear to operate quite differently from latherin and the ranaspumins. Unilever also uses hydrophobins in combination with antifreeze proteins to control the size of ice crystals in frozen desserts. Other potential applications
References 1 M. Oke et al, Angew. Chem. Int. Edn. 2008, 47, 7853.
2 D. C. Hissa et al, J. Exp. Biol., 2008, 211, 2707.
3 J. C. Castilla et al, Proc. Natl. Acad. Sci. U.S.A, 2007, 104, 18120.
4 R. E. McDonald et al, PLOS One, 2009, 4, e5726.
5 S. J. Vance et al, Biomol. NMR Assign., 2013, DOI 10.1007/s12104-013- 9485-3.
6 S. J. Vance et al, J. R. Soc. Interface, 2013, 10: 20130453. http://
dx.doi.org/10.1098/ rsif.2013.0453
7 M. Khalesi et al, Cerevisia, 2012, 37, 3.
of hydrophobins are as special biosurfactants or foaming agents, as well as for protein immobilisation in food production and in biosensor applications. They could also be used to stabilise flavours and encapsulate trace ingredients in beverages. Brewers and wine makers also hope that hydrophobin research will enable them to address a fungal spoiling phenomenon known as gushing.7 As the protein-based foams are biocompatible, biodegradable, water- based, bioadhesive, stable for many days, resistant to dehydration, and antimicrobial, they have potential for medical applications, including, for example, temporary wound dressings, coatings for artificial surgical implants and for the controlled release of drugs. Perhaps much more important, suggests Kennedy, is how researchers can accidentally discover new phenomena that can then be useful medically and commercially by being allowed to exercise pure curiosity.
Michael Gross is a science writer based in Oxford, UK
Chemistry&Industry • November 2013 27
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