Non-protein biological antifreeze molecule
Biological antifreeze molecules: Antifreeze molecules produced by certain vertebrates, plants, fungi and bacteria permit their survival in subzero environments. If ice crystals are formed inside an organism exposed to subzero temperatures then this ice crystal formation can draw so much water out of the organism’s cells that those cells will die and this will be fatal for the organism. In organisms that survival in subzero environments the antifreeze molecules keep small ice crystals from growing or prevent ice crystals from forming at all. Basically the antifreeze molecules help freeze-tolerant organisms survive by preventing freezing from penetrating into cells. Normally the antifreeze molecules are a class of polypeptides called antifreeze proteins (AFPs) or ice structuring proteins (ISPs). AFPs bind to small ice crystals to inhibit growth and recrystallization of ice. There is also increasing evidence that AFPs interact with mammalian cell membranes to protect from cold damage.
Thermal hysteresis: Antifreeze molecules create a difference between the melting point and freezing point known as thermal hysteresis. The addition of antifreeze molecules at the interface between solid ice and liquid water inhibits the thermodynamically favored growth of the ice crystal.
Non-protein biological antifreeze molecule
On 23 November 2009 it was reported that Kent Walters, a PhD student from the University of Notre Dame in Indiana and his collaborators at the University of Alaska Fairbank had discovered a new class of biological antifreeze molecules – the first that do not contain proteins. The molecule was termed xylomannan and is found in the Alaskan beetle – Upis ceramboides that is able to survive temperatures below minus 100 degrees Fahrenheit. Initially Kent Walters was attempting to isolate and purify the AFP from the freeze-tolerant beetle Upis ceramboides as part of his PhD thesis at the University of Notre Dame in Indiana. ‘He tried unsuccessfully to do so over the course of nearly two years,’ says his then supervisor Jack Duman. ‘The sample that he isolated had thermal hysteresis activity, but there was no protein associated with it.’ Working with colleagues from Notre Dame and the Institute of Arctic Biology at the University of Alaska Fairbanks, the researchers identified the novel thermal hysteresis factor (THF) as a xylomannan in association with a fatty acid, which was largely located on cell membranes.
According to Kent Walters and his collaborators at the University of Alaska Fairbank’s Institute of Arctic Biology, xylomannan has little or no protein. It is composed of a sugar and a fatty acid and may exist in new places within the cells of organisms. Brian Barnes, director of the institute said, “The most exciting part of this discovery is that this molecule is a whole new kind of antifreeze that may work in a different location of the cell and in a different way.” A possible advantage of this novel molecule comes from it having the same fatty acid that cells membranes do. This similarity, says Barnes, may allow the molecule to become part of a cell wall and protect the cell from internal ice crystal formation. Barnes also said, “There are many difficult studies ahead. To find out how common this biologic antifreeze is and how it actually prevents freezing and where exactly it’s located.”
UAF graduate student and project collaborator Todd Sformo found that the Alaska Upis beetle first freezes at about minus 18.5 degrees Fahrenheit in the lab and survives temperatures down to about 104 degrees below zero Fahrenheit. Barnes noted, “It seems paradoxical that we find an antifreeze molecule in an organism that wants to freeze and that’s adapted to freezing.”
Jeffrey Bale (who is an expert on how insects survive in cold conditions) at the University of Birmingham in the UK said, “These results take our understanding of this subject to a new level. Not only is this the first thermal hysteresis factor to be structurally characterised from a freeze-tolerant species, the study has identified a totally new class of compounds that can produce a hysteretic effect equivalent to that of proteins.” As a next step, Bale says it will be fascinating to investigate the co-existence of proteins and xylomannan-like compounds in insects, and whether the properties of thermal hysteresis factors could be exploited through genetic engineering.
The other members of the project were Anthony Serianni and John H. Duman of University of Notre Dame.
(Source: http://www.rsc.org/chemistryworld/News/2009/November/23110902.asp and http://www.iab.uaf.edu/news/index.php?newsrel=81&start=0&nitems=71)
December 15, 2009