Antifreeze proteins have now been found also in plants and are one of nature’s neatest tricks. But where did they come from? AFPS are a classic case of evolution of new functions by gene duplication. The original gene continues in its old function; the copy mutates and finds a new role. The AFPs in some fishes such as eelpouts, ocean pouts, and wolffishes are similar to an old enzyme present in most creatures from bacteria through vertebrates that catalyze intracellular synthesis of sialic acids. AFPs have evolved independently in different fishes from different genes.
In a recent paper in Science, researchers at Queen’s University, Kingston, Canada, describe the structure of the AFP from the winter flounder. The two helices of its polypeptide chains corral around 400 water molecules in a kind of cage structure. The water molecules also spill out of the overall molecule and certain sites on the protein orientate ice crystals which form within the cell. Crystal growth is halted before it can freeze the cell.
The fact that a protein can be fashioned, full of water, to create an antifreeze compound, from a gene derived from a much more humble biochemical housekeeping gene, demonstrates once again the astonishing versatility of proteins. DNA gets all the hype but it is just a string of letters. Life’s machinery – its chemical engineering – is protein-based. Proteins can gown into exquisite structures by self assembly, such as a bird’s feather, or ferry oxygen around the body in haemoglobin, or power the muscles of Usain Bolt, or regulate the body’s burning of sugar (insulin), or speed up the body’s vital chemical reactions by almost a billion billion times. Proteins, by human engineering standards, seem miraculous.
We consume AFPs regularly in the fish we eat (and some plants too). Engineered AFPs could have many uses. The first is fairly trivial – they are used to control ice crystal growth in ice cream – but in future, nature’s antifreeze proteins will almost certainly find more profound applications.
Science, 2014, 343, pp. 795-98