The temperature of arctic waters is as low as -1.8°C for many months of the
year. How is it that fish, such as the arctic cod (Figure 1), survive at this
temperature without their tissues freezing? The answer lies in antifreeze
proteins that bind to ice crystals and prevent the freezing of cells. The nature
of these proteins has been studied in detail, partly because of their usefulness
in industry. Ice cream manufacturers are using arctic fish antifreeze proteins
to avoid the formation of ice crystals during the cooling process. The proteins
have also been used successfully in the cold storage of sperm and other tissues,
with applications in agriculture, wildlife conservation, medical research, and
fertility treatments.
Figure 1: Arctic cod,
Boreogadus saida, manufactures special proteins that prevent its
tissues from freezing in the sub zero waters of the Arctic Ocean.
One aspect of research on antifreeze proteins has focused on the genetic code
that results in their formation. This research has not only increased understanding
of the protein's structure and function, but also its origins. Recently, a study
of a gene that encodes for an antifreeze protein in some Antarctic fishes has
shed light on the evolution of this adaptation1. The discovery is a
gene that encodes for both an antifreeze protein and a digestive protein, a protease.
The DNA sequences that encode for the antifreeze and the digestive protein are
side-by-side, with a segment of noncoding DNA in between them. This finding not
only confirms suspicions that these antifreezes arose from proteases, but also
sheds some light on the mechanism of evolution of these important molecules.
The basic structure of antifreeze molecules follows a simple repeat pattern (with
some variation between types) of three amino acids-threonine (Thr), proline (Pro),
and alanine (Ala) (Figure 2). The ability of these molecules to bind to ice crystals
is a consequence of this repeating pattern (Figure 3).
Figure 2: Three amino acids
- two alanine and one threonine - make up this chemical structure. A molecule
comprised of this unit, repeated in a chain, has antifreeze properties.
Figure 3: The side chains on
the antifreeze molecule bind to the ice crystal lattice, preventing it from growing.
Tellingly, a segment of the digestive enzyme DNA in the dual gene encodes for
a Thr-Ala-Ala sequence; a single event an accident of replication
would have resulted in a repeat of this element. The resulting protein would have
some ice-binding properties. This was likely the first step in the evolution of
the antifreeze proteins, at least in this group of fishes. Further events, in
subsequent generations, would have resulted in additional replications of the
element, and eventually, over evolutionary time, the DNA encoding for the antifreeze
protein split from the digestive enzyme DNA. The genome of the Antarctic fish
studied contains a record of these events with a gene that encodes for
a protease with a Thr-Ala-Ala element, a gene that encodes for both a protease
and an antifreeze, and others encode for just antifreezes. The ability of these
fish to thrive in subzero waters is a product of a genetic accident a glimpse
of evolution at work!
1 Cheng CC, Chen L. 1999. Evolution of an antifreeze glycoprotein.
Nature 401: 443444.